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  • 1. Agapitov, Oleksiy
    et al.
    Krasnoselskikh, Vladimir
    de Wit, Thierry Dudok
    Khotyaintsev, Yuri
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Pickett, Jolene S.
    Santolik, Ondrej
    Rolland, Guy
    Multispacecraft observations of chorus emissions as a tool for the plasma density fluctuations' remote sensing2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, A09222- p.Article in journal (Refereed)
    Abstract [en]

    Discrete ELF/VLF chorus emissions are the most intense electromagnetic plasma waves that are observed in the radiation belts and in the outer magnetosphere of the Earth. They are assumed to propagate approximately along the magnetic field lines and are generated in source regions in the vicinity of the magnetic equator and in minimum B pockets in the dayside outer zone of the magnetosphere. The presence of plasma density irregularities along the raypath causes a loss of phase coherence of the chorus wave packets. These irregularities are often present around the plasmapause and in the radiation belts; they occur at scales ranging from a few meters up to several hundred kilometers and can be highly anisotropic. Such irregularities result in fluctuations of the dielectric permittivity, whose statistical properties can be studied making use of intersatellite correlations of whistler waves' phases and amplitudes. We demonstrate how the whistler-mode wave properties can be used to infer statistical characteristics of the density fluctuations. The analogy between weakly coupled oscillators under the action of uncorrelated random forces and wave propagation in a randomly fluctuating medium is used to determine the wave phase dependence on the duration of signal recording time. We study chorus whistler-mode waves observed by the Cluster WBD instrument and apply intersatellite correlation analysis to determine the statistical characteristics of the waveform phases and amplitudes. We then infer the statistical characteristics of the plasma density fluctuations and evaluate the spatial distribution of the irregularities using the same chorus events observed by the four Cluster spacecraft.

  • 2. Agren, Anneli
    et al.
    Buffam, Ishi
    Bishop, Kevin
    Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden.
    Laudon, Hjalmar
    Modeling stream dissolved organic carbon concentrations during spring flood in the boreal forest: A simple empirical approach for regional predictions2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, no G1, G01012- p.Article in journal (Refereed)
    Abstract [en]

    Changes in dissolved organic carbon (DOC) concentration are clearly seen for streams in which chemistry is measured on a high-frequency/episode basis, but these high-frequency data are not available in long-term monitoring programs. Here we develop statistical models to predict DOC concentrations during spring flood from easily available geographic information system data and base flow chemistry. Two response variables were studied, the extreme DOC concentration and the concentration during peak flood. Ninety-seven streams in boreal Scandinavia in two different ecoregions with substantially different mean water chemistry and landscape characteristics (covering a large climatic gradient) were used to construct models where 56% of the extreme DOC concentration and 63% of the concentration during peak flood were explained by altitude. This highlights important regional drivers (gradients in altitude, runoff, precipitation, temperature) of material flux. Spring flood extreme DOC concentration could be predicted from only base flow chemistry (r(2) = 0.71) or from landscape data (r(2) = 0 .74) but combining them increased the proportion of explained variance to 87%. The "best" model included base flow DOC (positive), mean annual runoff (negative), and wetland coverage (positive). The root mean square error was 1.18 mg L-1 for both response variables. The different ecoregions were successfully combined into the same regression models, yielding a single approach that works across much of boreal Scandinavia.

  • 3.
    Almqvist, Bjarne
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Burlini, Luigi
    Mainprice, David
    Hirt, Ann
    Elastic properties of anisotropic synthetic calcite-muscovite aggregates2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115Article in journal (Refereed)
  • 4.
    Almqvist, Bjarne
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Mainprice, David
    Madonna, Claudio
    Burlini, Luigi
    Hirt, Ann
    Application of differential effective medium, magnetic pore fabric analysis, and X-ray microtomography to calculate elastic properties of porous and anisotropic rock aggregates2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116Article in journal (Refereed)
  • 5. Andreeova, K.
    et al.
    Pulkkinen, T. I.
    Laitinen, Tiera V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Prech, L.
    Shock propagation in the magnetosphere: Observations and MHD simulations compared2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A9, A09224- p.Article in journal (Refereed)
    Abstract [en]

    We examine the propagation of disturbances in the Earth's magnetosphere caused by fast forward shock interaction with the magnetopause. Our statistical study and event analyses show that the propagation speeds are larger in the magnetosphere than in the solar wind and are larger in the nightside magnetosphere than in the dayside magnetosphere. A case study of a double shock during 9 November 2002 is examined both observationally and using the GUMICS-4 global MHD simulation. Tracing the disturbance propagation allows us to confirm that the MHD simulation results are in good agreement with the in situ observations. The simulation results show that the propagation of the disturbance occurs in the antisunward direction at all clock angles simultaneously. However, changes in the magnetosheath are largest at high latitudes, while in the magnetotail the largest variations are seen in the plasma sheet.

  • 6.
    Andrews, David J.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Cowley, S. W. H.
    Dougherty, M. K.
    Lamy, L.
    Provan, G.
    Southwood, D. J.
    Planetary period oscillations in Saturn's magnetosphere: Evolution of magnetic oscillation properties from southern summer to post-equinox2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A04224- p.Article in journal (Refereed)
    Abstract [en]

    We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturn's magnetosphere over the interval from late 2004 to early 2011, spanning equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L <= 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the equinox interval, such that the beat period increases significantly from similar to 20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a similar to 10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.

  • 7. Auriac, A.
    et al.
    Spaans, K. H.
    Sigmundsson, F.
    Hooper, A.
    Schmidt, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Iceland rising: Solid Earth response to ice retreat inferred from satellite radar interferometry and visocelastic modeling2013In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, no 4, 1331-1344 p.Article in journal (Refereed)
    Abstract [en]

    A broad uplift occurs in Iceland in response to the retreat of ice caps, which began circa 1890. Until now, this deformation signal has been measured primarily using GPS at points some distance away from the ice caps. Here, for the first time we use satellite radar interferometry (interferometric synthetic aperture radar) to constrain uplift of the ground all the way up to the edge of the largest ice cap, Vatnajokull. This allows for improved constraints on the Earth rheology, both the thickness of the uppermost Earth layer that responds only in an elastic manner and the viscosity below it. The interferometric synthetic aperture radar velocities indicate a maximum displacement rate of 24 +/- 4 and 31 +/- 4 mm/yr at the edge of Vatnajokull, during 1995-2002 and 2004-2009, respectively. The fastest rates occur at outlet glaciers of low elevation where ice retreat is high. We compare the observations with glacial isostatic adjustment models that include the deglaciation history of the Icelandic ice caps since 1890 and two Earth layers. Using a Bayesian approach, we derived probability density functions for the average Earth model parameters for three satellite tracks. Based on our assumptions, the three best fit models give elastic thicknesses in the range of 15-40 km, and viscosities ranging from 4-10x1018 Pa s.

  • 8.
    Ayarza, P
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Juhlin, Christopher
    Brown, D
    Beckholmen, M
    Kimbell, G
    Pechnig, R
    Pevzner, L
    Pevzner, R
    Ayala, C
    Bliznetsov, M
    Glushkov, A
    Rybalka, A
    Integrated geological and geophysical studies in the SG4 borehole area, Tagil Volcanic Arc, Middle Urals: Location of seismic reflectors and source of the reflectivity2000In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 105, no B9, 21333-21352 p.Article in journal (Refereed)
    Abstract [en]

    Near-vertical incidence reflection seismic data acquired in the Tagil Volcanic Arc (Middle Urals) show the upper crust to be highly reflective. Two intersecting seismic lines located near the ongoing ∼5400 m deep SG4 borehole show that the main reflectivity strikes approximately N-S and dips ∼35°–55° to the east. Prominent reflections intercept the borehole at ∼1000, ∼1500, 2800–2900, ∼3400, and between ∼4000 and 5400 m, which correspond to intervals of low velocity/low density/low resistivity. The surface projections of these reflections lie parallel to the strike of magnetic anomaly trends. Multioffset vertical seismic profile (VSP) data acquired in the SG4 borehole show a seismic response dominated by P to S reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data. Modeling of the VSP data constrains the depth at which reflectors intercept the borehole and suggests that the P to S conversions are best explained by low-velocity porous intervals rather than higher-velocity mafic material. The most prominent east dipping reflection on the surface seismic data is only imaged on VSP shots that sample the crust closer to the E-W seismic line. This discrepancy between the VSP and the surface seismic data is attributed to rapid lateral changes in the physical properties of the reflector. Surface and borehole data suggest that the low-velocity/low-density/low-resistivity intervals are the most important source of reflectivity in the SG4 borehole area, although lithological contrasts may also play a role. Drill cores from the these zones contain hydrothermal alteration minerals indicating interaction with fluids. Tectonic criteria suggest that they might represent imbricated fracture zones often bounding different lithologies and/or intrusions. Some of them might also represent high-porosity lava flows or pyroclastic units, common in island arc environments.

  • 9. Bavassano Cattaneo, M. Bice
    et al.
    Marcucci, M. Federica
    Retinò, Alessandro
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Pallocchia, G.
    Rème, H.
    Dandouras, I.
    Kistler, L. M.
    Klecker, B.
    Carlson, C. W.
    Korth, A.
    McCarthy, M.
    Lundin, Rickard
    Balogh, A.
    Kinetic signatures during a quasi-continuous lobe reconnection event: Cluster Ion Spectrometer (CIS) observations2006In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, no A9, A09212- p.Article in journal (Refereed)
    Abstract [en]

    On 3 December 2001 the Cluster spacecraft observed a long-lasting lobe reconnection event in the southern high-latitude dusk magnetopause (MP) tailward of the cusp, during a 4 hour interval of mainly northward interplanetary magnetic field ( IMF) and of sub-Alfvenic magnetosheath flow. Almost all the MP encounters have accelerated flows ( for which the Walen test has been successfully verified by Retino et al. ( 2005)) as well as a large number of secondary populations related to reconnection, that is, ions of magnetosheath or magnetospheric origin which cross the MP either way. The detailed analysis of the distribution functions shows that the reconnection site frequently moves relative to the spacecraft, but simultaneous measurements by two spacecraft on opposite sides of the reconnection site indicate that the spacecraft's distance from the X line is small, i.e., below 3200 km. The vicinity to the X line throughout the event is probably the reason why the distribution functions characteristics agree with theoretical expectations on both sides of the reconnection site throughout this long event. Moreover, the detailed analysis of the distribution functions shows evidence, during a few time intervals, of dual reconnection, i.e., of reconnection simultaneously going on also in the northern hemisphere.

  • 10.
    Becerra, Marley
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Soula, Serge
    Chauzy, Serge
    Effect of the space charge layer created by corona at ground level on the inception of upward lightning leaders from tall towers2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no D12, D12205- p.Article in journal (Refereed)
    Abstract [en]

    Electric field measurements above ground have shown that the space charge layer created by corona at ground level shields the background electric field produced by the thundercloud. Therefore it is expected that this space charge layer can also influence the conditions required to initiate upward lightning from tall objects. For this reason, a numerical model that describes the evolution of the main electrical parameters below a thunderstorm is used to compute the space charge layer development. The time variation of the electric field measured at 600 m above ground during the 1989 rocket triggered lightning experiment at the Kennedy Space Center (Florida) is used to drive the model. The obtained space charge density profiles are used to compute the conditions required to initiate stable upward lightning positive leaders from tall towers. Corona at the tip of the tower is neglected. It is found that the space charge layer significantly affects the critical thundercloud electric fields required to initiate upward lightning leaders from tall objects. The neutral aerosol particle concentration is observed to have a significant influence on the space charge density profiles and the critical thundercloud electric fields, whereas the corona current density does not considerably affect the results for the cases considered in the analysis. It is found that a lower thundercloud electric field is required to trigger a lightning flash from a tall tower or other tall slender grounded structure in the case of sites with a high neutral aerosol particle concentration, like polluted areas or coastal regions.

  • 11.
    Bruneton, M.
    et al.
    Laboratoire de Ge´ophysique Interne et Tectonophysique, Observatoire des Sciences de l’Univers de Grenoble, Grenoble, France..
    et, al. Shomali, Z.H.
    Svekalapko working group.
    Complex lithospheric structure under the central Baltic Shield from surface wave tomography2004In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 109, no B10303, 1-15 p.Article in journal (Refereed)
  • 12. Byrne, P.K.
    et al.
    Van Wyk de Vries, B.
    Murray, J.B.
    Troll, Valentin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    A Volcanotectonic Survey of Ascraeus Mons, Mars2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117Article in journal (Refereed)
    Abstract [en]

    Ascraeus Mons is one of the largest volcanoes on Mars. It is replete with well-preserved features that can be used to understand its volcanotectonic evolution. Previous studies of this volcano focused on specific features, and were limited by the quality and coverage of contemporary data. Our objective is to review and enhance the existing developmental model for Ascraeus by considering all endogenic surface features on the volcano. We surveyed the volcano's caldera complex, flank terraces, pit structures, sinuous rilles, arcuate grabens, and small vents. We report the spatial and temporal distributions of these features, appraise their proposed formation mechanisms in light of our mapping results, and propose a detailed geological history for Ascraeus Mons. An initial shield-building phase was followed by the formation of a summit caldera complex and small parasitic cones, while compression due to flexure of the supporting basement led to extensive terracing of the shield flanks. An eruptive hiatus followed, ending with the construction of expansive rift aprons to the northeast and southwest. Against later, extensive flank resurfacing in the late Amazonian, continued flexure formed arcuate grabens concentric to the edifice. Localized eruption and surface flow of a fluid agent (lava and/or water) from within the volcano then produced a population of rilles on the lower flanks. Finally, in a change of flank tectonic regime from compression to extension, pit crater chains and troughs developed on the main shield and rift aprons, eventually coalescing to form large embayments at the northeast and southwest base of the volcano.

  • 13. Chaufray, J. Y.
    et al.
    Modolo, Ronan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Leblanc, F.
    Chanteur, G.
    Johnson, R. E.
    Luhmann, J. G.
    Mars solar wind interaction: Formation of the Martian corona and atmospheric loss to space2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no E9, E09009- p.Article, review/survey (Refereed)
    Abstract [en]

    A three- dimensional ( 3- D) atomic oxygen corona of Mars is computed for periods of low and high solar activities. The thermal atomic oxygen corona is derived from a collisionless Chamberlain approach, whereas the nonthermal atomic oxygen corona is derived from Monte Carlo simulations. The two main sources of hot exospheric oxygen atoms at Mars are the dissociative recombination of O-2(+) between 120 and 300 km and the sputtering of the Martian atmosphere by incident O+ pickup ions. The reimpacting and escaping fluxes of pickup ions are derived from a 3- D hybrid model describing the interaction of the solar wind with our computed Martian oxygen exosphere. In this work it is shown that the role of the sputtering crucially depends on an accurate description of the Martian corona as well as of its interaction with the solar wind. The sputtering contribution to the total oxygen escape is smaller by one order of magnitude than the contribution due to the dissociative recombination. The neutral escape is dominant at both solar activities ( 1 x 10(25) s(-1) for low solar activity and 4 x 10(25) s(-1) for high solar activity), and the ion escape flux is estimated to be equal to 2 x 10(23) s(-1) at low solar activity and to 3.4 x 10(24) s(-1) at high solar activity. This work illustrates one more time the strong dependency of these loss rates on solar conditions. It underlines the difficulty of extrapolating the present measured loss rates to the past solar conditions without a better theoretical and observational knowledge of this dependency.

  • 14. Chen, Li-Jen
    et al.
    Bessho, N.
    Lefebvre, B.
    Vaith, H.
    Fazakerley, A.
    Bhattacharjee, A.
    Puhl-Quinn, P. A.
    Runov, A.
    Khotyaintsev, Yuri
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vaivads, Andris
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Georgescu, E.
    Torbert, R.
    Evidence of an extended electron current sheet and its neighboring magnetic island during magnetotail reconnection2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A12, A12213- p.Article in journal (Refereed)
    Abstract [en]

    We have identified a spatially extended electron current sheet (ECS) and its adjacent magnetic island during a magnetotail reconnection event with no appreciable guide field. This finding is based on data from the four Cluster spacecraft and is enabled by detailed maps of electron distribution functions and DC electric fields within the diffusion region. The maps are developed using two-dimensional particle-in-cell simulations with a mass ratio m(i)/m(e) = 800. One spacecraft crossed the ECS earthward of the reconnection null and, together with the other three spacecraft, registered the following properties: (1) The ECS is colocated with a layer of bipolar electric fields normal to the ECS, pointing toward the ECS, and with a half width less than 8 electron skin depths. (2) In the inflow region up to the ECS and separatrices, electrons have a temperature anisotropy (Te-parallel to/Te-perpendicular to > 1), and the anisotropy increases toward the ECS. (3) Within about 1 ion skin depth (d(i)) above and below the ECS, the electron density decreases toward the ECS by a factor of 3-4, reaching a minimum at edges of the ECS, and has a local distinct maximum at the ECS center. (4) A di-scale magnetic island is attached to the ECS, separating it from another reconnection layer. Our simulations established that the electric field normal to the ECS is due to charge imbalance and is of the ECS scale, and ions exhibit electron-scale structures in response to this electric field.

  • 15. Collier, A. B.
    et al.
    Delport, B
    Hughes, W
    Lichtenberger, J
    Steinbach, P
    Oster, J
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Rodger, J
    Correlation between global lightning and whistlers observed at Tihany, Hungary2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, A07210- p.Article in journal (Refereed)
    Abstract [en]

    Although the generation and propagation mechanisms for whistlers are fairly well understood, the location and extent of the lightning source region for the whistlers observed at a given station are currently unknown. The correlation of whistler observations against global lightning data allows an estimate of the size and position of the source region. For whistlers detected at Tihany, Hungary, an area of positive correlation with radius of similar to 1000 km was found to be centered on the conjugate point. Although the maximal sample correlation coefficient was relatively low, r = 0.065, it has a high statistical significance, indicating that it is extremely improbable that the whistlers and lightning in this region are actually uncorrelated. Other smaller areas of positive correlation were found further afield in South America and the Maritime Continent. Lightning in the northern hemisphere displayed a negative correlation with whistlers at Tihany.

  • 16. Cravens, T. E.
    et al.
    Richard, M.
    Ma, Y. -J
    Bertucci, C.
    Luhmann, J. G.
    Ledvina, S.
    Robertson, I. P.
    Wahlund, Jan Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Cui, J.
    Muller-Wodarg, I.
    Waite, J. H.
    Dougherty, M.
    Bell, J.
    Ulusen, D.
    Dynamical and magnetic field time constants for Titan's ionosphere: Empirical estimates and comparisons with Venus2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, no 8, A08319- p.Article in journal (Refereed)
    Abstract [en]

    Plasma in Titan's ionosphere flows in response to forcing from thermal pressure gradients, magnetic forces, gravity, and ion-neutral collisions. This paper takes an empirical approach to the ionospheric dynamics by using data from Cassini instruments to estimate pressures, flow speeds, and time constants on the dayside and nightside. The plasma flow speed relative to the neutral gas speed is approximately 1 m s(-1) near an altitude of 1000 km and 200 m s(-1) at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m s(-1). The ionospheric plasma is strongly coupled to the neutrals below an altitude of about 1300 km. Transport, vertical or horizontal, becomes more important than chemistry in controlling ionospheric densities above about 1200-1500 km, depending on the ion species. Empirical estimates are used to demonstrate that the structure of the ionospheric magnetic field is determined by plasma transport (including neutral wind effects) for altitudes above about 1000 km and by magnetic diffusion at lower altitudes. The paper suggests that a velocity shear layer near 1300 km could exist at some locations and could affect the structure of the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation are shown to be locally important in controlling the structure of Titan's ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.

  • 17. Cui, J.
    et al.
    Galand, M.
    Yelle, R. V.
    Vuitton, V.
    Wahlund, Jan Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Lavvas, P. P.
    Mueller-Wodarg, I. C. F.
    Cravens, T. E.
    Kasprzak, W. T.
    Waite, J. H., Jr.
    Diurnal variations of Titan's ionosphere2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, no 6, A06310- p.Article in journal (Refereed)
    Abstract [en]

    We present our analysis of the diurnal variations of Titan's ionosphere (between 1000 and 1300 km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in the Open Source Ion (OSI) mode obtained from eight close encounters of the Cassini spacecraft with Titan. Although there is an overall ion depletion well beyond the terminator, the ion content on Titan's nightside is still appreciable, with a density plateau of similar to 700 cm(-3) below similar to 1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan's nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through "fast'' ion-neutral chemistry and the latter through "slow'' electron dissociative recombination. The strong correlation between the observed night-to-day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan's dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time-dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long-lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short-lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn's magnetosphere may be an additional and important contributor to the densities of the short-lived ions observed on Titan's nightside.

  • 18. Cui, J.
    et al.
    Galand, M.
    Yelle, R. V.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Ågren, K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Waite, J. H., Jr.
    Dougherty, M. K.
    Ion transport in Titan's upper atmosphere2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, A06314- p.Article in journal (Refereed)
    Abstract [en]

    Based on a combined Cassini data set including Ion Neutral Mass Spectrometer, Radio Plasma Wave Science, and Magnetometer measurements made during nine close encounters of the Cassini spacecraft with Titan, we investigate the electron ( or total ion) distribution in the upper ionosphere of the satellite between 1250 and 1600 km. A comparison of the measured electron distribution with that in diffusive equilibrium suggests global ion escape from Titan with a total ion loss rate of similar to(1.7 +/- 0.4) x 10(25) s(-1). Significant diurnal variation in ion transport is implied by the data, characterized by ion outflow at the dayside and ion inflow at the nightside, especially below similar to 1400 km. This is interpreted as a result of day-to-night ion transport, with a horizontal transport rate estimated to be similar to(1.4 +/- 0.5) x 10(24) s(-1). Such an ion flow is likely to be an important source for Titan's nightside ionosphere, as proposed in Cui et al. [2009a].

  • 19. Cully, C.M.
    et al.
    Ergun, R. E.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Electrostatic structure around spacecraft in tenuous plasmas2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no A9, A09211- p.Article in journal (Refereed)
    Abstract [en]

    Most satellite-based in situ plasma experiments are affected in some manner by the electrostatic structure surrounding the spacecraft. In order to better understand this structure, we have developed a fully three-dimensional self-consistent model that can accept realistic spacecraft geometry, including both thin (similar to 10(-4) m) wires and long (similar to 10(2) m) booms, with open boundary conditions. The model uses an integral formulation incorporating boundary element, multigrid and fast multipole methods to overcome problems associated with the large range in scale sizes and inherently three-dimensional structure. By applying the model to the Cluster spacecraft, we show that the electric potential structure is dominated by the charge on the wire booms, with the spacecraft body contributing at small distances. Consequently, the potential near the EFW ( Electric Fields and Waves experiment) probes at the end of the wire booms is typically significantly above the true plasma potential. For the Cluster spacecraft, we show that this effect causes a 19% underestimation of the spacecraft potential and 13% underestimation of the ambient electric field. We further assess the electric field due to the sunward-oriented photoelectron cloud, showing that the cloud contributes little to the observed spurious sunward field in the EFW data.

  • 20. De Conti, Alberto
    et al.
    Visacro, Silverio
    Theethayi, Nelson
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    A comparison of different approaches to simulate a nonlinear channel resistance in lightning return stroke models2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no D14, D14129- p.Article in journal (Refereed)
    Abstract [en]

    Different physical models that describe the time variation of the channel resistance are investigated in a lightning return stroke model. Such models consider one of the three following hypotheses: (1) the channel resistance decays exponentially with time, (2) the channel resistance decays with the radial expansion of the channel core, which is assumed to be described by the strong-shock approximation, or (3) the channel resistance varies with time according to three different arc resistance models (defined by Toepler, Barannik and Kushner et al.). Analyses illustrate the effect of a time-varying channel resistance on channel currents and corresponding electromagnetic fields. It is shown that the strong-shock approximation is able to predict typical features of experimentally observed lightning electromagnetic fields and return stroke speed profiles. It is also shown that results predicted by the strong-shock approximation can be qualitatively reproduced by either using simplified arc resistance equations (such as Toepler's and Barannik's ones) or considering an exponential decay of the channel resistance with attenuation constants linearly increasing with height.

  • 21. Deng, X. H.
    et al.
    Zhou, M.
    Li, S. Y.
    Baumjohann, W.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Cornilleau, N.
    Santolik, O.
    Pontin, D. I.
    Reme, H.
    Lucek, E.
    Fazakerley, A. N.
    Decreau, P.
    Daly, P.
    Nakamura, R.
    Tang, R. X.
    Hu, Y. H.
    Pang, Y.
    Buechner, J.
    Zhao, H.
    Vaivads, Andris
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Pickett, J. S.
    Ng, C. S.
    Lin, X.
    Fu, S.
    Yuan, Z. G.
    Su, Z. W.
    Wang, J. F.
    Dynamics and waves near multiple magnetic null points in reconnection diffusion region2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, no 7, A07216- p.Article in journal (Refereed)
    Abstract [en]

    Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three-dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler-mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for the whistler wave regime of reconnection.

  • 22. Divine, D. V.
    et al.
    Isaksson, E.
    Pohjola, V.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Meijer, H.
    de Wal, R. S. W. van
    Martma, T.
    Moore, J.
    Sjögren, B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Godtliebsen, F.
    Deuterium excess record from a small Arctic ice cap2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no D19, D19104- p.Article in journal (Refereed)
    Abstract [en]

    In this paper we present a deuterium excess (d) record from an ice core drilled on a small ice cap in Svalbard in 1997. The core site is located at Lomonosovfonna at 1255 m asl, and the analyzed time series spans the period 1400-1990 A. D. The record shows pronounced multidecadal to centennial-scale variations coherent with sea surface temperature changes registered in the subtropical to southern middle-latitude North Atlantic during the instrumental period. We interpret the negative trend in the deuterium excess during the 1400s and 1500s as an indication of cooling in the North Atlantic associated with the onset of the Little Ice Age. Consistently positive anomalies of d after 1900, peaking at about 1950, correspond with well-documented contemporary warming. Yet the maximum values of deuterium excess during 1900-1990 are not as high as in the early part of the record (pre-1550). This suggests that the sea surface temperatures during this earlier period of time in the North Atlantic to the south of approximately 45 degrees N were at least comparable with those registered in the 20th century before the end of the 1980s. We examine the potential for a cold bias to exist in the deuterium excess record due to increased evaporation from the local colder sources of moisture having isotopically cold signature. It is argued that despite a recent oceanic warming, the contribution from this local moisture to the Lomonosovfonna precipitation budget is still insufficient to interfere with the isotopic signal from the primary moisture region in the midlatitude North Atlantic.

  • 23. Eastwood, J.P.
    et al.
    Phan, T.-D.
    Moser, F.S.
    Shay, M.A.
    Fujimoto, M.
    Retino, Alessandro
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Dandouras, I.
    Multi-point observations of the Hall electromagnetic field and secondary island formation during magnetic reconnection2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no A6, A06235- p.Article in journal (Refereed)
    Abstract [en]

    A key feature of collisionless magnetic reconnection is the formation of Hall magnetic and electric field structure in the vicinity of the diffusion region. Here we present multi‐point Cluster observations of a reconnection event in the near‐Earth magnetotail where the diffusion region was nested by the Cluster spacecraft; we compare observations made simultaneously by different spacecraft on opposite sides of the magnetotail current sheet. This allows the spatial structure of both the electric and magnetic field to be probed. It is found that, close to the diffusion region, the magnetic field displays a symmetric quadrupole structure. The Hall electric field is symmetric, observed to be inwardly directed on both sides of the current sheet. It is large (∼40 mV m−1) on the earthward side of the diffusion region, but substantially weaker on the tailward side, suggesting a reduced reconnection rate reflected by a similar reduction in Ey. A small‐scale magnetic flux rope was observed in conjunction with these observations. This flux rope, observed very close to the reconnection site and entrained in the plasma flow, may correspond to what have been termed secondary islands in computer simulations. The core magnetic field inside the flux rope is enhanced by a factor of 3, even though the lobe guide field is negligible. Observations of the electric field inside the magnetic island show extremely strong (∼100 mV m−1) fields which may play a significant role in the particle dynamics during reconnection.

  • 24.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Lester, M.
    Cowley, S. W. H.
    Brain, D. A.
    Fraenz, M.
    Barabash, S.
    Magnetosonic Mach number effect of the position of the bow shock at Mars in comparison to Venus2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, A07203- p.Article in journal (Refereed)
    Abstract [en]

    We study the effect of the magnetosonic Mach number on the position of the bow shock (BS) at Mars. The magnetosonic Mach number is calculated from solar wind data obtained by the ACE satellite upstream of Earth and extrapolated to Mars during two intervals, starting in 2005 and 2007, when Mars and Earth were close to opposition. An increased Mach number is observed to cause the Martian BS to move to lower altitudes and the variation in the terminator altitude is proportional to the Mach number change. When the Mach number is lowered, the BS flares more. We also compare our results to previous studies at Venus. The variation in BS altitude with magnetosonic Mach number is found to be very similar to the variation of the Venusian BS, which has previously been shown to decrease linearly in altitude with increasing Mach number.

  • 25.
    Edberg, Niklas J. T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Nilsson, H.
    Futaana, Y.
    Stenberg, G.
    Lester, M.
    Cowley, S. W. H.
    Luhmann, J. G.
    McEnulty, T. R.
    Opgenoorth, Herman J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Fedorov, A.
    Barabash, S.
    Zhang, T. L.
    Atmospheric erosion of Venus during stormy space weather2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, A09308- p.Article in journal (Refereed)
    Abstract [en]

    We study atmospheric escape from Venus during solar minimum conditions when 147 corotating interaction regions (CIRs) and interplanetary coronal mass ejections (ICMEs) combined impact on the planet. This is the largest study to date of the effects of stormy space weather on Venus and we show for the first time statistically that the atmosphere of Venus is significantly affected by CIRs and ICMEs. When such events impact on Venus, as observed by the ACE and Venus Express satellites, the escape rate of Venus's ionosphere is measured to increase by a factor of 1.9, on average, compared to quiet solar wind times. However, the increase in escape flux during impacts can occasionally be significantly larger by orders of magnitude. Taking into account the occurrence rate of such events we find that roughly half (51%) of the outflow occurs during stormy space weather. Furthermore, we particularly discuss the importance of the increased solar wind dynamic pressure as well as the polarity change of the interplanetary magnetic field (IMF) in terms of causing the increase escape rate. The IMF polarity change across a CIR/ICME could cause dayside magnetic reconnection processes to occur in the induced magnetosphere of Venus, which would add to the erosion through associated particle acceleration.

  • 26.
    Edberg, Niklas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Lester, M.
    Cowley, S. W. H.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Statistical analysis of the location of the Martian magnetic pileup boundary and bow shock and the influence of crustal magnetic fields2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A8, A08206- p.Article in journal (Refereed)
    Abstract [en]

    We use the data set from the magnetometer and electron reflectometer instruments on board the Mars Global Surveyor spacecraft to show that the crustal magnetic fields of Mars affect the location of the magnetic pileup boundary (MPB) and bow shock (BS) globally. We search for crossings of the MPB and BS in the data that were observed over the first 16 months of the mission. To identify the influence of the crustal magnetic fields, all crossings are extrapolated to the terminator plane in order to remove the solar zenith angle (SZA) dependence, and to make it possible to compare crossings independently of location. The MPB crossings that were observed over regions on Mars, which contain strong crustal magnetic fields, are on average located further out than crossings observed over regions with weak crustal fields. This is shown in three separate longitude intervals. We also find that the dayside BS crossings observed over the southern hemisphere of Mars are on average located further out than the BS crossings observed over the northern hemisphere, possibly because of the influence of the crustal fields. We also study the magnetic field strength and its variation at the inside of the MPB and their dependence on the SZA and altitude. We find that the magnitude of the magnetic field in the MPB is closely linked to the altitude of the MPB, with the magnitude increasing as the MPB is observed closer to the planet.

  • 27. Eliasson, B.
    et al.
    Thidé, Bo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A2, A02313- p.Article in journal (Refereed)
    Abstract [en]

    [1] We have performed a simulation study of Langmuir turbulence in the Earth's ionosphere by means of a Zakharov model with parameters relevant for the F layer. The model includes dissipative terms to model collisions and Landau damping of the electrons and ions, and a linear density profile, which models the ionospheric plasma inhomogeneity whose length scale is of the order 10-100 km. The injection of energy into the system is modeled by a constant source term in the Zakharov equation. Langmuir turbulence is excited "on-demand" in controlled ionospheric modification experiments where the energy is provided by an HF radio beam injected into the overhead ionospheric plasma. The ensuing turbulence can be studied with radars and in the form of secondary radiation recorded by ground-based receivers. We have analyzed spectral signatures of the turbulence for different sets of parameters and different altitudes relative to the turning point of the linear Langmuir mode where the Langmuir frequency equals the local plasma frequency. By a parametric analysis, we have derived a simple scaling law, which links the spectral width of the turbulent frequency spectrum to the physical parameters in the ionosphere. The scaling law provides a quantitative relation between the physical parameters ( temperatures, electron number density, ionospheric length scale, etc.) and the observed frequency spectrum. This law may be useful for interpreting experimental results.

  • 28. Eriksson, S.
    et al.
    Hasegawa, H.
    Teh, W. -L
    Sonnerup, B. U. Oe.
    McFadden, J. P.
    Glassmeier, K. -H
    Le Contel, O.
    Angelopoulos, V.
    Cully, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Larson, D. E.
    Ergun, R. E.
    Roux, A.
    Carlson, C. W.
    Magnetic island formation between large-scale flow vortices at an undulating postnoon magnetopause for northward interplanetary magnetic field2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, no 2, A00C17- p.Article in journal (Refereed)
    Abstract [en]

    Time History of Events and Macroscale Interactions during Substorms multispacecraft observations are presented for a similar to 2-h-long postnoon magnetopause event on 8 June 2007 that for the first time indicate that the trailing (sunward) edges of Kelvin-Helmholtz (KH) waves are commonly related to small-scale < 0.56 R-E magnetic islands or flux transfer events (FTE) during the growth phase of these surface waves. The FTEs typically show a characteristic bipolar B-N structure with enhanced total pressure at their center. Most of the small-scale FTEs are not related to any major plasma acceleration. TH-A observations of one small FTE at a transition from the low-latitude boundary layer (LLBL) into a magnetosheath plasma depletion layer were reconstructed using separate techniques that together confirm the presence of a magnetic island within the LLBL adjacent to the magnetopause. The island was associated with a small plasma vortex and both features appeared between two large-scale (similar to 1 R-E long and 2000 km wide) plasma vortices. We propose that the observed magnetic islands may have been generated from a time-varying reconnection process in a low ion plasma beta (beta(i) < 0.2) and low 8.3 degrees field shear environment at the sunward edge of the growing KH waves where the local magnetopause current sheet may be compressed by the converging flow of the large-scale plasma vortices as suggested by numerical simulations of the KH instability.

  • 29.
    Erlandsson, Martin
    et al.
    Department of Aquatic and Environmental Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden.
    Cory, Neil
    Department of Forest Resource Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
    Kohler, Stephan
    Department of Aquatic and Environmental Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden.
    Bishop, Kevin
    Department of Aquatic and Environmental Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden.
    Direct and indirect effects of increasing dissolved organic carbon levels on pH in lakes recovering from acidification2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, no G3, G03004- p.Article in journal (Refereed)
    Abstract [en]

    In this study, we examine the impact of increasing concentrations of dissolved organic carbon (DOC) on the recovery from acidification for 66 lakes in Southern Sweden. The lakes are small, weakly buffered, and have all been affected by acidifying deposition. A majority of the lakes (similar to 75%) showed an increase in DOC concentrations between 1990 and 2008. The method used in this study was to model pH in 2008 from DOC, acid neutralizing capacity, pCO(2) (partial carbon dioxide pressure), and Al speciation, using both the DOC observed in 2008 and the "unelevated" DOC of 1990. Furthermore, we consider the indirect effects of increasing DOC on acidity, i.e., the ancillary effects from DOC on pCO(2), Al transport and speciation, and release of base cations (BCs). Our results indicate that the DOC increase in the latest decades has retarded the recovery from acidification by 0.13 pH units (median for all lakes) and by more than 1 unit for individual lakes. The effects of elevated pCO(2) and BC concentrations accompanying the DOC increase compensated for each other for the average lake, whereas the effects of higher Al transport on pH were minor. The estimate of the amount of BCs released with the organic anions is however uncertain, and further studies on this topic are needed.

  • 30. Farrugia, C. J.
    et al.
    Chen, Li-Jen
    Torbert, R. B.
    Southwood, D. J.
    Cowley, S. W. H.
    Vrublevskis, A.
    Mouikis, C.
    Vaivads, Andris
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Decreau, P.
    Vaith, H.
    Owen, C. J.
    Sibeck, D. J.
    Lucek, E.
    Smith, C. W.
    "Crater" flux transfer events: Highroad to the X line?2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, no 2, A02204- p.Article in journal (Refereed)
    Abstract [en]

    We examine Cluster observations of a so-called magnetosphere "crater FTE," employing data from five instruments (FGM, CIS, EDI, EFW, and WHISPER), some at the highest resolution. The aim of doing this is to deepen our understanding of the reconnection nature of these events by applying recent advances in the theory of collisionless reconnection and in detailed observational work. Our data support the hypothesis of a stratified structure with regions which we show to be spatial structures. We support the bulge-like topology of the core region (R3) made up of plasma jetting transverse to reconnected field lines. We document encounters with a magnetic separatrix as a thin layer embedded in the region (R2) just outside the bulge, where the speed of the protons flowing approximately parallel to the field maximizes: (1) short (fraction of a sec) bursts of enhanced electric field strengths (up to similar to 30 mV/m) and (2) electrons flowing against the field toward the X line at approximately the same time as the bursts of intense electric fields. R2 also contains a density decrease concomitant with an enhanced magnetic field strength. At its interface with the core region, R3, electric field activity ceases abruptly. The accelerated plasma flow profile has a catenary shape consisting of beams parallel to the field in R2 close to the R2/R3 boundary and slower jets moving across the magnetic field within the bulge region. We detail commonalities our observations of crater FTEs have with reconnection structures in other scenarios. We suggest that in view of these properties and their frequency of occurrence, crater FTEs are ideal places to study processes at the separatrices, key regions in magnetic reconnection. This is a good preparation for the MMS mission.

  • 31. Forsyth, C.
    et al.
    Fazakerley, A. N.
    Walsh, A. P.
    Watt, C. E. J.
    Garza, K. J.
    Owen, C. J.
    Constantinescu, D.
    Dandouras, I.
    Fornacon, K. -H
    Lucek, E.
    Marklund, G. T.
    Sadeghi, S. S.
    Khotyaintsev, Yuri
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Masson, A.
    Doss, N.
    Temporal evolution and electric potential structure of the auroral acceleration region from multispacecraft measurements2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A12203- p.Article in journal (Refereed)
    Abstract [en]

    Bright aurorae can be excited by the acceleration of electrons into the atmosphere in violation of ideal magnetohydrodynamics. Modeling studies predict that the accelerating electric potential consists of electric double layers at the boundaries of an acceleration region but observations suggest that particle acceleration occurs throughout this region. Using multispacecraft observations from Cluster, we have examined two upward current regions on 14 December 2009. Our observations show that the potential difference below C4 and C3 changed by up to 1.7 kV between their respective crossings, which were separated by 150 s. The field-aligned current density observed by C3 was also larger than that observed by C4. The potential drop above C3 and C4 was approximately the same in both crossings. Using a novel technique of quantitively comparing the electron spectra measured by Cluster 1 and 3, which were separated in altitude, we determine when these spacecraft made effectively magnetically conjugate observations, and we use these conjugate observations to determine the instantaneous distribution of the potential drop in the AAR. Our observations show that an average of 15% of the potential drop in the AAR was located between C1 at 6235 km and C3 at 4685 km altitude, with a maximum potential drop between the spacecraft of 500 V, and that the majority of the potential drop was below C3. Assuming a spatial invariance along the length of the upward current region, we discuss these observations in terms of temporal changes and the vertical structure of the electrostatic potential drop and in the context of existing models and previous single- and multispacecraft observations. Citation: Forsyth, C., et al. (2012), Temporal evolution and electric potential structure of the auroral acceleration region from multispacecraft measurements, J. Geophys. Res., 117, A12203, doi: 10.1029/2012JA017655.

  • 32.
    Fu, H. S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Khotyaintsev, Y. V.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vaivads, A.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Andre, M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Sergeev, V. A.
    Huang, S. Y.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Kronberg, E. A.
    Daly, P. W.
    Pitch angle distribution of suprathermal electrons behind dipolarization fronts: A statistical overview2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A12221- p.Article in journal (Refereed)
    Abstract [en]

    We examine the pitch angle distribution (PAD) of suprathermal electrons (> 40 keV) inside the flux pileup regions (FPRs) that are located behind the dipolarization fronts (DFs), in order to better understand the particle energization mechanisms operating therein. The 303 earthward-propagating DFs observed during 9 years (2001-2009) by Cluster 1 have been analyzed and divided into two groups according to the differential fluxes of the > 40 keV electrons inside the FPR. One group, characterized by the low flux (F < 500/cm(2) , s . sr . keV), consists of 153 events and corresponds to a broad distribution of IMF Bz components. The other group, characterized by the high flux (F >= 500/cm(2) . s . sr . keV), consists of 150 events and corresponds to southward IMF Bz components. Only the high-flux group is considered to investigate the PAD of the > 40 keV electrons as the low-flux situation may lead to large uncertainties in computing the anisotropy factor that is defined as A = F-perpendicular to/F-parallel to - 1 for F-perpendicular to > F-parallel to, and A = -F-parallel to/F-perpendicular to + 1 for F-perpendicular to < F-parallel to. We find that, among the 150 events, 46 events have isotropic distribution (vertical bar A vertical bar <= 0.5); 60 events have perpendicular distribution (A > 0.5), and 44 events have field-aligned distribution inside the FPR (A < -0.5). The perpendicular distribution appears mainly inside the growing FPR, where the flow velocity is increasing and the local flux tube is compressed. The field-aligned distribution occurs mainly inside the decaying FPR, where the flow velocity is decreasing and the local flux tube is expanding. Inside the steady FPR, we observed primarily the isotropic distribution of suprathermal electrons. This statistical result confirms the previous case study and gives an overview of the PAD of suprathermal electrons behind DFs.

  • 33.
    Fu, Huishan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Cao, J. B.
    Mozer, F. S.
    Lu, H. Y.
    Yang, B.
    Chorus intensification in response to interplanetary shock2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A01203- p.Article in journal (Refereed)
    Abstract [en]

    On 3 September 2009, the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites observed a significant intensification of chorus in response to the interplanetary shock in the Earth's dayside plasma trough. We analyze the wave-particle interaction and reveal that the chorus intensification can be caused by the gyroresonance between the chorus and the energetic electrons. When the electrons are scattered from resonance points to low-density regions along the diffusion curves, a part of their energy can be lost and then transferred to amplify the chorus. During the compression of the magnetosphere, the temperature anisotropy of electrons is enhanced. This makes the electron diffusion and chorus intensification very effective. The maximum growth rate after the shock is about 50% greater than that before the shock. The lower-energy (15-25 keV) electrons contribute more to the growth of chorus due to the larger density gradient along the diffusion curve. The < 10 keV electrons are almost isotropic, so they contribute little to the amplification of chorus. We investigate the free energy for the chorus intensification and find that it can be generated through the local betatron acceleration and radial diffusion processes. The local betatron acceleration results from the shock-induced compression of the magnetosphere. The linear and nonlinear growth rates are also compared. We find that the linear diffusion process works well for the present case.

  • 34.
    Fu, Huishan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Cao, J. B.
    Yang, B.
    Lu, H. Y.
    Electron loss and acceleration during storm time: The contribution of wave-particle interaction, radial diffusion, and transport processes2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, A10210- p.Article in journal (Refereed)
    Abstract [en]

    During the period 19-22 November 2007, the near-equatorial satellites THEMIS D (ThD) and E (ThE) traversed the Earth's morningside magnetosphere once per day and for nearly 2 h the orbits tracked close to each other, providing an excellent opportunity to investigate the evolution of energetic electrons fluxes (EEFs) on two time scales. By analyzing the electrons in the energy range 100-300 keV, we have found that the EEFs undergo different evolutions in the different subregions of Earth's morningside magnetosphere during a moderate storm. The evolutions at three specific locations, showing, respectively, the features of electron loss, acceleration, and conservation, have been analyzed in detail. Our observations reveal that, during storm time, the evolution of EEFs involves five processes: (1) the resonant interaction between chorus and energetic electrons, which can contribute to both loss and acceleration of electrons depending on the distribution of phase space density, (2) the radial diffusion, which is indicated by the good coherence between ULF waves and EEFs and dominates in the region where the chorus is relatively weak; (3) the adiabatic transport, which affects the EEFs at L > 6 during the recovery phase and prefers to work on large time scale (>1 d); (4) the magnetopause shadowing, which can evacuate electrons at L > 7 during the storm main phase but play minor roles during the recovery phase, when the magnetopause was moving outward; (5) the magnetospheric convection, which can significantly affect the dynamics of the <100 keV but not the >100 keV electrons. All these five processes couple to each other and determine the EEFs together.

  • 35. Galand, Marina
    et al.
    Yelle, Roger
    Cui, Jun
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Vuitton, Veronique
    Wellbrock, Anne
    Coates, Andrew
    Ionization sources in Titan's deep ionosphere2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, A07312- p.Article in journal (Refereed)
    Abstract [en]

    We analyze a multi-instrumental data set from four Titan encounters by the Cassini spacecraft to investigate in detail the formation of the ionosphere. The data set includes observations of thermospheric and ionospheric species and suprathermal electrons. A model describing the solar and electron energy deposition is used as an organizing element of the Cassini data set. We first compare the calculated secondary electron production rates with the rates inferred from suprathermal electron intensity measurements. We then calculate an effective electron dissociative recombination coefficient, applying three different approaches to the Cassini data set. Our findings are threefold: (1) The effective recombination coefficient derived under sunlit conditions in the deep ionosphere (< 1200 km) is found to be independent of solar zenith angle and flyby. Its value ranges from 6.9 x 10(-7) cm(3) s(-1) at 1200 km to 5.9 x 10(-6) cm(3) s(-1) at 970 km at 500 K. (2) The presence of an additional, minor source of ionization is revealed when the solar contribution is weak enough. The contribution by this non-solar source-energetic electrons most probably of magnetospheric origin-becomes apparent for secondary electron production rates, due to solar illumination alone, close to or smaller than about 3 x 10(-1) cm(-3) s(-1). Such a threshold is reached near the solar terminator below the main solar-driven electron production peak (< 1050 km). (3) Our ability to model the electron density in the deep ionosphere is very limited. Our findings highlight the need for more laboratory measurements of electron dissociative recombination coefficients for heavy ion species at high electron temperatures (especially near 500 K).

  • 36. Gao, G.
    et al.
    Chen, D.
    Xu, Chong-yu
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Air and Water Science.
    Simelton, E.
    Trend of estimated actual evapotranspiration over China during 1960-20022007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no D11, D11120- p.Article in journal (Refereed)
    Abstract [en]

    In this study, the water balance methodology introduced by Thornthwaite and Mather (1955) is modified to estimate monthly actual evapotranspiration for 686 stations over China during 1960–2002. The modification is done by replacing the Thornthwaite potential evapotranspiration estimation with the Penman-Monteith method. Temporal trend and spatial distribution of the estimated annual actual evapotranspiration during the past 43 years are analyzed. The results show that (1) the actual evapotranspiration had a decreasing trend in most areas east of 100°E, and there was an increasing trend in the west and the north parts of northeast China; (2) the spatial distribution of the trend for the actual evapotranspiration is similar to that of the potential evapotranspiration in south China, while the trends are opposite in north China; (3) for most parts of China, the change in precipitation played a key role for the change of estimated actual evapotranspiration, while in southeast China, the change of potential evapotranspiration appeared to be the major factor; and (4) in general, the hydrological cycle was intensified in western China, whereas it was weakened from the Yellow River basin northward.

  • 37.
    Garnier, P.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Dandouras, I.
    Toublanc, D.
    Roelof, E. C.
    Brandt, P. C.
    Mitchell, D. G.
    Krimigis, S. M.
    Krupp, N.
    Hamilton, D. C.
    Dutuit, O.
    Wahlund, Jan Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    The lower exosphere of Titan: Energetic neutral atoms absorption and imaging2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A10, A10216- p.Article in journal (Refereed)
    Abstract [en]

    The Saturn magnetosphere interacts with the Titan atmosphere through various mechanisms. One of them leads, by charge exchange reactions between the energetic Saturnian ions and the exospheric neutrals of Titan, to the production of energetic neutral atoms (ENAs). The Ion and Neutral Camera (INCA), one of the three sensors that comprise the Magnetosphere Imaging Instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions in the Saturnian magnetosphere. This study focuses on the ENA imaging of Titan (for 20-50 keV H ENAs), with the example of the Ta Titan flyby (26 October 2004): our objective is to understand the positioning of the ENA halo observed around Titan. Thus we investigate the main ENA loss mechanisms, such as the finite gyroradii effects for the parent ions, or the charge stripping with exospheric neutrals. We show that multiple stripping and charge exchange reactions have to be taken into account to understand the ENA dynamics. The use of an analytical approach, taking into account these reactions, combined with a reprocessing of the INCA data, allows us to reproduce the ENA images of the Ta flyby and indicates a lower limit for ENA emission around the exobase. However, the dynamics of energetic particle through the Titan atmosphere remains complex, with an inconsistency between the ENA imaging at low and high altitudes.

  • 38. Garnier, P.
    et al.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Holmberg, Madeleine K. G.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Morooka, M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Grimald, S.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Schippers, P.
    Gurnett, D. A.
    Krimigis, S. M.
    Krupp, N.
    Coates, A.
    Crary, F.
    Gustafsson, Georg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    The detection of energetic electrons with the Cassini Langmuir probe at Saturn2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A10202- p.Article in journal (Refereed)
    Abstract [en]

    The Cassini Langmuir probe, part of the Radio and Plasma Wave Science (RPWS) instrument, has provided a wealth of information about the cold and dense plasma in the Saturnian system. The analysis of the ion side current (current for negative potentials) measured by the probe from 2005 to 2008 reveals also a strong sensitivity to energetic electrons (250-450 eV). These electrons impact the surface of the probe, and generate a detectable current of secondary electrons. A broad secondary electrons current region is inferred from the observations in the dipole L Shell range of similar to 6-10, with a peak full width at half maximum (FWHM) at L = 6.4-9.4 (near the Dione and Rhea magnetic dipole L Shell values). This magnetospheric flux tube region, which displays a large day/night asymmetry, is related to the similar structure in the energetic electron fluxes as the one measured by the onboard Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer (CAPS). It corresponds spatially to both the outer electron radiation belt observed by the Magnetosphere Imaging Instrument (MIMI) at high energies and to the low-energy peak which has been observed since the Voyager era. Finally, a case study suggests that the mapping of the current measured by the Langmuir probe for negative potentials can allow to identify the plasmapause-like boundary recently identified at Saturn, and thus potentially identify the separation between the closed and open magnetic field lines regions.

  • 39. Geirsson, Halldor
    et al.
    LaFemina, Peter
    Arnadottir, Thora
    Sturkell, Erik
    Sigmundsson, Freysteinn
    Travis, Matthew
    Schmidt, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Hreinsdottir, Sigrun
    Bennett, Rick
    Volcano deformation at active plate boundaries: Deep magma accumulation at Hekla volcano and plate boundary deformation in south Iceland2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, B11409- p.Article in journal (Refereed)
    Abstract [en]

    Most magmatic systems on Earth are located at actively deforming plate boundaries. In these systems, the magmatic and plate boundary deformation signals are intertwined and must be deconvolved to properly estimate magma flux and source characteristics of the magma plumbing system. We investigate the inter-rifting and inter-seismic deformation signals at the Eastern Volcanic Zone (EVZ) - South Iceland Seismic Zone (SISZ) ridge - transform intersection and estimate the location, depth, and volume rate for magmatic sources at Hekla and Torfajokull volcanoes, which are located at the intersection. We solve simultaneously for the source parameters of the tectonic and volcanic deformation signals using a new ten-year velocity field derived from a dense network of episodic and continuous GPS stations in south Iceland. We find the intersection of the axes of the EVZ and the SISZ is located within the Torfajokull caldera, which itself is subsiding. Deformation at Hekla is statistically best described in terms of a horizontal ellipsoidal magma chamber at 24(2)(+4) km depth aligned with the volcanic system and increasing in volume by 0.017(-0.002)(+0.007) km(3) per year. A spherical magma chamber centered at 24(-2)(+5) km depth with a volume rate of 0.019(-0.002)(+0.011) km(3) per year, or a vertical pipe-shaped magma chamber between 10(-1)(+3) km and 21(-4)(+7) km with a volume rate of 0.008(-0.001)(+0.003) km(3) per year are also plausible models explaining the deformation at Hekla. All three models indicate magma accumulation in the lower crust or near the Moho under Hekla.

  • 40. Golovchanskaya, I. V.
    et al.
    Kullen, Anita
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Maltsev, Y. P.
    Biernat, H.
    Ballooning instability at the plasma sheet-lobe interface and its implications for polar arc formation2006In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 111, no A11, A11216- p.Article in journal (Refereed)
    Abstract [en]

    Huang et al. ( 1987, 1989) reported hot filaments of plasma sheet origin filling the magnetospheric lobes during northward interplanetary magnetic field ( IMF). On the other hand, cold plasma transients of presumably lobe origin are often observed in the plasma sheet. These features can be interpreted in terms of plasma exchange at the plasma sheet - lobe interface (PSLI) proceeding in a filamentary manner. We present a description of this process within ballooning destabilizing of the near-Earth curved segment of the PSLI. Although the basic ballooning instability condition is not typically met inside the plasma sheet, it is satisfied at this segment. The PSLI always separates the cold lobe population from the hot plasma sheet, thus providing a pressure gradient favorable for the instability; its near-Earth part has a nonnegligible magnetic curvature. A solution for the least stable ballooning harmonics is found, which satisfies the finite conductivity boundary condition in the ionosphere and the outgoing Alfven wave condition at the tailward end of the near-Earth curved segment of the plasma sheet boundary. We show that this part of the PSLI may be a generator region launching filamentation. The background convection is imposed on the ballooning motions. The large-scale convection associated with southward IMF B-z suppresses hot filament progression into the lobes, while promoting penetration of lobe transients into the plasma sheet. However, during northward IMF, the convection favors at certain magnetic local times the extension of plasma sheet filaments into the lobes and their subsequent protrusion toward noon. This process is signified in the ionosphere by the occurrence of nightside originating polar arcs. Several polar arc events are shown that develop from the nightside oval boundary into the polar cap on timescales of approximately 10 - 15 min, consistent with the growth rates of the studied instability.

  • 41. Goren, L
    et al.
    Aharonov, E.
    Mulugeta, G.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Koyi, Hemin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Mart, Y.
    Ductile Deformation of Passive Margins: A New Mechanism for Subduction Initiation2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no B8, B08411- p.Article in journal (Refereed)
    Abstract [en]

    [1] The onset of subduction at passive margins has been extensively investigated and debated. However, the force constellations and mechanisms that enable the development of a subduction system from a passive margin remain unclear. This study presents new insights into the conditions and processes by which lateral density differences between oceanic and continental lithospheres in passive margins may lead to initiation of a low-angle subduction system. The presented study consists of (1) analytical calculations of flow fields generated in passive margins, and (2) analogue experiments of mature passive margins performed in a centrifuge. The analytical formulation predicts temporal and spatial evolution of an interface between the oceanic and continental lithospheres, and demonstrates that oceanic underthrusting may occur by rotation of this interface. The analogue experiments show that incipient subduction may develop by ductile deformation within the lithosphere, involving no sliding along the ocean-continent interface, so that the frictional resistance between the plates need not be overcome. The force induced by the negative buoyancy of the oceanic plate with respect to the asthenosphere, is found to be in some cases irrelevant to subduction nucleation. Results of both the analogue experiments and the analytical calculation are compared to the south-east Australian passive margin, and show an excellent fit to its geometry and stress distribution. The proposed mechanism is also applied to the only two cases of subduction of Atlantic tectonic system, the Lesser-Antilles and the South Sandwich subduction systems.

  • 42. Gusmeroli, Alessio
    et al.
    Murray, Tavi
    Jansson, Peter
    Pettersson, Rickard
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Aschwanden, Andy
    Booth, Adam D.
    Vertical distribution of water within the polythermal Storglaciären, Sweden2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, no F4, F04002- p.Article in journal (Refereed)
    Abstract [en]

    Knowledge of water content and its distribution in polythermal glaciers is required to model their flow and thermal state. However, observ of water content variations with depth in polythermal glaciers are scarce. Water content can be estimated from radio wave speed because they depend on one another. We obtained continuous profiles of radio wave speed variations with depth from zero-offset radar profiles collected in boreholes approximately 80 m deep in the upper ablation area of Storglaciaren, northern Sweden. These profiles show that the microcrystalline water system in the temperate ice is relatively homogeneous. The overall hydrothermal structure at this location is composed of a 20 m thick upper layer of cold, water-free ice, underlain by a temperate ice layer whose average water content is 0.6% +/- 0.3%. These results are corroborated by surface radar and thermistor measurements, which show that the depth of the cold temperate transition is 21 m and the calculated water content at that transition is 0.6% +/- 0.1%. These findings imply that the whole temperate ice layer is from 3 to 4 times softer than the cold ice and, consequently, that realistic ice flow models of polythermal glaciers should include the effect of water content on viscosity.

  • 43. Haaland, S.
    et al.
    Eriksson, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Engwall, E.
    Lybekk, B.
    Nilsson, H.
    Pedersen, A.
    Svenes, K.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Foerster, M.
    Li, K.
    Johnsen, C.
    Ostgaard, N.
    Estimating the capture and loss of cold plasma from ionospheric outflow2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A07311- p.Article in journal (Refereed)
    Abstract [en]

    An important source of magnetospheric plasma is cold plasma from the terrestrial ionosphere. Low energy ions travel along the magnetic field lines and enter the magnetospheric lobes where they are convected toward the tail plasma sheet. Recent observations indicate that the field aligned ion outflow velocity is sometimes much higher than the convection toward the central plasma sheet. A substantial amount of plasma therefore escapes downtail without ever reaching the central plasma sheet. In this work, we use Cluster measurements of cold plasma outflow and lobe convection velocities combined with models of the magnetic field in an attempt to determine the fate of the outflowing ions and to quantify the amount of plasma lost downtail. The results show that both the circulation of plasma and the direct tailward escape of ions varies significantly with magnetospheric conditions. For strong solar wind driving with a southward interplanetary magnetic field, also typically associated with high geomagnetic activity, most of the outflowing plasma is convected to the plasma sheet and recirculated. For periods with northward interplanetary magnetic field, the convection is nearly stagnant, whereas the outflow, although limited, still persists. The dominant part of the outflowing ions escape downtail and are directly lost into the solar wind under such conditions.

  • 44. Hamrin, M.
    et al.
    Marghitu, O.
    Norqvist, P.
    Buchert, Stephan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    Energy conversion regions as observed by Cluster in the plasma sheet2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, A00K08- p.Article in journal (Refereed)
    Abstract [en]

    In this article we present a review of recent studies of observations of localized energy conversion regions (ECRs) observed by Cluster in the plasma sheet at altitudes of 15-20R(E). By examining variations in the power density, E . J, where E is the electric field and J is the current density, we show that the plasma sheet exhibits a high level of fine structure. Approximately three times as many concentrated load regions (CLRs) (E . J > 0) as concentrated generator regions (CGRs) (E . J < 0) are identified, confirming the average load character of the plasma sheet. Some ECRs are found to relate to auroral activity. While ECRs are relevant for the energy conversion between the electromagnetic field and the particles, bursty bulk flows (BBFs) play a central role for the energy transfer in the plasma sheet. We show that ECRs and BBFs are likely to be related, although details of this relationship are yet to be explored. The plasma sheet energy conversion increases rather simultaneously with increasing geomagnetic activity in both CLRs and CGRs. Consistent with large-scale magnetotail simulations, most of the observed ECRs appear to be rather stationary in space but varying in time. We estimate that the ECR lifetime and scale size are a few minutes and a few R(E), respectively. It is conceivable that ECRs rise and vanish locally in significant regions of the plasma sheet, possibly oscillating between load and generator character, while some energy is transmitted as Poynting flux to the ionosphere.

  • 45. Hamrin, M.
    et al.
    Marghitu, O.
    Norqvist, P.
    Buchert, Stephan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    The role of the inner tail to midtail plasma sheet in channeling solar wind power to the ionosphere2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A06310- p.Article in journal (Refereed)
    Abstract [en]

    In this article we use Cluster power density (E . J) data from 2001, 2002, and 2004 to investigate energy conversion and transfer in the plasma sheet. We show that a southward IMF B-z is favorable for plasma sheet energy conversion, and that there is an increased particle and Poynting flux toward the Earth at times when Cluster observes an enhanced energy conversion in the plasma sheet. Conversion from electromagnetic to kinetic energy is increasingly dominant farther down-tail, while the generation of electromagnetic power from kinetic energy becomes important toward the Earth with a maximum at roughly 10 R-E. By linking observations of the key quantity E . J to observations of the solar wind input and earthward energy flux, our results demonstrate the role of the inner tail to midtail plasma sheet as a mediator between the solar wind energy input into the magnetosphere and the auroral dissipation in the ionosphere.

  • 46. Hasegawa, H.
    et al.
    Nakamura, R.
    Fujimoto, M.
    Sergeev, V. A.
    Lucek, E. A.
    Reme, H.
    Khotyaintsev, Yuri
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Reconstruction of a bipolar magnetic signature in an earthward jet in the tail: Flux rope or 3D guide-field reconnection?2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no A11, A11206- p.Article in journal (Refereed)
    Abstract [en]

    Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar B-z ( magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At similar to 2313 UT on 13 August 2002, Cluster encountered a bipolar B-z at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of Grad-Shafranov ( GS) reconstruction, the technique for recovery of two-dimensional ( 2D) magnetohydrostatic structures, suggests that a flux rope with diameter of similar to 2 R-E was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a three-dimensional ( 3D) MHD simulation, in which a bipolar B-z can be produced through localized ( 3D) reconnection in the presence of guide field B-y ( Shirataka et al., 2006) without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar B-z thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some ( e. g., force-free, or magnetohydrostatic) model-based results.

  • 47. Hasegawa, H.
    et al.
    Retino, A.
    Vaivads, Andris
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Khotyaintsev, Yuri
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    André, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Nakamura, T. K. M.
    Teh, W. -L
    Sonnerup, B. U. Oe.
    Schwartz, S. J.
    Seki, Y.
    Fujimoto, M.
    Saito, Y.
    Reme, H.
    Canu, P.
    Kelvin-Helmholtz waves at the Earth's magnetopause: Multiscale development and associated reconnection2009In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 114, no 12, A12207- p.Article in journal (Refereed)
    Abstract [en]

    We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at similar to 1500 magnetic local time (MLT) by the Geotail spacecraft and at similar to 1900 MLT by the Cluster spacecraft, which detected rolled-up MHD-scale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 x 10(4) km) is considerably longer than predicted by the linear theory from the thickness (similar to 1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length similar to 100 km and of magnetic shear similar to 60 degrees existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvenic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.

  • 48.
    Hieronymus, C. F.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Goes, S.
    Department of Earth Science and Engineering, Imperial College, London, UK.
    Sargent, M.
    Max-Planck Institut für Astronomie, Heidelberg, Germany.
    Morra, G.
    Department of Earth Sciences, Institute of Geophysics, ETH Zürich, Zürich, Switzerland.
    A dynamical model for generating Eurasian lithospheric stress and strain rate fields: Effect of rheology and cratons2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no B7, B07404- p.Article in journal (Refereed)
    Abstract [en]

    For most continents, stress models driven by plate boundary forces have successfully reproduced the main characteristics of the stress field. However, Eurasia has remained a challenge due to its large areas of intraplate deformation. We present a set of three-dimensional models of the upper mantle lithosphere system for a simplified geometry of the Eurasian plate where we try to match the first-order characteristics of the stress and strain rate fields simultaneously. For typical elastic, viscous, or plastic rheologies, high stress levels are required in order to produce realistic convergence rates between India and Asia. Our models show robustly that such stresses are transmitted throughout most of the plate, dominating locally generated stresses even in distal regions such as Europe in a manner that is not compatible with observations. Cratons with roots that extend deep into the mantle are unable to provide a significant stress-shielding effect unless the viscosity contrast between the asthenosphere and the underlying mantle is around 100 or greater. A damage rheology for the lithosphere with history-dependent behavior and material softening by a viscosity reduction of several orders of magnitude is shown to eliminate this conundrum. Continental convergence at high velocity but low stress is facilitated by the formation of long-lived shear zones similar to those observed north of the Himalayas. The low stress associated with the collision, together with the decoupling effect of the shear zones, causes the distal stress field in Europe to be controlled by the effects of the neighboring boundaries in agreement with observations.

  • 49.
    Hieronymus, C.F.
    et al.
    Danish Lithosphere Centre.
    Bercovici, D.
    University of Hawaii.
    A theoretical model of hotspot volcanism: Control on volcanic spacing and patterns via magma dynamics and lithospheric stresses2001In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 106, no B1, 683-702 p.Article in journal (Refereed)
    Abstract [en]

    Many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots. The occurrence of discrete, nearly regularly spaced volcanoes, rather than continuous ridges, are assumed to be caused by the interaction of flexural stresses (due to the volcanic loads) with magma percolation. A parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion. It is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge. The intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere. The size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization. If an initial off-axis edifice is introduced, the model is able to preserve this asymmetry and produce an alternating series of volcanoes. A small initial perturbation grows over time, resulting in double lines or wider patterns depending on the width of the magma source region. Single lines of volcanoes therefore indicate very narrow magma source regions.

  • 50. Hill, T. W.
    et al.
    Thomsen, M. F.
    Tokar, R. L.
    Coates, A. J.
    Lewis, G. R.
    Young, D. T.
    Crary, F. J.
    Baragiola, R. A.
    Johnson, R. E.
    Dong, Y.
    Wilson, R. J.
    Jones, G. H.
    Wahlund, Jan-Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Mitchell, D. G.
    Horanyi, M.
    Charged nanograins in the Enceladus plume2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, A05209- p.Article in journal (Refereed)
    Abstract [en]

    There have been three Cassini encounters with the south-pole eruptive plume of Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the spacecraft ram direction. In each case, CAPS detected a cold dense population of heavy charged particles having mass-to-charge (m/q) ratios up to the maximum detectable by CAPS (similar to 10(4) amu/e). These particles are interpreted as singly charged nanometer-sized water-ice grains. Although they are detected with both negative and positive net charges, the former greatly outnumber the latter, at least in the m/q range accessible to CAPS. On the most distant available encounter (E3, March 2008) we derive a net (negative) charge density of up to similar to 2600 e/cm(3) for nanograins, far exceeding the ambient plasma number density, but less than the net (positive) charge density inferred from the RPWS Langmuir probe data during the same plume encounter. Comparison of the CAPS data from the three available encounters is consistent with the idea that the nanograins leave the surface vents largely uncharged, but become increasingly negatively charged by plasma electron impact as they move farther from the satellite. These nanograins provide a potentially potent source of magnetospheric plasma and E-ring material.

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