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Holmberg, Madeleine K. G.
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Holmberg, M., Wahlund, J.-E., Vigren, E., Cassidy, T. & Andrews, D. (2016). Transport and chemical loss rates in Saturn's inner plasma disk. Journal of Geophysical Research - Space Physics, 121(3), 2321-2334
Öppna denna publikation i ny flik eller fönster >>Transport and chemical loss rates in Saturn's inner plasma disk
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2016 (Engelska)Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 3, s. 2321-2334Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The Kronian moon Enceladus is constantly feeding its surrounding with new gas and dust, from cryovolcanoes located in its south polar region. Through photoionization and impact ionization of the neutrals a plasma disk is created, which mainly contains hydrogen ions H+ and water group ions W+. This paper investigates the importance of ion loss by outward radial transport and ion loss by dissociative recombination, which is the dominant chemical loss process in the inner plasma disk. We use plasma densities derived from several years of measurements by the Cassini Radio and Plasma Wave Science (RPWS) electric field spectrums and Langmuir probe (LP), to derive the total flux tube content NL2. Our calculation show that NL2 agrees well with earlier estimates within L shell 8. We also show that loss by transport dominates chemical loss in between L shell 2.5 and 10. The loss rate by transport is ∼5 times larger at 5 Saturn radii (1 RS = 60,268 km) and the difference is increasing as L7.7 for larger radial distances, for the total ion population. Chemical loss may still be important for the structure of the plasma disk in the region closest to Enceladus (∼±0.5 RS) at 3.95 RS, since the transport and chemical loss rates only differ by a factor of ∼2 in this region. We also derive the total plasma content of the plasma disk from L shell 4 to 10 to be 1.9×10^33 ions, and the total ion source rate for the same region to be 5.8×10^27 s^−1. The equatorial ion production rate P, ranges from 2.6×10^−5 cm^−3s^−1 (at L = 10) to 1.1×10^−4 cm^−3s^−1 (at L = 4.8). The net mass loading rate is derived to be 123 kg/s for L shell 4 to 10. 

Nationell ämneskategori
Fusion, plasma och rymdfysik
Identifikatorer
urn:nbn:se:uu:diva-263274 (URN)10.1002/2015JA021784 (DOI)000374730900032 ()
Forskningsfinansiär
Rymdstyrelsen, DNR 162/14 DNR 166/14Vetenskapsrådet, DNR 621-2014-450 5526
Tillgänglig från: 2015-09-29 Skapad: 2015-09-29 Senast uppdaterad: 2017-12-01Bibliografiskt granskad
Holmberg, M. (2015). A study of the structure and dynamics of Saturn's inner plasma disk. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Öppna denna publikation i ny flik eller fönster >>A study of the structure and dynamics of Saturn's inner plasma disk
2015 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

This thesis presents a study of the inner plasma disk of Saturn. The results are derived from measurements by the instruments on board the Cassini spacecraft, mainly the Cassini Langmuir probe (LP), which has been in orbit around Saturn since 2004. One of the great discoveries of the Cassini spacecraft is that the Saturnian moon Enceladus, located at 3.95 Saturn radii (1 RS = 60,268 km), constantly expels water vapor and condensed water from ridges and troughs located in its south polar region. Impact ionization and photoionization of the water molecules, and subsequent transport, creates a plasma disk around the orbit of Enceladus. The plasma disk ion components are mainly hydrogen ions H+ and water group ions W+ (O+, OH+, H2O+, and H3O+). The Cassini LP is used to measure the properties of the plasma. A new method to derive ion density and ion velocity from Langmuir probe measurements has been developed. The estimated LP statistics are used to derive the extension of the plasma disk, which show plasma densities above ~20 cm-3 in between 2.7 and 8.8 RS. The densities also show a very variable plasma disk, varying with one order of magnitude at the inner part of the disk. We show that the density variation could partly be explained by a dayside/nightside asymmetry in both equatorial ion densities and azimuthal ion velocities. The asymmetry is suggested to be due to the particle orbits being shifted towards the Sun that in turn would cause the whole plasma disk to be shifted. We also investigate the ion loss processes of the inner plasma disk and conclude that loss by transport dominates loss by recombination in the entire region. However, loss by recombination is still important in the region closest to Enceladus (~±0.5 RS) where it differs with only a factor of two from ion transport loss. 

Ort, förlag, år, upplaga, sidor
Uppsala: Acta Universitatis Upsaliensis, 2015. s. 53
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1298
Nyckelord
Planetary magnetospheres, Saturn, magnetospheric dynamics, Saturn's inner plasma disk, ring plasma, ion densities, ion velocities, dayside/nightside asymmetry, ion loss rates, Cassini, Langmuir probe, RPWS
Nationell ämneskategori
Fusion, plasma och rymdfysik
Forskningsämne
Rymdfysik
Identifikatorer
urn:nbn:se:uu:diva-263278 (URN)978-91-554-9353-0 (ISBN)
Disputation
2015-11-19, Lägerhyddsvägen 1, Uppsala, 13:00 (Engelska)
Opponent
Handledare
Forskningsfinansiär
Rymdstyrelsen
Tillgänglig från: 2015-10-27 Skapad: 2015-09-29 Senast uppdaterad: 2015-11-10
Edberg, N. J. T., Andrews, D. J., Bertucci, C., Gurnett, D. A., Holmberg, M. K. G., Jackman, C. M., . . . Wahlund, J.-E. (2015). Effects of Saturn's magnetospheric dynamics on Titan's ionosphere. Journal of Geophysical Research - Space Physics, 120(10), 8884-8898
Öppna denna publikation i ny flik eller fönster >>Effects of Saturn's magnetospheric dynamics on Titan's ionosphere
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2015 (Engelska)Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 10, s. 8884-8898Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We use the Cassini Radio and Plasma Wave Science/Langmuir probe measurements of the electron density from the first 110 flybys of Titan to study how Saturn's magnetosphere influences Titan's ionosphere. The data is first corrected for biased sampling due to varying solar zenith angle and solar energy flux (solar cycle effects). We then present results showing that the electron density in Titan's ionosphere, in the altitude range 1600-2400km, is increased by about a factor of 2.5 when Titan is located on the nightside of Saturn (Saturn local time (SLT) 21-03h) compared to when on the dayside (SLT 09-15 h). For lower altitudes (1100-1600km) the main dividing factor for the ionospheric density is the ambient magnetospheric conditions. When Titan is located in the magnetospheric current sheet, the electron density in Titan's ionosphere is about a factor of 1.4 higher compared to when Titan is located in the magnetospheric lobes. The factor of 1.4 increase in between sheet and lobe flybys is interpreted as an effect of increased particle impact ionization from approximate to 200eV sheet electrons. The factor of 2.5 increase in electron density between flybys on Saturn's nightside and dayside is suggested to be an effect of the pressure balance between thermal plus magnetic pressure in Titan's ionosphere against the dynamic pressure and energetic particle pressure in Saturn's magnetosphere.

Nyckelord
Titan, Saturn's magnetosphere, Cassini, ionosphere
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Identifikatorer
urn:nbn:se:uu:diva-272411 (URN)10.1002/2015JA021373 (DOI)000366135200052 ()
Forskningsfinansiär
Vetenskapsrådet
Tillgänglig från: 2016-01-25 Skapad: 2016-01-13 Senast uppdaterad: 2017-11-30Bibliografiskt granskad
Holmberg, M., Wahlund, J.-E. & Morooka, M. (2014). Dayside/nightside asymmetry of ion densities and velocities in Saturn's inner magnetosphere [Letter to the editor]. Geophysical Research Letters, 41(11), 3717-3723
Öppna denna publikation i ny flik eller fönster >>Dayside/nightside asymmetry of ion densities and velocities in Saturn's inner magnetosphere
2014 (Engelska)Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 41, nr 11, s. 3717-3723Artikel i tidskrift, Letter (Refereegranskat) Published
Abstract [en]

We present Radio and Plasma Wave Science Langmuir probe measurements from 129 Cassini orbits, which show a day/night asymmetry in both ion density and ion velocity in the radial region 4–6 RS (1 RS = 60,268 km) from the center of Saturn. The ion densities ni vary from an average of ∼35 cm−3 around noon up to ∼70 cm−3 around midnight. The ion velocities vi,θ vary from ∼28–32 km/s at the lowest dayside values to ∼36–40 km/s at the highest nightside values. The day/night asymmetry is suggested to be due to the radiation pressure force acting on negatively charged nanometer-sized dust of the E ring. This force will introduce an extra grain and ion drift component equivalent to the force of an additional electric field of 0.1–2 mV/m for 10–50 nm sized grains.

Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Identifikatorer
urn:nbn:se:uu:diva-227095 (URN)10.1002/2014GL060229 (DOI)000339280200005 ()
Tillgänglig från: 2014-06-24 Skapad: 2014-06-24 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
Gamier, P., Holmberg, M. K. G., Wahlund, J.-E. -., Lewis, G. R., Schippers, P., Coates, A., . . . Dandouras, I. (2014). Deriving the characteristics of warm electrons (100-500 eV) in the magnetosphere of Saturn with the Cassini Langmuir probe. Planetary and Space Science, 104, 173-184
Öppna denna publikation i ny flik eller fönster >>Deriving the characteristics of warm electrons (100-500 eV) in the magnetosphere of Saturn with the Cassini Langmuir probe
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2014 (Engelska)Ingår i: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 104, s. 173-184Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Though Langmuir probes (LP) are designed to investigate cold plasma regions (e.g. ionospheres), a recent analysis revealed a strong sensitivity of the Cassini LP measurements to hundreds of eV electrons. These warm electrons impact the surface of the probe and generate a significant current of secondary electrons, that impacts both the DC level and the slope of the current-voltage curve of the LP (for negative potentials) through energetic contributions that may be modeled with a reasonable precision. We show here how to derive information about the incident warm electrons from the analysis of these energetic contributions, in the regions where the cold plasma component is small with an average temperature in the range similar to [100-500] eV. First, modeling the energetic contributions (based on the incident electron flux given by a single anode of the CAPS spectrometer) allows us to provide information about the pitch angle anisotropies of the incident hundreds of eV electrons. The modeling reveals indeed sometimes a large variability of the estimated maximum secondary electron yield (which is a constant for a surface material) needed to reproduce the observations. Such dispersions give evidence for strong pitch angle anisotropies of the incident electrons, and using a functional form of the pitch angle distribution even allows us to derive the real peak angle of the distribution. Second, rough estimates of the total electron temperature may be derived in the regions where the warm electrons are dominant and thus strongly influence the LP observations, i.e. when the average electron temperature is in the range similar to [100-500] eV. These regions may be identified from the LP observations through large positive values of the current-voltage slope at negative potentials. The estimated temperature may then be used to derive the electron density in the same region, with estimated densities between similar to 0.1 and a few particles/cm(3) (cc). The derived densities are in better agreement with the CAPS measurements than the values derived from the proxy technique (Morooka et al., 2009) based on the floating potential of the LP. Both the electron temperature and the density estimates lie outside the classical capabilities of the LP, which are essentially n(e) > 5 cc and T-e <5 eV at Saturn. This approximate derivation technique may be used in the regions where the cold plasma component is small with an average temperature in the range similar to [100-500] eV, which occurs often in the L range 6.4-9.4 R-S when Cassini is off the equator, but may occur anywhere in the magnetosphere. This technique may be all the more interesting since the CAPS instrument was shut down, and, though it cannot replace the CAPS instrument, the technique can provide useful information about the electron moments, with probably even better estimates than CAPS in some cases (when the plasma is strongly anisotropic). Finally, a simple modeling approach allows us to predict the impact of the energetic contributions on LP measurements in any plasma environment whose characteristics (density, temperature, etc.) are known. LP observations may thus be influenced by warm electrons in several planetary plasma regions in the solar system, and ambient magnetospheric electron density and temperature could be estimated in some of them (e.g. around several galilean satellites) through the use of Langmuir probes.

Nyckelord
Langmuir probe, Cassini, Electron density, Electron temperature, Energetic plasma, Pitch angle anisotropies
Nationell ämneskategori
Fysik
Identifikatorer
urn:nbn:se:uu:diva-244585 (URN)10.1016/j.pss.2014.09.008 (DOI)000347606200003 ()
Tillgänglig från: 2015-03-06 Skapad: 2015-02-18 Senast uppdaterad: 2017-12-04Bibliografiskt granskad
Sakai, S., Watanabe, S., Morooka, M. W., Holmberg, M. K. G., Wahlund, J.-E., Gurnett, D. A. & Kurth, W. S. (2013). Dust-plasma interaction through magnetosphere-ionosphere coupling in Saturn's plasma disk. Planetary and Space Science, 75(1), 11-16
Öppna denna publikation i ny flik eller fönster >>Dust-plasma interaction through magnetosphere-ionosphere coupling in Saturn's plasma disk
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2013 (Engelska)Ingår i: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 75, nr 1, s. 11-16Artikel, forskningsöversikt (Refereegranskat) Published
Abstract [en]

The ion bulk speeds in the equatorial region of Saturn's inner magnetosphere, according to data from the Langmuir Probe (LP) on board the Cassini spacecraft, are about 60% of the ideal co-rotation speed; the ion speeds are between the co-rotation and Keplerian speeds (Holmberg et al.; Ion densities and velocities in the inner plasma torus of Saturn, Planetary and Space Science). These findings suggest that sub-micrometer negatively charged E ring dust contributes to the plasma dynamics in the plasma disk. We calculated the ion speeds by using a multi-species fluid model, taking into account dust interactions to investigate the effects of ion-dust coulomb collision, mass loading, as well as taking into account magnetosphere-ionosphere coupling to investigate the effect of the magnetospheric electric field. The results show that the ion speeds can be significantly reduced by the electric fields generated by the collisions between ions and dusts when the dust density is high and the thickness of dust distribution is large. We also show that the ion speeds from our model are consistent with the LP observations when the maximum density of dust is larger than ∼105 m-3.

Nyckelord
Co-rotation lag, Dust-plasma interaction, Dusty plasma, Magnetosphere-ionosphere coupling, Saturn
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:uu:diva-195037 (URN)10.1016/j.pss.2012.11.003 (DOI)000315136600002 ()
Tillgänglig från: 2013-02-20 Skapad: 2013-02-20 Senast uppdaterad: 2017-12-06Bibliografiskt granskad
Holmberg, M. (2013). On the structure and dynamics of Saturn's inner plasma disk. (Licentiate dissertation). Uppsala: Uppsala universitet
Öppna denna publikation i ny flik eller fönster >>On the structure and dynamics of Saturn's inner plasma disk
2013 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

This licentiate thesis presents our investigation of Saturn's inner plasma disk. The thesis gives an overview of the Cassini-Huygens project, what a plasma is and how we use the Langmuir probe to investigate it, various difficulties related to the measurements, the structure of the magnetosphere of Saturn, with special focus on the inner magnetosphere and the region around the Saturnian moon Enceladus. For our investigation we use the Cassini Langmuir probe to derive ion density and ion velocity in the region from 2.5 to 12 Saturn radii. We show that the dominant part of the plasma torus, ion density above ~15 particle/cm3, is located in between 2.5 and 8 Saturn radii (1 RS = 60,268 km) from the planet, with a north-southward extension of 2 RS. The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm-3, only present at the south side of the ring plane, as expected since the Enceladus plumes are located in the south polar region. Also the azimuthal ion velocity vi,Θ is estimated, showing a clear general trend in the region between 3 and 7 RS, described by vi,Θ =1.5R2-8.7R+39. The average vi,Θ starts to deviate from corotation speed at around 3 RS and reaches down to ~68 % of corotation close to 5 RS. The Langmuir probe data show a clear day/night side asymmetry in both ion density and ion velocity, most prominent in the radial region 4-6 RS from the center of Saturn. The ion densities ni varies from an average of ~35 cm-3 for the lowest dayside values close to noon up to ~70 cm-3 for the highest nightside values around midnight. The azimuthal ion velocities vi,Θ varies from ~28-32 km/s at the lowest dayside values around noon to ~36-40 km/s at the highest nightside values around midnight. This gives an azimuthal ion velocity difference between noon and midnight of Δvi,Θ ~5-10 km/s. The day/night asymmetry is suggested to be due to dust-plasma interaction.

Ort, förlag, år, upplaga, sidor
Uppsala: Uppsala universitet, 2013. s. 57
Nyckelord
Saturn, E-ring, plasma disk
Nationell ämneskategori
Teknik och teknologier
Forskningsämne
Rymd- och plasmafysik
Identifikatorer
urn:nbn:se:uu:diva-218772 (URN)
Presentation
2013-10-07, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 15:00 (Engelska)
Opponent
Handledare
Tillgänglig från: 2014-06-24 Skapad: 2014-02-17 Senast uppdaterad: 2014-06-24Bibliografiskt granskad
Garnier, P., Holmberg, M., Wahlund, J.-E., Lewis, G. R., Grimald, S. R., Thomsen, M. F., . . . Dandouras, I. (2013). The influence of the secondary electrons induced by energetic electrons impacting the Cassini Langmuir probe at Saturn. Journal of geophysical research Space Physics, 118(11), 7054-7073
Öppna denna publikation i ny flik eller fönster >>The influence of the secondary electrons induced by energetic electrons impacting the Cassini Langmuir probe at Saturn
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2013 (Engelska)Ingår i: Journal of geophysical research Space Physics, ISSN 2169-9402, Vol. 118, nr 11, s. 7054-7073Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

The Cassini Langmuir Probe (LP) onboard the Radio and Plasma Wave Science experiment has provided much information about the Saturnian cold plasma environment since the Saturn Orbit Insertion in 2004. A recent analysis revealed that the LP is also sensitive to the energetic electrons (250–450 eV) for negative potentials. These electrons impact the surface of the probe and generate a current of secondary electrons, inducing an energetic contribution to the DC level of the current-voltage (I-V) curve measured by the LP. In this paper, we further investigated this influence of the energetic electrons and (1) showed how the secondary electrons impact not only the DC level but also the slope of the (I-V) curve with unexpected positive values of the slope, (2) explained how the slope of the (I-V) curve can be used to identify where the influence of the energetic electrons is strong, (3) showed that this influence may be interpreted in terms of the critical and anticritical temperatures concept detailed by Lai and Tautz (2008), thus providing the first observational evidence for the existence of the anticritical temperature, (4) derived estimations of the maximum secondary yield value for the LP surface without using laboratory measurements, and (5) showed how to model the energetic contributions to the DC level and slope of the (I-V) curve via several methods (empirically and theoretically). This work will allow, for the whole Cassini mission, to clean the measurements influenced by such electrons. Furthermore, the understanding of this influence may be used for other missions using Langmuir probes, such as the future missions Jupiter Icy Moons Explorer at Jupiter, BepiColombo at Mercury, Rosetta at the comet Churyumov-Gerasimenko, and even the probes onboard spacecrafts in the Earth magnetosphere.

Nyckelord
Energetic particles, Planetary magnetospheres, Saturn, Spacecraft sheaths, wakes, charging
Nationell ämneskategori
Teknik och teknologier
Forskningsämne
Rymd- och plasmafysik
Identifikatorer
urn:nbn:se:uu:diva-218777 (URN)10.1002/2013JA019114 (DOI)000329992900023 ()
Tillgänglig från: 2014-02-17 Skapad: 2014-02-17 Senast uppdaterad: 2014-03-19Bibliografiskt granskad
Roussos, E., Kollmann, P., Krupp, N., Paranicas, C., Krimigis, S. M., Mitchell, D. G., . . . Holmberg, M. K. G. (2012). Energetic electron observations of Rhea's magnetospheric interaction. Icarus, 221(1), 116-134
Öppna denna publikation i ny flik eller fönster >>Energetic electron observations of Rhea's magnetospheric interaction
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2012 (Engelska)Ingår i: Icarus, ISSN 0019-1035, E-ISSN 1090-2643, Vol. 221, nr 1, s. 116-134Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Saturn's moon Rhea is thought to be a simple plasma absorber, however, energetic particle observations in its vicinity show a variety of unexpected and complex interaction features that do not conform with our current understanding about plasma absorbing interactions. Energetic electron data are especially interesting, as they contain a series of broad and narrow flux depletions on either side of the moon's wake. The association of these dropouts with absorption by dust and boulders orbiting within Rhea's Hill sphere was suggested but subsequently not confirmed, so in this study we review data from all four Cassini flybys of Rhea to date seeking evidence for alternative processes operating within the moon's interaction region. We focus on energetic electron observations, which we put in context with magnetometer, cold plasma density and energetic ion data. All flybys have unique features, but here we only focus on several structures that are consistently observed. The most interesting common feature is that of narrow dropouts in energetic electron fluxes, visible near the wake flanks. These are typically seen together with narrow flux enhancements inside the wake. A phase-space-density analysis for these structures from the first Rhea flyby (R1) shows that Liouville's theorem holds, suggesting that they may be forming due to rapid transport of energetic electrons from the magnetosphere to the wake, through narrow channels. A series of possibilities are considered to explain this transport process. We examined whether complex energetic electron drifts in the interaction region of a plasma absorbing moon (modeled through a hybrid simulation code) may allow such a transport. With the exception of several features (e.g. broadening of the central wake with increasing electron energy), most of the commonly observed interaction signatures in energetic electrons (including the narrow structures) were not reproduced. Additional dynamical processes, not simulated by the hybrid code, should be considered in order to explain the data. For the small scale features, the possibility that a flute (interchange) instability acts on the electrons is discussed. This instability is probably driven by strong gradients in the plasma pressure and the magnetic field magnitude: magnetometer observations show clearly signatures consistent with the (expected) plasma pressure loss due to ion absorption at Rhea. Another potential driver of the instability could have been gradients in the cold plasma density, which are, however, surprisingly absent from most crossings of Rhea's plasma wake. The lack of a density depletion in Rhea's wake suggests the presence of a local cold plasma source region. Hybrid plasma simulations show that this source cannot be the ionized component of Rhea's weak exosphere. It is probably related to accelerated photoelectrons from the moon's negatively charged surface, indicating that surface charging may play a very important role in shaping Rhea's magnetospheric interaction region. (C) 2012 Elsevier Inc. All rights reserved.

Nyckelord
Saturn, Magnetosphere, Saturn, Satellites, Moon
Nationell ämneskategori
Fysik
Identifikatorer
urn:nbn:se:uu:diva-184658 (URN)10.1016/j.icarus.2012.07.006 (DOI)000309620200011 ()
Tillgänglig från: 2012-11-12 Skapad: 2012-11-12 Senast uppdaterad: 2023-03-28Bibliografiskt granskad
Holmberg, M. K. G., Wahlund, J.-E., Morooka, M. W. & Persoon, A. M. (2012). Ion densities and velocities in the inner plasma torus of Saturn. Planetary and Space Science, 73(1), 151-160
Öppna denna publikation i ny flik eller fönster >>Ion densities and velocities in the inner plasma torus of Saturn
2012 (Engelska)Ingår i: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 73, nr 1, s. 151-160Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We present plasma data from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP), mapping the ion density and velocity of Saturn's inner plasma torus. Data from 129 orbits, recorded during the period from the 1st of February 2005 to the 27th of June 2010, are used to map the extension of the inner plasma torus. The dominant part of the plasma torus is shown to be located in between 2.5 and 8 Saturn radii (1 RS=60,268 km) from the planet, with a north-southward extension of ±2RS. The plasma disk ion density shows a broad maximum in between the orbits of Enceladus and Tethys. Ion density values vary between 20 and 125 cm-3 at the location of the density maximum, indicating considerable dynamics of the plasma disk. The equatorial density structure, |z|&lt;0.5RS, shows a slower decrease away from the planet than towards. The outward decrease, from 5 R S, is well described by the relation neq=2.2×10 4(1/R)3.63. The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm-3, only present at the south side of the ring plane. A less prominent density peak, of 115 cm-3, is also detected at the orbit of Tethys, at ∼4.9 RS. No density peaks are recorded at the orbits of the moons Mimas, Dione, and Rhea. The presented ion velocity vi,θ shows a clear general trend in the region between 3 and 7 RS, described by vi, θ=1.5R2-8.7R+39. The average vi,θ starts to deviate from corotation at around 3 RS, reaching ∼68% of corotation close to 5 RS.

Ort, förlag, år, upplaga, sidor
Elsevier, 2012
Nyckelord
Cassini, E-ring, Ion density, Ion velocity, Plasma disk, Saturn magnetosphere, Alpha particles, Magnetosphere, Orbits, Plasma waves, Plasmas, Velocity, Ions
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:uu:diva-192893 (URN)10.1016/j.pss.2012.09.016 (DOI)000314007400024 ()
Tillgänglig från: 2013-01-28 Skapad: 2013-01-25 Senast uppdaterad: 2017-12-06Bibliografiskt granskad
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