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  • 1.
    de la Cruz Rodriguez, Jaime
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    De Pontieu, B.
    Carlsson, M.
    van der Voort, L. H. M. Rouppe
    Heating of the Magnetic Chromosphere: Observational Constraints from Ca II lambda 8542 Spectra2013In: The Astrophysical Journal Letters, ISSN 2041-8205, Vol. 764, no 1, p. L11-Article in journal (Refereed)
    Abstract [en]

    The heating of the Sun's chromosphere remains poorly understood. While progress has been made on understanding what drives the quiet-Sun internetwork chromosphere, chromospheric heating in strong magnetic field regions continues to present a difficult challenge, mostly because of a lack of observational constraints. We use high-resolution spectropolarimetric data from the Swedish 1 m Solar Telescope to identify the location and spatio-temporal properties of heating in the magnetic chromosphere. In particular, we report the existence of raised-core spectral line profiles in the Ca II lambda 8542 line. These profiles are characterized by the absence of an absorption line core, showing a quasi-flat profile between. lambda approximate to +/- 0.5 angstrom, and are abundant close to magnetic bright points and plage. Comparison with three-dimensional MHD simulations indicates that such profiles occur when the line of sight goes through an "elevated temperature canopy" associated with the expansion with height of the magnetic field of flux concentrations. This temperature canopy in the simulations is caused by ohmic dissipation where there are strong magnetic field gradients. The raised-core profiles are thus indicators of locations of increased chromospheric heating. We characterize the location and temporal and spatial properties of such profiles in our observations, thus providing much stricter constraints on theoretical models of chromospheric heating mechanisms than before.

  • 2.
    de la Cruz Rodriguez, Jaime
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Socas-Navarro, H.
    Carlsson, M.
    Leenaarts, J.
    Chromospheric Magnetic Fields: Observations, Simulations and their Interpretation2012In: SECOND ATST-EAST MEETING: MAGNETIC FIELDS FROM THE PHOTOSPHERE TO THE CORONA, SAN FRANCISCO: ASTRONOMICAL SOC PACIFIC , 2012, Vol. 463, p. 15-23Conference paper (Refereed)
    Abstract [en]

    The magnetic field of the quiet-Sun chromosphere remains a mystery for solar physicists. The reduced number of chromospheric lines are intrinsically hard to model and only a few of them are magnetically sensitive. In this work, we use a 3D numerical simulation of the outer layers of the solar atmosphere, to asses the reliability of non-LTE inversions, in this case applied to the Ca II lambda 8542 angstrom line. We show that NLTE inversions provide realistic estimates of physical quantities from synthetic observations.

  • 3.
    de la Cruz Rodriguez, Jaime
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Socas-Navarro, H.
    Carlsson, M.
    Leenaarts, J.
    Non-local thermodynamic equilibrium inversions from a 3D magnetohydrodynamic chromospheric model2012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 543, p. A34-Article in journal (Refereed)
    Abstract [en]

    Context. The structure of the solar chromosphere is believed to be governed by magnetic fields, even in quiet-Sun regions that have a relatively weak photospheric field. During the past decade inversion methods have emerged as powerful tools for analyzing the chromosphere of active regions. The applicability of inversions to infer the stratification of the physical conditions in a dynamic 3D solar chromosphere has not yet been studied in detail.

    Aims. This study aims to establish the diagnostic capabilities of non-local thermodynamical equilibrium (NLTE) inversion techniques of Stokes profiles induced by the Zeeman effect in the Ca II lambda 8542 angstrom line.

    Methods. We computed the Ca II atomic level populations in a snapshot from a 3D radiation-MHD simulation of the quiet solar atmosphere in non-LTE using the 3D radiative transfer code Multi3d. These populations were used to compute synthetic full-Stokes profiles in the Ca II lambda 8542 angstrom line using 1.5D radiative transfer and the inversion code Nicole. The profiles were then spectrally degraded to account for finite filter width, and Gaussian noise was added to account for finite photon flux. These profiles were inverted using Nicole and the results were compared with the original model atmosphere.

    Results. Our NLTE inversions applied to quiet-Sun synthetic observations provide reasonably good estimates of the chromospheric magnetic field, line-of-sight velocities and somewhat less accurate, but still very useful, estimates of the temperature. Three-dimensional scattering of photons cause cool pockets in the chromosphere to be invisible in the line profile and consequently they are also not recovered by the inversions. To successfully detect Stokes linear polarization in this quiet snapshot, a noise level below 10(-3.5) is necessary.

  • 4. Ortiz, Ada
    et al.
    Bellot Rubio, Luis R.
    Hansteen, Viggo H.
    Rodriguez, Jaime de la Cruz
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    van der Voort, Luc Rouppe
    Emergence of Granular-Sized Magnetic Bubbles Through the Solar Atmosphere. I. Spectropolarimetric Observations and Simulations2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 781, no 2Article in journal (Refereed)
    Abstract [en]

    We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in 2009 July. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0".14. Simultaneous full Stokes observations of the two photospheric Fe i lines at 630.2 nm and the chromospheric Ca ii 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on three-dimensional (3D) semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. Several phenomena occur simultaneously, namely, abnormal granulation, separation of opposite-polarity legs, and brightenings at chromospheric heights. However, the most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca ii 854.2 nm line. There is a clear coincidence between the emergence of horizontal magnetic field patches and the formation of the dark bubble. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca ii 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km s-1 and expands at a horizontal speed of 4 kms-1. In about 3.5minutes it travels some 1100 km to reach themid chromosphere, implying an average ascent speed of 5.2 km s-1. The maximum separation attained by the magnetic legs is 6".6. From an inversion of the observed Stokes spectra with the SIR code, we find maximum photospheric field strengths of 480 G and inclinations of nearly 90. in the magnetic bubble interior, along with temperature deficits of up to 250 K at log t = -2 and above. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere, the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes.

  • 5.
    Rodriguez, Jaime de la Cruz
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Piskunov, Nikolai
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Delo-bezier formal solutions of the polarized radiative transfer equation2013In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 764, no 1, p. 33-Article in journal (Refereed)
    Abstract [en]

    We present two new accurate and efficient methods to compute the formal solution of the polarized radiative transfer equation. In this work, the source function and the absorption matrix are approximated using quadratic and cubic Bezier spline interpolants. These schemes provide second- and third-order approximations, respectively, and do not suffer from erratic behavior of the polynomial approximation (overshooting). The accuracy and the convergence of the new method are studied along with other popular solutions of the radiative transfer equation, using stellar atmospheres with strong gradients in the line-of-sight velocity and in the magnetic-field vector.

  • 6.
    Rodriguez, Jamiel de la Cruz
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    van der Voort, L. Rouppe
    Socas-Navarro, H.
    van Noort, M.
    Physical properties of a sunspot chromosphere with umbral flashes2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, p. A115-Article in journal (Refereed)
    Abstract [en]

    We present new high-resolution spectro-polarimetric Ca II lambda 8542 observations of umbral flashes in sunspots. At nearly 0 ''.18, and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves both LTE and non-LTE inversions (but includes also a weak field analysis as a sanity check) to quantify temperatures, mass flows and the full magnetic field vector geometry. We confirm earlier reports that UFs have very fine structure with hot and cool material intermixed at sub-arcsecond scales. The shock front is roughly 1000 K hotter than the surrounding material. We do not observe significant fluctuations of the field in the umbra. In the penumbra, however, the passage of the running penumbral waves alter the magnetic field strength by some 200 G (peak-to-peak amplitude) but it does not change the field orientation (at least not significantly within our sensitivity of a few degrees). From a fast Fourier transform analysis, we find a trend of decreasing power at high temporal frequencies at those locations with more horizontal magnetic fields, for the line-of-sight velocity and magnetic field strength. In the outer penumbra we find an absence of high frequency power while there is increasingly more power at high frequencies towards the umbra.

  • 7. Scharmer, G. B.
    et al.
    de la Cruz Rodriguez, Jaime
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics.
    Sutterlin, P.
    Henriques, V. M. J.
    Opposite polarity field with convective downflow and its relation to magnetic spines in a sunspot penumbra2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 553, p. A63-Article in journal (Refereed)
    Abstract [en]

    We discuss NICOLE inversions of Fe I 630.15 nm and 630.25 nm Stokes spectra from a sunspot penumbra recorded with the CRISP imaging spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial resolution close to 0.'' 15. We report on narrow, radially extended lanes of opposite polarity field, located at the boundaries between areas of relatively horizontal magnetic field (the intra-spines) and much more vertical field (the spines). These lanes harbor convective downflows of about 1 km s(-1). The locations of these downflows close to the spines agree with predictions from the convective gap model (the "gappy penumbra") proposed six years ago, and more recent three-dimensional magnetohydrodynamic simulations. We also confirm the existence of strong convective flows throughout the entire penumbra, showing the expected correlation between temperature and vertical velocity, and having vertical root mean square velocities of about 1.2 km s(-1).

  • 8. van der Voort, L. Rouppe
    et al.
    Rodriguez, Jaime de la Cruz
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Short Dynamic Fibrils In Sunspot Chromospheres2013In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 776, no 1, p. 56-Article in journal (Refereed)
    Abstract [en]

    Sunspot chromospheres display vigorous oscillatory signatures when observed using chromospheric diagnostics such as the strong Ca II lines and H alpha. New high-resolution sunspot observations from the Swedish 1 m Solar Telescope show the ubiquitous presence of small-scale, periodic, jet-like features that move up and down. This phenomenon has not been described before. The typical width of these features is about 0 ''.3 and they display clear parabolic trajectories in space-time diagrams. The maximum extension of the top of the jets is lowest in the umbra, a few 100 km, and progressively longer further away from the umbra in the penumbra, with the longest extending more than 1000 km. These jets resemble the dynamic fibrils found in plage regions but at smaller extensions. Local thermodynamic equilibrium inversion of spectropolarimetric Ca II 8542 observations enabled a comparison of the magnetic field inclination and properties of these short jets. We find that the most extended of these jets also have longer periods and tend to be located in regions with more horizontal magnetic fields. These results are direct observational confirmation of the mechanism of long-period waves propagating along inclined magnetic fields into the solar chromosphere. This mechanism was identified earlier as the driver of dynamic fibrils in plage, part of the mottles in the quiet Sun, and the type I spicules at the limb. The sunspot dynamic fibrils that we report here represent a new class of manifestation of this mechanism, distinct from the transient penumbral and umbral micro-jets reported earlier.

  • 9. Watanabe, Hiroko
    et al.
    Bellot Rubio, Luis R.
    Rodriguez, Jaime de la Cruz
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    van der Voort, Luc Rouppe
    Temporal evolution of velocity and magnetic field in and around umbral dots2012In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 757, no 1, p. 49-Article in journal (Refereed)
    Abstract [en]

    We study the temporal evolution of umbral dots (UDs) using measurements from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope. Scans of the magnetically sensitive 630 nm iron lines were performed under stable atmospheric conditions for 71 minutes with a cadence of 63 s. These observations allow us to investigate the magnetic field and velocity in and around UDs at a resolution approaching 0.'' 13. From the analysis of 339 UDs, we draw the following conclusions: (1) UDs show clear hints of upflows, as predicted by magnetohydrodynamic simulations. By contrast, we could not find systematic downflow signals. Only in very deep layers, we detect localized downflows around UDs, but they do not persist in time. (2) We confirm that UDs exhibit weaker and more inclined fields than their surroundings, as reported previously. However, UDs that have strong fields above 2000 G or are in the decay phase show enhanced and more vertical fields. (3) There are enhanced fields at the migration front of UDs detached from penumbral grains, as if their motion were impeded by the ambient field. (4) Long-lived UDs travel longer distances with slower proper motions. Our results appear to confirm some aspects of recent numerical simulations of magnetoconvection in the umbra (e. g., the existence of upflows in UDs), but not others (e. g., the systematic weakening of the magnetic field at the position of UDs).

  • 10. Wedemeyer-Bohm, Sven
    et al.
    Scullion, Eamon
    Steiner, Oskar
    van der Voort, Luc Rouppe
    de la Cruz Rodriguez, Jaime
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Fedun, Viktor
    Erdelyi, Robert
    Magnetic tornadoes as energy channels into the solar corona2012In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 486, no 7404, p. 505-508Article in journal (Refereed)
    Abstract [en]

    Heating the outer layers of the magnetically quiet solar atmosphere to more than one million kelvin and accelerating the solar wind requires an energy flux of approximately 100 to 300 watts per square metre(1-6), but how this energy is transferred and dissipated there is a puzzle and several alternative solutions have been proposed. Braiding and twisting of magnetic field structures, which is caused by the convective flows at the solar surface, was suggested as an efficient mechanism for atmospheric heating(7). Convectively driven vortex flows that harbour magnetic fields are observed(8-10) to be abundant in the photosphere (the visible surface of the Sun). Recently, corresponding swirling motions have been discovered(11) in the chromosphere, the atmospheric layer sandwiched between the photosphere and the corona. Here we report the imprints of these chromospheric swirls in the transition region and low corona, and identify them as observational signatures of rapidly rotating magnetic structures. These ubiquitous structures, which resemble super-tornadoes under solar conditions, reach from the convection zone into the upper solar atmosphere and provide an alternative mechanism for channelling energy from the lower into the upper solar atmosphere.

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