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A New MHD-assisted Stokes Inversion Technique
Riethmü,ller, T. L.,Solanki, S. K.,Barthol, P.,Gandorfer, A.,Gizon, L.,Hirzberger, J.,Noort, M. van,Rodrí,guez, J. Blanco,Iniesta, J. C. Del Toro,Suá,rez, D. Orozco,Schmidt, W.,Pillet, American Astronomical Society 2017 The Astrophysical journal Supplement series Vol.229 No.1
<P>We present a new method of Stokes inversion of spectropolarimetric data and evaluate it by taking the example of a SUNRISE/IMaX observation. An archive of synthetic Stokes profiles is obtained by the spectral synthesis of stateof- the-art magnetohydrodynamics (MHD) simulations and a realistic degradation to the level of the observed data. The definition of a merit function allows the archive to be searched for the synthetic Stokes profiles that best match the observed profiles. In contrast to traditional Stokes inversion codes, which solve the Unno-Rachkovsky equations for the polarized radiative transfer numerically and fit the Stokes profiles iteratively, the new technique provides the full set of atmospheric parameters. This gives us the ability to start an MHD simulation that takes the inversion result as an initial condition. After a relaxation process of half an hour solar time we obtain physically consistent MHD data sets with a target similar to the observation. The new MHD simulation is used to repeat the method in a second iteration, which further improves the match between observation and simulation, resulting in a factor of 2.2 lower mean chi(2) value. One advantage of the new technique is that it provides the physical parameters on a geometrical height scale. It constitutes a first step toward inversions that give results consistent with the MHD equations.</P>
Oba, T.,Riethmü,ller, T. L.,Solanki, S. K.,Iida, Y.,Quintero Noda, C.,Shimizu, T. American Astronomical Society 2017 The Astrophysical journal Vol.849 No.1
<P>Solar granules are bright patterns surrounded by dark channels, called intergranular lanes, in the solar photosphere and are a manifestation of overshooting convection. Observational studies generally find stronger upflows in granules and weaker downflows in intergranular lanes. This trend is, however, inconsistent with the results of numerical simulations in which downflows are stronger than upflows through the joint action of gravitational acceleration/deceleration and pressure gradients. One cause of this discrepancy is the image degradation caused by optical distortion and light diffraction and scattering that takes place in an imaging instrument. We apply a deconvolution technique to Hinode/SP data in an attempt to recover the original solar scene. Our results show a significant enhancement in both the convective upflows and downflows but particularly for the latter. After deconvolution, the up- and downflows reach maximum amplitudes of -3.0 km s(-1) and + 3.0 km s(-1) at an average geometrical height of roughly 50 km, respectively. We found that the velocity distributions after deconvolution match those derived from numerical simulations. After deconvolution, the net LOS velocity averaged over the whole field of view lies close to zero as expected in a rough sense from mass balance.</P>
Intensity contrast of solar plage as a function of magnetic flux at high spatial resolution
Kahil, F.,Riethmü,ller, T. L.,Solanki, S. K. Springer-Verlag 2019 Astronomy and astrophysics Vol.621 No.-
<P>Magnetic elements have an intensity contrast that depends on the type of region they are located in (for example quiet Sun, or active region plage). Observed values also depend on the spatial resolution of the data. Here we investigate the contrast-magnetic field dependence in active region plage observed near disk center with SUNRISE during its second flight in 2013. The wavelengths under study range from the visible at 525 nm to the near ultraviolet (NUV) at 300 nm and 397 nm. We use quasi-simultaneous spectropolarimetric and photometric data from the Imaging Magnetograph eXperiment (IMaX) and the Sunrise Filter Imager (SuFI), respectively. We find that in all wavelength bands, the contrast exhibits a qualitatively similar dependence on the line-of-sight magnetic field, <I>B</I>LOS, as found in the quiet Sun, with the exception of the continuum at 525 nm. There, the contrast of plage magnetic elements peaks for intermediate values of <I>B</I>LOS and decreases at higher field strengths. By comparison, the contrast of magnetic elements in the quiet Sun saturates at its maximum value at large <I>B</I>LOS. We find that the explanation of the turnover in contrast in terms of the effect of finite spatial resolution of the data is incorrect with the evidence provided by the high-spatial resolution SUNRISE data, as the plage magnetic elements are larger than the quiet Sun magnetic elements and are well-resolved. The turnover comes from the fact that the core pixels of these larger magnetic elements are darker than the quiet Sun. We find that plages reach lower contrast than the quiet Sun at disk center at wavelength bands formed deep in the photosphere, such as the visible continuum and the 300 nm band. This difference decreases with formation height and disappears in the Ca II H core, in agreement with empirical models of magnetic element atmospheres.</P>
Brightness of Solar Magnetic Elements As a Function of Magnetic Flux at High Spatial Resolution
Kahil, F.,Riethmü,ller, T. L.,Solanki, S. K. American Astronomical Society 2017 The Astrophysical journal Supplement series Vol.229 No.1
<P>We investigate the relationship between the photospheric magnetic field of small-scale magnetic elements in the quiet-Sun (QS) at disk. center. and the brightness at 214, 300, 313, 388, 397, and 525.02 nm. To this end, we analyzed spectropolarimetric and imaging time series acquired simultaneously by the Imaging Magnetograph eXperiment magnetograph and the SuFI filter imager on board the balloon-borne observatory SUNRISE during its first science flight in 2009, with high spatial and temporal resolution. We find a clear dependence of the contrast in the near ultraviolet and the visible on the line-of-sight component of the magnetic field, BLOS, which is best described by a logarithmic model. This function effectively. represents the relationship between the Ca II H-line emission and BLOS. and works better than the. power-law fit adopted by previous studies. This, along with the high contrast reached at these wavelengths, will help with determining the contribution of small-scale elements in the QS to the irradiance changes for wavelengths below 388 nm. At all wavelengths, including the continuum at 525.40 nm, the intensity contrast does not decrease with increasing BLOS. This result also strongly supports the fact that SUNRISE has resolved small strong magnetic field elements in the internetwork, resulting in constant contrasts for large magnetic fields in our continuum contrast at 525.40 nm versus. the. BLOS scatterplot, unlike the turnover obtained in previous observational studies. This turnover is due to the intermixing of the bright magnetic features with the dark intergranular lanes surrounding them.</P>
Moving Magnetic Features Around a Pore
Kaithakkal, A. J.,Riethmü,ller, T. L.,Solanki, S. K.,Lagg, A.,Barthol, P.,Gandorfer, A.,Gizon, L.,Hirzberger, J.,vanNoort, M.,Rodrí,guez, J. Blanco,Iniesta, J. C. Del Toro,Suá,rez, D. American Astronomical Society 2017 The Astrophysical journal, Supplement series Vol.229 No.1
<P>Spectropolarimetric observations from SUNRISE/IMaX, obtained in 2013 June, are used for a statistical analysis to determine the physical properties of moving magnetic features (MMFs) observed near a pore. MMFs of the same and opposite polarity, with respect to the pore, are found to stream from its border at an average speed of 1.3 km s(-1) and 1.2 km s(-1), respectively, with mainly same-polarity MMFs found further away from the pore. MMFs of both polarities are found to harbor rather weak, inclined magnetic fields. Opposite-polarity MMFs are blueshifted, whereas same-polarity MMFs do not show any preference for up-or downflows. Most of the MMFs are found to be of sub-arcsecond size and carry a mean flux of similar to 1.2 x 10(17) Mx.</P>
Solanki, S. K.,Riethmü,ller, T. L.,Barthol, P.,Danilovic, S.,Deutsch, W.,Doerr, H.-P.,Feller, A.,Gandorfer, A.,Germerott, D.,Gizon, L.,Grauf, B.,Heerlein, K.,Hirzberger, J.,Kolleck, M.,Lagg, A.,Me American Astronomical Society 2017 The Astrophysical journal, Supplement series Vol.229 No.1
<P>The SUNRISE balloon-borne solar observatory, consisting of a 1 m aperture telescope that provides a stabilized image to a UV filter imager and an imaging vector polarimeter, carried out its second science flight in 2013 June. It provided observations of parts of active regions at high spatial resolution, including the first high-resolution images in the Mg II. k line. The obtained data are of very high quality, with the best UV images reaching the diffraction limit of the telescope at 3000 angstrom after Multi-Frame Blind Deconvolution reconstruction accounting for phasediversity information. Here a brief update is given of the instruments and the data reduction techniques, which includes an inversion of the polarimetric data. Mainly those aspects that evolved compared with the first flight are described. A tabular overview of the observations is given. In addition, an example time series of a part of the emerging active region NOAA AR. 11768 observed relatively close to disk center is described and discussed in some detail. The observations cover the pores in the trailing polarity of the active region, as well as the polarity inversion line where flux emergence was ongoing and a small flare-like brightening occurred in the course of the time series. The pores are found to contain magnetic field strengths ranging up to 2500 G, and while large pores are clearly darker and cooler than the quiet Sun in all layers of the photosphere, the temperature and brightness of small pores approach or even exceed those of the quiet Sun in the upper photosphere.</P>
QUIET-SUN INTENSITY CONTRASTS IN THE NEAR-ULTRAVIOLET AS MEASURED FROM SUNRISE
Hirzberger, J.,Feller, A.,Riethmü,ller, T. L.,Schü,ssler, M.,Borrero, J. M.,Afram, N.,Unruh, Y. C.,Berdyugina, S. V.,Gandorfer, A.,Solanki, S. K.,Barthol, P.,Bonet, J. A.,Martí,nez Pille IOP Publishing 2010 ASTROPHYSICAL JOURNAL LETTERS - Vol.723 No.2