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Register a new userMagnetohydrostatic modeling of AR11768 based on a SUNRISE/IMaX vector magnetogram
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Context. High resolution magnetic field measurements are routinely done only in the solar photosphere. Higher layers like the chromosphere and corona can be modeled by extrapolating the photospheric magnetic field upward. In the solar corona, plasma forces can be neglected and the Lorentz force vanishes. This is not the case in the upper photosphere and chromosphere where magnetic and non-magnetic forces are equally important. One way to deal with this problem is to compute the plasma and magnetic field self-consistently with a magnetohydrostatic (MHS) model. Aims. We aim to derive the magnetic field, plasma pressure and density of AR11768 by applying the newly developed extrapolation technique to the SUNRISE/IMaX data. Methods. An optimization method is used for the MHS modeling. The initial conditions consist of a nonlinear force-free field (NLFFF) and a gravity-stratified atmosphere. Results. In the non-force-free layer, which is spatially resolved by the new code, Lorentz forces are effectively balanced by the gas pressure gradient force and the gravity force. The pressure and density are depleted in strong field regions, which is consistent with observations. Denser plasma, however, is also observed at some parts of the active region edges. In the chromosphere, the fibril-like plasma structures trace the magnetic field nicely. Bright points in SUNRISE/SuFI 3000 {$AA$} images are often accompanied by the plasma pressure and electric current concentrations. In addition, the average of angle between MHS field lines and the selected chromospheric fibrils is $11.8^circ$, which is smaller than those computed from the NLFFF model ($15.7^circ$) and linear MHS model ($20.9^circ$). This indicates that the MHS solution provides a better representation of the magnetic field in the chromosphere.
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Our aim is to model the 3D magnetic field structure of the upper solar atmosphere, including regions of non-negligible plasma beta. We use high-resolution photospheric magnetic field measurements from SUNRISE/IMaX as boundary condition for a magneto-static magnetic field model. The high resolution of IMaX allows us to resolve the interface region between photosphere and corona, but modelling this region is challenging for the following reasons. While the coronal magnetic field is thought to be force-free (the Lorentz-force vanishes), this is not the case in the mixed plasma $beta$ environment in the photosphere and lower chromosphere. In our model, pressure gradients and gravity forces are taken self-consistently into account and compensate the non-vanishing Lorentz-force. Above a certain height (about 2 Mm) the non-magnetic forces become very weak and consequently the magnetic field becomes almost force-free. Here we apply a linear approach, where the electric current density consists of a superposition of a field-line parallel current and a current perpendicular to the Suns gravity field. We illustrate the prospects and limitations of this approach and give an outlook for an extension towards a non-linear model.
We study the photospheric evolution of an exploding granule observed in the quiet Sun at high spatial ($0.3^{primeprime}$) and temporal (31.5 s) resolution by the imaging magnetograph Sunrise/IMaX in June 2009. These observations show that the exploding granule is cospatial to a magnetic flux emergence event occurring at mesogranular scale (up to 12 Mm$^{2}$ area). Using a modified version of the SIR code for inverting the IMaX spectropolarimetric measurements, we obtain information about the magnetic configuration of this photospheric feature. In particular, we find evidence of highly inclined emerging fields in the structure, carrying a magnetic flux content up to $4 times 10^{18}$ Mx. The balance between gas and magnetic pressure in the region of flux emergence, compared with a very quiet region of the Sun, indicates that the additional pressure carried by the emerging flux increases by about 5% the total pressure and appears to allow the granulation to be modified, as predicted by numerical simulations. The overall characteristics suggest that a multi-polar structure emerges into the photosphere, resembling an almost horizontal flux sheet. This seems to be associated with exploding granules. Finally, we discuss the origin of such flux emergence events.
Magneto-static models may overcome some of the issues facing force-free magnetic field extrapolations. So far they have seen limited use and have faced problems when applied to quiet-Sun data. Here we present a first application to an active region. We use solar vector magnetic field measurements gathered by the IMaX polarimeter during the flight of the sunrise{} balloon-borne solar observatory in June 2013 as boundary condition for a magneto-static model of the higher solar atmosphere above an active region. The IMaX data are embedded in active region vector magnetograms observed with SDO/HMI. This work continues our magneto-static extrapolation approach, which has been applied earlier ({it Paper I}) to a quiet Sun region observed with sunrise{} I. In an active region the signal-to-noise-ratio in the measured Stokes parameters is considerably higher than in the quiet Sun and consequently the IMaX measurements of the horizontal photospheric magnetic field allow us to specify the free parameters of the model in a special class of linear magneto-static equilibria. The high spatial resolution of IMaX (110-130 km, pixel size 40 km) enables us to model the non-force-free layer between the photosphere and the mid chromosphere vertically by about 50 grid points. In our approach we can incorporate some aspects of the mixed beta layer of photosphere and chromosphere, e.g., taking a finite Lorentz force into account, which was not possible with lower resolution photospheric measurements in the past. The linear model does not, however, permit to model intrinsic nonlinear structures like strongly localized electric currents.
We characterize the observational properties of the convectively driven vortex flows recently discovered on the quiet Sun, using magnetograms, Dopplergrams and images obtained with the 1-m balloon-borne Sunrise telescope. By visual inspection of time series, we find some 3.1e-3 vortices/(Mm^2 min), which is a factor of 1.7 larger than previous estimates. The mean duration of the individual events turns out to be 7.9 min, with a standard deviation of 3.2 min. In addition, we find several events appearing at the same locations along the duration of the time series (31.6 min). Such recurrent vortices show up in the proper motion flow field map averaged over the time series. The typical vertical vorticities are <= 6e-3 1/sec, which corresponds to a period of rotation of some 35 min. The vortices show a preferred counterclockwise sense of rotation, which we conjecture may have to do with the preferred vorticity impinged by the solar differential rotation.
Using the IMaX instrument on-board the Sunrise stratospheric balloon-telescope we have detected extremely shifted polarization signals around the Fe I 5250.217 {AA} spectral line within granules in the solar photosphere. We interpret the velocities associated with these events as corresponding to supersonic and magnetic upflows. In addition, they are also related to the appearance of opposite polarities and highly inclined magnetic fields. This suggests that they are produced by the reconnection of emerging magnetic loops through granular upflows. The events occupy an average area of 0.046 arcsec$^2$ and last for about 80 seconds, with larger events having longer lifetimes. These supersonic events occur at a rate of $1.3times10^{-5}$ occurrences per second per arcsec$^{2}$.
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