Other
Scientific paper
Oct 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........16r&link_type=abstract
PhD Thesis, Albert-Ludwigs Univeristy, Freiburg, Germany
Other
1
Sun, Chromosphere, Photosphere, Spectropolarimetry, Magnetic Field
Scientific paper
The solar surface outside sunspots and active regions, i.e., the quiet Sun, shows the ubiquitous pattern of granulation in the photosphere. The quiet solar photosphere harbors small-scale magnetic structures inside and between granules. This thesis presents thermodynamic properties of the small-scale magnetic flux concentrations in the quiet Sun using high spatial and temporal resolution observations along with numerical simulations. Spectral line profiles of the Fe I 630 nm pair and Ca II H were used to trace the photospheric and chromospheric layers of the magnetic elements.
In the presence of magnetic field spectral lines split and are polarized via the Zeeman effect. The difference of a spectral line profile, measured in left and right circular polarized light, is a Stokes-V profile with two lobes. In the absence of any gradients of velocity or magnetic field along the line of sight, Stokes-V profiles are anti-symmetric. The different area of the two lobes, the Stokes-V area asymmetry, provides information about the gradients of the magnetic and velocity fields along the line of sight.
Comparing high resolution spectropolarimetric data with synthetic maps of a 3D MHD simulation, we found several magnetic elements in the photosphere showing a central region of negative Stokes-V area asymmetry surrounded by a peripheral region with larger positive asymmetry. This finding was the first observational confirmation of the existence of a sharp boundary layer between magnetic elements and their immediate surroundings. Such boundary layers had been theoretically predicted ten years go. Furthermore, we found for the first time two Stokes-V profiles of the Fe I 630 nm line pair in a single spectrum showing opposite magnetic polarities. These two lines form in slightly different layers, so they trace the magnetic field in different geometrical heights.
The temporal evolution of these profiles showed a magnetic flux cancellation, suggesting a magnetic reconnection in the photosphere. A 1D numerical model that reproduced the observed profiles was interpreted as an indication for the magnetic reconnection. To verify the existence of vigorous gradients in the magnetic and thermal properties of the atmosphere as suggested by the cancellation event, extreme cases of asymmetry in Stokes-V profiles, i.e., profiles with only one lobe instead of two, were found in a large sample of data. We find strong evidence for concentrated magnetic flux structures with sharp boundaries sustaining a strong gradient or a discontinuity in thermodynamic parameters. In other words, we find current sheets in boundary layers which separate magnetic from non-magnetic plasma. This supports the existence of structured magnetic entities like flux tubes.
The second part of this thesis was devoted to study the thermal structure of the chromosphere and its relation to underlying photospheric magnetic flux concentrations. The Ca II H line is one of the strongest absorption lines in the visible solar spectrum. Generally, there are two emission peaks (H2v and H2r) on either side of the line core which form in the chromosphere. The existence of the emission peaks implies a temperature rise in the chromosphere. Parts of the line outside these two emission peaks mainly form in the photosphere. The integrated intensity in a 0.1 nm band around the core which contains the two emission peaks is a measure for the chromospheric emission. The minimum chromospheric emission is as large as half of the average emission. We found that the chromospheric emission in excess to the minimum emission scales with the magnetic flux density. To establish a relation between the chromospheric emission and temperature, we looked for the Ca II H line profiles without any emission peak at H2v and H2r wavelengths. A quarter of the observed calcium profiles did not show any emission peak. A comparison of these profiles with synthesized calcium profiles of one-dimensional time-independent models provides strong indication for chromospheric temperatures cooler than that of the spatially and temporally averaged quiet Sun. Our results suggest that a large fraction of the solar chromosphere consists of hot plasma, T > 10000 K. Beside this, there is a smaller fraction which is cooler than the underlying photosphere, T < 5000 K, but cannot be much cooler than, e.g., 2000 K.
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