Volume 556, August 2013
|Number of page(s)||15|
|Published online||06 August 2013|
Physical properties of a sunspot chromosphere with umbral flashes⋆
1 Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
2 Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
3 Instituto de Astrofísica de Canarias, Avda vía Láctea S/N, 38205 La Laguna, Tenerife, Spain
4 Max-Planck Institute for Solar System Research, Max-Planck Straße 1, 37412 Katlenburg-Lindau, Germany
Received: 2 April 2013
Accepted: 21 June 2013
We present new high-resolution spectro-polarimetric Ca iiλ8542 observations of umbral flashes in sunspots. At nearly , 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.
Key words: Sun: chromosphere / sunspots / Sun: magnetic topology / radiative transfer / polarization / Sun: infrared
Movie and Appendices A and B are available in electronic form at http://www.aanda.org
© ESO, 2013
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