Aims: The aim of this paper is to demonstrate that millimeter wave data can be used to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some contr
oversy. Methods: We use observations of the temperature contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm observations from Lindsey & Kopp (1995) and 2 cm observations with the Very Large Array. These are compared with the umbral contrast calculated from various atmospheric models of sunspots. Results: Current mm and submm observational data suggest that the brightness observed at these wavelengths is low compared to the most widely used sunspot models. These data impose strong constraints on the temperature and density stratifications of the sunspot umbral atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region. Conclusions: A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K). Better spatial resolution as well as better wavelength coverage are needed for a more complete determination of the chromospheric temperature stratification above sunspot umbrae.
Aims. We analyze observational data from 4 instruments to study the correlations between chromospheric emission, spanning the heights from the temperature minimum region to the middle chromosphere, and photospheric magnetic field. Methods: The data c
onsist of radio images at 3.5 mm from the Berkeley-Illinois-Maryland Array (BIMA), UV images at 1600 A from TRACE, Ca II K-line filtergrams from BBSO, and MDI/SOHO longitudinal photospheric magnetograms. For the first time interferometric millimeter data with the highest currently available resolution are included in such an analysis. We determine various parameters of the intensity maps and correlate the intensities with each other and with the magnetic field. Results: The chromospheric diagnostics studied here show a pronounced similarity in their brightness structures and map out the underlying photospheric magnetic field relatively well. We find a power law to be a good representation of the relationship between photospheric magnetic field and emission from chromospheric diagnostics at all wavelengths. The dependence of chromospheric brightness on magnetic field is found to be different for network and internetwork regions.
