![]() However, while there are numerous radio temperature measurements of the quiet-Sun atmosphere, there are very few for sunspots, particularly at millimeter wavelengths, since good spatial resolution is required to isolate the brightness temperature of the sunspot from its surroundings, and single-dish measurements generally do not have enough resolution. The temperature, at which a given frequency is optically thick, is sensitive to density and temperature, and hence the radio data provide important constraints for modeling (e.g., Loukitcheva et al. ![]() The radio data are particularly valuable for solar diagnostics because the measurements are in the Rayleigh-Jeans limit, meaning that measured brightness temperatures actually represent thermal electron temperatures in the optically thick atmosphere. 1986 Obridko & Staude 1988, for a review see Solanki 2003) and, when available, radio measurements of brightness spectra, together with ionization equilibrium and radiative transfer calculations that include heat transfer down from the corona as well as other factors (e.g., Fontenla et al. Deriving such models is a complicated process that tries to balance observations of a range of optical and UV lines (mostly formed in non-local thermodynamical equilibrium (LTE) conditions, see, e.g., Avrett 1981 Maltby et al. There have been numerous attempts to build a comprehensive semiempirical model of the atmosphere above a sunspot umbra. Key words: Sun: chromosphere / Sun: radio radiation / sunspots Better spatial resolution as well as better wavelength coverage are needed for a more complete determination of the chromospheric temperature stratification above sunspot umbrae. A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K). 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.Ĭonclusions. 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 are compared with the umbral contrast calculated from various atmospheric models of sunspots. 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. 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 controversy. Petersburg, Russiaģ School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, KoreaĤ Space Vehicles Directorate, Air Force Research Laboratory, Kirtland AFB, NM, USAĪims. Petersburg University, Universitetskii pr. White 4ġ Max-Planck-Institut for Sonnensystemforschung, 37191 Katlenburg-Lindau, GermanyĮ-mail: Astronomical Institute, St. Astronomical objects: linking to databases. ![]()
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