On the recording of an emission with a reduced brightness in the region of a strong sunspot magnetic field

Abstract

Observations are reviewed of active regions where radio emission depressions in radio sources above large sunspots is observed. The depression value can be significant and can reach 2000–4000 K relative to the temperature around a quiet Sun. However, the number of cases of strong depression is small, which is apparently related to the specific features and conditions of sunspot observations and the limited or restrict telescope possibilities of the present day. Usage of the RATAN-600 radio telescope with a high spectral resolution (1%) made it possible to establish that this phenomenon is observed in a limited wavelength range (1.7–3.0) cm. Owing to the special method of RATAN-600 polarization measurements, it has been indicated that the emission depression effect takes place in the ordinary o-mode emission, whereas the emission source above a sunspot is always brighter than the background in the extraordinary e-mode. Two new active regions where the depression phenomenon was registered have been considered, and a comparison with the data from the NoRH radioheliograph, SSRT, and spacecraft has been performed. The values of the magnetic fields above the sunspots at which the region of ordinary and extraordinary wave generation penetrates in the coronal temperature region have been measured. A depression phenomenon modeling, indicating that electron density decreases and the situation is similar to coronal holes, has been performed based on the set of observational data. Possible directions in the study of this phenomenon are discussed.