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Bogod V.M.,Russian Academy of Sciences | Peterova N.G.,Russian Academy of Sciences | Ryabov B.I.,Ventspils International Radio Astronomy Center | Topchilo N.A.,Saint Petersburg State University
Cosmic Research

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. © 2015, Pleiades Publishing, Ltd. Source

Ryabov B.I.,Ventspils International Radio Astronomy Center | Gary D.E.,New Jersey Institute of Technology | Peterova N.G.,Russian Academy of Sciences | Shibasaki K.,Nobeyama Solar Radio Observatory | Topchilo N.A.,Saint Petersburg State University
Solar Physics

We analysed specific regions of reduced soft X-ray and microwave emission in five large isolated sunspots. The Nobeyama Radioheliograph 17 GHz observations reveal a local depression of microwave brightness in the peripheral area of the sunspots. The depression regions appear light (weak absorption) in the He 10830 Å line in areas with extended (open) field lines, as indicated by potential field source surface model (PFSS) extrapolations up to 1.5 R⊙. The observed depressions of 3 – 8 % in ordinary mode at 17 GHz are interpreted as resulting from free–free emission when the plasma density is lower by 5 – 10 %. Our model estimates show that the decrease in density in both the coronal and the lower layers above the depression region accounts for the depression. These depression regions lend themselves well to marking the location of outward plasma motions. © 2014, Springer Science+Business Media Dordrecht. Source

Bezrukov D.,Ventspils International Radio Astronomy Center
Baltic Astronomy

The article describes microwave observations of the Sun with the radio telescope RT-32 of the Ventspils International Radio Astronomy Center. The observations were performed using a multichannel spectral Polarimeter for the 6.3-9.4 GHz frequency range. A set of 2D microwave emission maps of the Sun for the Stokes I and V parameters has been obtained and analyzed. Source

Bezrukovs D.,Ventspils International Radio Astronomy Center
Baltic Astronomy

A brief historical review of the development of solar physics and solar radio astronomy in Latvia and its current status is presented. Source

Bezrukov D.A.,Ventspils International Radio Astronomy Center | Ryabov B.I.,Ventspils International Radio Astronomy Center | Shibasaki K.,Nobeyama Radio Observatory
Baltic Astronomy

On the base of the 17 GHz radio maps of the Sun taken with the Nobeyama Radio Heliograph we estimate plasma parameters in the specific region of the sunspot atmosphere in the active region AR 11312. This region of the sunspot atmosphere is characterized by the depletion in coronal emission (soft X-ray and EUV lines) and the reduced absorption in the a chromospheric line (HeI 1.083 μm). In the ordinary normal mode of 17 GHz emission the corresponding dark patch has the largest visibility near the central solar meridian. We infer that the reduced coronal plasma density of about 5 × 108 cm -3 is the characteristic feature of the dark patch. Source

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