No abstract

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.

No abstract

No abstract

The first successful radio astronomical results of measurements of the magnetic field of solar prominences are presented. The accuracy of polarization and magnetic field observations is 3 . lo-* and 2 G, respectively. The observations were made by the 22-meter radio telescope of the Crimean Astrophysical Observatory at wavelengths of 8 and 13.5 mm. It has been found that the value of the magnetic fields coincides with the optical one (from 7 to 30 G), but the image of radio polarization differs from the intensity. K e y words: prominences -magnetic field -radio measurements AAA subject classification: 073; 077 IntroductionThe term prominence is used to describe an object in the chromosphere or corona that is denser and cooler than its surroundings. Certain prominences are relatively stable structures with lifetimes of many months, some others are transient phenomena that last several hours or less. The most important parameter to study prominences is the magnetic field. These prominences are over neutral lines of the photospheric magnetic field and mark the magnetic field topology in the corona where the magnetic fields are not measured directly. The most scenarios of solar flares to be discussed are related to the prominence-flare coupling. The optical magnetic field measurements based on the Zeeman effect are well known (Tandberg-Hanssen 1974, Kim 1985, Leroy 1989. The errors of magnetic field measurements by the Zeeman magnetographs are found to be of the order 1-10 G for the longitudinal effect and about 50 G for the transversal effect.The next progress comes from another spectroscopic effect, namely the Hanle effect, the modification of coherent scattering in spectral lines, which occurs in the presence of a magnetic field. The polarization observed by the Zeeman effect consists 0.8%, by the Hanle effect 5 2%. The Hanle method gives no way to measure magnetic fields at more than some tens of Gauss because of the total depolarization of lines.The radioastronomical method of magnetic field determination was proposed by Korolkov et al. (1958) and developed by Gelfreikh (1972) and Apushkinskij et al. (1996). In the presence of a magnetic field the optical depth T for thermal bremsstrahlung is different for extraordinary and ordinary radio waves and depends on the gyrofrequency f H : 4-Here (Y is the angle between the magnetic field and ray directions.The brightness temperatures are determined by the formula TB,,, = 7° T e --T e , O dr,,, 0 Thus, T B~ for fi = f + f H I cosal is equal TB. for fi = f -f H I cosal.