Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.446..185b&link_type=abstract
8th SOHO Workshop: Plasma Dynamics and Diagnostics in the Solar Transition Region and Corona. Proceedings of the Conference hel
Astronomy and Astrophysics
Astrophysics
1
Scientific paper
The multiwavelength radio observations of the coronal holes (CH) have been carried out on the RATAN-600 radiotelescope at microwaves during the last minimum of solar activity (1995-1996). These data are compared with those obtained by the same method on RATAN-600 during the previous period of low solar activity in 1984-1985. The southern sector of the RATAN-600 radio telescope in combination with a plane periscopic mirror was used. The total intensity and circularly polarized component of the solar radio emission have been recorded daily simultaneously at 30-40 wavelengths in the range of 2-30 cm using the Panoramic Analyser of Spectra (Bogod, V.M., et al., 1992, Proceedings of IAU Colloquim N.141, p.79). The radio telescope provided resolution of 17" x 13' at 2.0 cm. We considered 9 equatorial isolated coronal holes which were observed during several solar rotations (according to He I 10830 AA data). 1. The contrasts of brightness temperature of CH against the quiet Sun in 1995-96 are turned to be lower than those in the previous cycle. But the absolute values of brightness temperature of CH and their spectra in the range of 2-30 cm are the same for two periods. 2. The coronal holes in 1995-1996 had the different dependence of their effective radio sizes on the wavelength in the range of 8-30 cm. The most coronal holes at previous activity minimum showed the increase of the radio size with wavelength increasing in the range of 8-32 cm. At the last minimum two coronal holes had the constant radio sizes in the range of 8-30 cm, and one CH showed the decrease of the radio size with wavelength increasing. The different relations between sizes of EUV, SXR and microwave images of coronal holes has been found also. 3. The coincidence of calculated and observed limb effects detected with RATAN-600 during the rise of one coronal hole (March, 1996) confirmed the validity of the empirical homogeneous models of CH and quiet Sun atmospheres constructed on the base of RATAN-600 data obtained in the previous minimum. 4. At first time the circular polarized emission associated with the isolated equatorial coronal hole (CMPs - 13.10.96, 9.11.96) has been registered in the range of 9-30 cm. The degree of polarisation was 0.2-3%. In the framework of bremsstrahlung mechanism of radio emission it means that the magnetic field strength (longitudinal component) in CH is of 2-10 G at the level of upper chromosphere - low corona (Borovik, V.N., Medar,V.G., Korzhavin A.N., 1999. Pisma v Astron.Zh.,25, N 1, 1-9, in russ.) 5. The examples of the different effects of the presence of neighbouring active regions on the radio characteristics of CH are given. 6. The small scale structures at microwaves in the quiet Sun and in coronal holes are considered. We thank the Yohkoh/SXT Team and the SOHO/EIT
Borovik Valeria N.
Medar' V. G.
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