The effects of large- and small-scale density structures on the radio from coronal streamers

Details The effects of large- and small-scale density structures on the radio from coronal streamers The effects of large- and small-scale density structures on the radio from coronal streamers

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994soph..149...31t&link_type=abstract

Solar Physics (ISSN 0038-0938), vol. 149, no. 1, p. 31-49

Astronomy and Astrophysics

Astronomy

11

Astronomical Maps, Brightness Distribution, Brightness Temperature, Density Distribution, Radio Astronomy, Solar Activity, Solar Corona, Solar Radio Emission, Coronagraphs, Correlation, Image Analysis, Line Of Sight, Polarimeters, Radio Telescopes, Ray Tracing, Stellar Models

Scientific paper

The radio observations of the coronal streamers obtained using Clark Lake radioheliograph at 73.8, 50.0, and 38.5 MHz during a period of minimum activity in September 1986 are presented. Streamers appear to correlate with two prominent disk sources whose intensites fluctuated randomly. The variations in half-power diameter of the radio Sun are found to correspond with the variations in the white-light extents of the coronal streamers. It appears that the shape of the radio Sun is not a function of the phase of the solar cycle; instead it depends on the relative positions of the streamers in the corona. The observed peak brightness temperatures, TB, of the streamers are found to be very low, being approximately equal to 6 x 104 K. We compute the brightness temperature distribution along the equator by tracing the rays in the coronal plasma. The rays are deflected away by the streamers before reaching the critical density level, whereas they penetrate deeper into the coronal hole for small angles between the line of sight and the streamer axis. As a consequence, it is found that the streamers and coronal holes appear in the calculated equatorial brightness distribution as irregular brightness depressions and enhancements, respectively. The fine structures are found to disappear when the scattering due to small-scale density inhomogeneities is included in the ray-tracing calculations. The required relative level of density fluctuations, epsilon1 = (delta N)/N, is found to be greater than 12% to reduce the peak brightness temperature from 106 K to 6 x 104 K for all the three frequencies.

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