Interferometric Studies of the Sun at Microwave and Millimeter Wavelengths

Details Interferometric Studies of the Sun at Microwave and Millimeter Wavelengths Interferometric Studies of the Sun at Microwave and Millimeter Wavelengths

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........25b&link_type=abstract

Thesis (PH.D.)--UNIVERSITY OF COLORADO AT BOULDER, 1995.Source: Dissertation Abstracts International, Volume: 57-02, Section: B,

Computer Science

4

Solar Radio Emissions

Scientific paper

The work presented in this thesis relied mainly upon two radio interferometers, the Owens Valley Solar Array and the Owens Valley Millimeter Array. I used the Owens Valley Millimeter Array during the eclipse of July 11, 1991 to observe the brightness temperature profile of the solar limb. These observations showed that photospheric -temperature gas exists at an altitude of about 5500 km above the visible photosphere, far beyond the expected location of the transition region where the temperature increases to about one million degrees. I proposed that this result can be explained in terms of a two-component model of the upper chromosphere. Spicules would compose a relatively cool and dense component embedded in a hot, tenuous medium like that described by the so-called VAL model (Vernazza, Avrett, and Loeser 1981). Later I used data from the Solar Array to make a detailed investigation of the gyrosynchrotron spectrum of the flare of July 16 1992. The results of this investigation led me to propose a solution to a long-standing problem in solar microwave bursts, that of the constant peak frequency of the bursts as they evolve in brightness temperature, and the steep slope on the low-frequency side of the spectrum. I proposed that the Razin effect is at work, and I developed the theory of Razin suppression for solar microwave burst conditions. The Razin effect is the suppression of radiation from an electron in a medium in which the index of refraction is less than unity. I demonstrated that in a medium with density 2 times 10^ {11}cm^{-3} and magnetic field 300 Gauss, conditions not uncommon for solar microwave bursts, the gyrosynchroton spectrum can be suppressed for frequencies up to at least 10 GHz. Finally, I made use of the X-ray data from the Yohkoh spacecraft to learn more about this particular flare, and to check some of the results from the investigation of the radio spectrum. I found that the ambient density and electron spectral index inferred from the X-ray data matched those deduced from the microwave data.

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