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Multifrequency observations of a remarkable solar radio burst
White, S. M.; Kundu, M. R.; Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Kucera, T.; Bieging, J. H.
AA(Maryland, University, College Park), AB(Maryland, University, College Park), AC(National Radio Astronomy Observatory, Socorro, NM), AD(California Institute of Technology, Pasadena), AE(California Institute of Technology, Pasadena), AF(Colorado, University, Boulder), AG(California, University, Berkeley; Steward Observatory, Tucson, AZ)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 384, Jan. 10, 1992, p. 656-664. Research supported by NOAA. (ApJ Homepage)
Publication Date:
Solar Physics
NASA/STI Keywords:
Cyclotron Radiation, Radio Spectra, Solar Flares, Solar Radio Bursts, Synchrotron Radiation, Bremsstrahlung, Radio Astronomy, Radio Interferometers, Solar Spectra, Very Large Array (Vla)
Bibliographic Code:


Observations of an impulsive solar-radio burst from three observatories are presented. The striking observational aspects of this flare are that the time profile was identical throughout at 8.6, 15, and 86 GHz, that the spectrum was apparently flat from 15 to 86 GHz, and that there was a sharp cutoff in the spectrum between 5.0 and 8.6 GHz. The simplest interpretation of the cutoff, namely as a plasma frequency effect, leads to the conclusion that there was exceptionally high-density material in the solar corona (of about 5 x 10 exp 11/cu cm). Very Large Array images at 15 GHz show a single-loop structure which brightened uniformly and showed little change in size during the whole impulsive phase. The flat spectrum is consistent with optically thin thermal bremsstrahlung emission, but the lack of observed soft X-ray emission and other properties of the flare cannot easily be accommodated by this mechanism. The possibility is explored that the emission is optically thick due to thermal absorption of nonthermal gyrosynchrotron emission, or optically thin gyrosynchrotron emission absorbed by high-density material intervening along the line of sight. Both of these explanations also face difficulties.

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