Flat microwave spectra seen at X-class flares

Details Flat microwave spectra seen at X-class flares Flat microwave spectra seen at X-class flares

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994soph..152..409l&link_type=abstract

Solar Physics, vol. 152, no. 2, p. 409-428

Astronomy and Astrophysics

Astronomy

21

Magnetic Field Configurations, Mathematical Models, Microwave Emission, Microwave Spectra, Radio Astronomy, Radio Spectra, Solar Electrons, Solar Flares, Solar Radio Emission, Solar X-Rays, Spectrum Analysis, Synchrotrons, X Ray Astronomy, X Ray Spectra, Correlation, Goes Satellites, Power Spectra

Scientific paper

We report peculiar spectral activity of four large microwave bursts as obtained from the Solar Arrays at the Owens Valley Radio Observatory during observations of X-class flares on 24 May 1990 and 7, 8, 22 March 1991. Main observational points that we newly uncovered are: (1) flat flux spectra over 1-18 GHz in large amounts of flux ranging from 102 to 104 s.f.u. at the maximum phase, (2) a common evolutionary pattern in which the spectral region of dominant flux shifts from high frequencies at the initial rise to low frequencies at the decaying phase, and (3) unusual time profiles that are impulsive at high frequencies but more extended at lower frequencies. We carry out the model calculations of microwave spectra under assumptions of gyrosynchrotron mechanism and a dipole field configuration to reproduce the observational characteristics. Our results are summarized as follows. First, a flat microwave spectrum reaching up to 102 - 104 s.f.u. may occur in a case where a magnetic loop is extended to an angular size of approximately (0.7-7.0) x 10-7 sterad and contains a huge number (N(E greater than 10 keV) approx. 1036 - 1038) of nonthermal electrons with power-law index approx. 3-3.5 over the entire volume. Second, the observed spectral activity could adequately be accounted for by the shrinking of the region of nonthermal electrons to the loop top and by the softening of the power-law spectrum of electrons in a time scale ranging 3-45 min depending on the event. Third, the extended microwave activity at lower frequencies is probably due to electrons trapped in the loop top where magnetic fields are low. Finally, we clarify the physical distinction between these large, extended microwave bursts and the gradual/post-microwave bursts often seen in weak events, both of which are characterized by long-period activity and broadband spectra.

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