Flat microwave spectra seen at X-class flares
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 1990 May 24 and 1991 March 7, 8, and 22. 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. In an attempt to elucidate these new properties, 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 ∼(0.7–7.0) × 10⁻⁷ sterad and contains a huge number (N(E > 10 keV) ∼ 10³⁶– 10³⁸) of nonthermal electrons with power-law index δ ∼ 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.
© 1994 Kluwer Academic Publishers. Provided by the NASA Astrophysics Data System. Received 27 October, 1993; in revised form 28 February, 1994. We thank the referee, Dr R. Ramaty, for helpful comments, particularly on the presence of ultrarelativistic electrons and on the use of simplified expressions for gyrosynchrotron radiation. We also thank Dr L. G. Kocharov for suggesting the possible two-loop system in the 1990 May 24 flare. We wish to thank Dr A. Krüger and Dr N. Vilmer for helpful discussions. We thank Dr C. Barat and Dr O. Terekhov for the information of Phebus data and Ms Kim Tolbert for providing the GOES data. Observation at OVRO was funded by NSF grants ATM-9013173 and AST-8919770 to the California Institute of Technology. One of us (JWL) has been supported by NASA grant NAGW-3005 and NAGW-1972 during preparation of this paper.
Published - 1994SoPh__152__409L.pdf