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Flat microwave spectra seen at X-class flaresWe report peculiar spectral activity of four large microwave bursts as obtained from the Solar Arrays at the Owens Valey 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 10(exp 2) to 10(exp 4) 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 10(exp 2) - 10(exp 4) 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(exp -7) sterad and contains a huge number (N(E greater than 10 keV) approx. 10(exp 36) - 10(exp 38)) 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.
Document ID
19950033299
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Lee, Jeongwoo W.
(California Inst. of Tech., Pasadena, CA United States)
Gary, Dale E.
(California Inst. of Tech., Pasadena, CA United States)
Zirin, H.
(California Inst. of Tech., Pasadena, CA United States)
Date Acquired
August 16, 2013
Publication Date
July 1, 1994
Publication Information
Publication: Solar Physics
Volume: 152
Issue: 2
ISSN: 0038-0938
Subject Category
Solar Physics
Accession Number
95A64898
Funding Number(s)
CONTRACT_GRANT: NSF ATM-90-13173
CONTRACT_GRANT: NSF ATM-89-19770
CONTRACT_GRANT: NAGW-3005
CONTRACT_GRANT: NAGW-1972
Distribution Limits
Public
Copyright
Other

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