Some studies on solar microwave bursts in relation to the phases of the associated H?-flares and their spectral nature

Abstract

Occurrences of the flare-associated microwave bursts as well as their peak flux and energy excess spectra have been examined in relation to the pre-and post-maximum phases of the respective flares during the period 1969-72. Results obtained are: (i) about 76 % of the flare-associated bursts occur in the pre-maximum phase and the remaining 24 % occurs in the post-maximum phase irrespective of the flare classification, intensity-wise or area-wise; (ii) 'impulsive' and 'gradual rise and fall' bursts are relatively more important in the pre-maximum phase while 'post burst increase' bursts show comparatively higher occurrences in the post-maximum phase; (iii) peak flux and energy excess spectra of the concurrent microwave bursts in the pre-maximum phase of the flare are mostly of 'inverted U' and 'increasing with frequency' spectral types. Of these, 'impulsive' bursts are predominantly of the 'inverted U' and the 'grf' bursts are of the 'increasing with frequency' spectral type.Solar microwave bursts associated with He-flares are generally believed to occur in the flash phase (Ellison, 1949) or in the explosive phase (Covington and Harvey, 1961 ; Sakurai, 1966;Harvey, 1971) of the flare. However, it appears that the relative occurrences and the nature of these bm sts in relation to the different phases of the life time of the associated flares of different classes are not precisely known. This note presents the results of an analysis in regard to the distribution and spectral nature of the microwave bursts occurring in the pre-maximum and post-maximum phases of the associated flares reported under dual-classification.Group reports of 2340 confirmed and isolated flares (time separation~>2 min) having durations not less than 10 min and reported by at least three observatories were collected from Solar Geophysical Data Bulletins issued by NOAA, U.S.A., for the years 1969-1972. Of these, 1085 flares had associated microwave bursts. Microwave bursts' data at 2800 MHz (Ottawa observatory); at 1415, 2695, 4995, 8800, 15400 MHz (Sagamore Hill and Manila observatories) and those at 1000, 2000, 3750, 9400 MHz (Toyokawa observatory) were also taken from the same source. In this analysis a microwave burst has been considered to be associated with a flare if the former occurred within the actual life time of the flare. The life time of the flare has been subdivided in the present analysis into two phases, viz. (i) pre-maximum phase -the time interval between the starting and the peak of the flare and (ii) post-maximum phase -the time interval between the peak and the end of the flare. The percentage Solar Physics 40 (1975) 411-415. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordrecht-Holland