Solar soft X-rays and solar activity

Abstract

Soft solar X-rays (8 ~< 2 ~< 12 A) were observed from OSO-III. An analysis of the X-ray enhancements associated with 165 solar flares revealed that there is a tendency for a weak soft X-ray enhancement to precede the cm-2 burst and Ha flare. The peak soft X-ray flux follows the cm-2 peak by about 4 rain, on the average. Additionally, it was found that flare-rich active centers tend to produce flares which are stronger X-ray and cm-/. emitters than are flares which take place in flare- poor active centers. The association between optical, radio and X-ray emission from solar flares has been scrutinized recently by several observers. DONNZLI~u (1968) compared the timing of soft X-ray bursts with the onset and maximum of the He flare and with the accom- panying microwave burst, concluding that there is a tendency of the soft X-ray flux to be enhanced prior to the detectable onset of other phenomena. In his study, Donnelly used preliminary data furnished by several experimenters. NEUVERT (1968) compared flux curves at 1.87 A obtained on OSO-III with radio flux-density records (2695 and 2700 MHz) for three flare events. The maximum rate of solar line emission at 1.87 A (Fexxv?) occurred �89 min after the peak microwave emission, while little or no change in flux from lower stages of ionization (Felx-FexvI) took place. The data are consistent with a model in which X-ray emission is produced by ther- realization of the fast electrons which give rise to the impulsive radio burst. The McMath-Hulbert Observatory has on OSO-III a soft X-ray ion chamber. In this communication we propose to describe, for 165 flare events observed by that instrument between March 9 and December 31, 1967, the statistical relations between timing of He, X-ray and cm-2 events accompanying flare phenomena. The relation- ships which are found are discussed in terms of a schematic model for X-ray produc- tion prior to and during flares. 2. Observations Soft solar X-rays between 8 and 12 A~ were observed from the wheel of the OSO-III satellite with an ion chamber having an aluminium foil window and nitrogen gas filling (see AcToN et al., 1963). X-ray fluxes, calculated assuming a black-body energy distribution at T=2 x 106 K, were observed once each rotation of the wheel (1.7-sec period) and measured in discrete steps. Up to E(8, 12) ~- 0.0044 ergs cm -2 sec- 2, the interval between steps is about 0.000 035 ergs cm-2 sec-2. Above E(8, 12)