Solar flare X-ray burst on September 28,1961

Abstract

The bright chromospheric flare of September 28, 1961, emitted a burst of X rays observed by high-altitude balloon-borne detectors. The burst had a small precursor, a very rapid rise, and a three-component decay. The main part of the burst is therefore interpreted as being made up of three separate pulses. The x- ray burst shows two remarkable associations with the flare's radio noise emission: (1) microwave bursts at the highest radio frequencies coincide precisely in time with the precursor and the main peak; and (2) groups of type III bursts occur at or near the time of discontinuities in the decay of the X-ray burst and thus are associated with the leading edges of the three energeti c electron pulses whose bremsstrahlung constitutes the observed X-ray flux. The appearance of both type III radiation originating in the corona at about 10/sup 5/ km and bremsstrahlung X rays that cannot reasonably be produced above roughly 5000-km height in the chromosphere gives strong evidence that energetic electrons undergo large-scale motions in the solar atmosphere during chromospheric flares. For this particular flare a filamentary structure appears to be rooted near the sunspot where the flare occurs. Although no direct observations exist on thismore » point, the filament no doubt extends up into the lower corona perhaps to heights of 10/sup 5/ km. This leads to the possibility that the electrons are contained and guided by this structure. The photon energy spectrum is found to be nearly constant in form, whereas the X-ray intensity varies greatly. This fact makes it extremely unlikely that the bremsstrahlung originates from a thermal distribution of high-tempernture electrons. A detailed analysis of the spectral time variation shows that the solar electrons must spend most of their time above the 15,000km level. (auth)« less