An upper limit to the anisotropy of solar hard X-ray emission

A statistical study of center-to-limb variations in the frequency of occurrence of solar hard X-ray bursts is used to look for directivity in the emission. The X-ray data consist of 148 bursts observed by the University of California, San Diego, solar X-ray instrument on the OSO-7 satellite. No center-to-limb variation in hard X-ray burst occurrence was found. The upper limit on any limb darkening or limb brightening of 20 keV X-ray emission is 40% at the 95% confidence level. This result rules out downward-streaming thick-target models for the motion of the X-ray emitting electrons. Furthermore, strong east-west streaming is also ruled out when the observed center-to-limb spectral variation is combined with the directivity result. Either the motions of the electrons which emit hard X-rays are predominantly random, or a variety of different emitting-region geometries commonly occur.