A number of solar X-ray events above 10 keV and 20 keV were compiled in order to test for evidence of anisotropic emission. The results are not definite, although the two samples show apparently different behaviours.
Among the mechanism of X-ray emission from solar bursts and of related phenomena, like for instance Compton X-ray scattering, some are in principle apt to introduce a dependence of intensity on the direction of observation. This in turn could be evidenced by a centre-to-limb modulation on the number of events detected at different heliographic longitudes (HL).
It was once pointed out by Ohki (1969) that the frequency of detection of hard X-ray (hv~ 10 keV) solar bursts appeared to decrease from the centre to the limb of the solar disk. On the other hand, a maximum of detection frequency around 40-50 deg HL was claimed by Pint6r (1969), closing in towards 20-30 deg at 1-10 keV and near the centre at 0.6-1.5 keV. This pattern seemed in agreement with a Takakura-Kai flare model.
We attempt here to reconsider the matter with a larger compilation of events above 10 keV, and a sample above 20 keV. In the > 10 keV band, the OGO-1 and OGO-3 Atlases of X-ray bursts (Arnoldy et al., 1968;Kane and Winckler, 1969) were examined, covering the period from 5 Sept. 1964 to 31 Dec. 1967. Fourteen events from OGO-5 were collected among those reported by Kane (1969) and by Kane and Anderson (1970). In the ~>20 keV band, the events were selected from the list of 'pre-OGO' measurements reported by Arnoldy et al. (1968), from the list of OSO-3 events which McKenzie (1971) labelled 'Channel 4' (~>20 keV), and from the events given by Brini et aI. (1973) from the Bologna experiment on OSO-6.
A comprehensive list of all the events considered is given in Tables I andII. The heliographic locations are based as usual on associations with Ha flares reported in Solar-Geophysical Data, subject to the condition that no other flares be reported with end time later than, or start time earlier than the time of X-ray maximum. Exceptions were made when the overlapping flares occurred at approximately the same HL, whereby no serious ambiguity should arise within the context of this procedure. The X-ray time of maximum for the OGO-1 and -3 events was deduced with an accuracy of typically one minute from the profile time scales on the Atlases.
The events were distributed in 10 deg bins of HL by splitting in two halves the contributions of those falling at exact multiples of 10 deg, except for including all events at 90 deg in the 80-90 deg bin (Figure 1).