A Monte Carlo technique has been used to predict the relative visibility of solar hard X-ray flares as a function of solar longitude assuming the model of Takakura and Kai to be realistic. Comparison is made with previous statistical studies of observations. A discernable longitudinal variation in the relative visibility of flares is shown to be expected but the probability of flares being visible towards the limb is shown to be higher than had previously been evident.
The effect of the possible downward inclination of the particle beam with respect to the solar surface is considered.
It is now generally assumed that origin of the high energy (> 10 keV) X-ray emission in many solar flares is due to some non-thermal mechanism. This mechanism is often thought to be electron-proton bremsstrahlung caused when a directed beam of high energy particles interacts with the ambient coronal plasma, for example, in the way suggested by the model of Takakura and Kai (1966).
As Elwert and Haug (1970) have pointed out, the emission from such a process should exhibit both polarization and directivity. Tindo et al. (1970) have detected the existence of polarization in certain flares around 15 keV and it would be of interest to learn if the directivity exists too. Elwert and Haug (1971) have suggested that absolute measurements of directivity could be made by the simultaneous use of two or more satellite experiments at widely differing heliographic positions. Until such measurements are made, the only way in which information about directivity can be obtained seems to be by means of inference from statistical studies of the visibility of high energy flares as a function of their solar longitude. Two such studies have been made; namely those by Pint6r (1969) and by Ohki (1969). Results on the distribution of soft (<6 keV) X-ray flares have also been published by Drake (1971) and by Culhane and Philips (1970), however since the emission in this region is thought to be predominantly quasi-thermal in origin, no directivity is to be expected.
Both Pint6r (1969) and Ohki (1969) used observations of 46 flares to determine the longitudinal distribution of the flares. Their results have already been discussed by Elwert and Haug (1971) but are worth considering once more. Pint6r found peaks to occur between 40 ~ and 50 ~ both E and W, whilst Ohki found a monotonic centre to limb decrease. Pint6r's interpretation of his results is that they are evidence for the directivity to be expected from a mechanism where the preferred direction of the electrons is parallel to the solar surface. Such an interpretation does not necessarily follow, and * Present address; MBS,