Search for high energy gamma rays from the sun

An attempt to measure the flux of high energy solar gamma rays was made by comparing the angular distribution of the intensity from the direction of the sun and that of the intensity from the symmetrical direction about the zenith using two stacks of oriented nuclear emulsions flown by balloon on May 16, 1970, from Sioux Falls, South Dakota. An upper limit to the flux of solar gamma rays was estimated to be 4.3 × 10 −4 photons cm −2 s −1 in the energy region above 10 MeV. On the day of the flight the sun was moderately active and the relative sun spot number was 161. Three importance-2 flares (2N, 2N, and 2B) and five importance-1 flares were reported during the period of the balloon flight.