Particle Arrival Directions as Seen from a Cosmic-Ray Instrument in Low Earth Orbit

The French-Danish cosmic ray instrument on HEAO-3 has the ability to measure charge and arrival directions of cosmic rays and their energy above 500 MeV/nucleon. Arrival directions for silicon to nickel nuclei are displayed in a polar dot diagram, which shows particles arriving beyond the earth horizon viewed by the satellite (zenith greater than 112 deg, earth shadow). Results show that at the altitude of the HEAO-3 satellite, arrival directions with zenith angles lower than 30 deg are unambiguously assigned; for larger zenith angles the error rate is typically .10 or less for most of the charge range. Particles with zenith angles beyond the earth horizon of the satellite still reach the instrument (earth shadow). For such directions, the rigidities above the high rigidity limit are forbidden, because they are blocked by the solid earth. The earth shadow also makes it possible to estimate the error rate of the arrival directions assigned by the instrument (0.0010 for the iron group nuclei).