HARD X-RAY AND GAMMA-RAY OBSERVATIONS OF SOLAR-FLARES WITH THE PHEBUS EXPERIMENT

The Phebus experiment on board the GRANAT satellite provides temporal and spectral observations of solar and cosmic gamma-ray bursts in the 0.1 congruent-to 100 MeV nominal energy range. The experiment was turned on January 8, 1990 and is still in operation. In this paper we present the main characteristics of the Phebus experiment and we describe and discuss some of the observational properties of the 18 solar hard X-ray/gamma-ray events detected during the first semester of the Phebus operation. It is found that: (i) events of a few minutes duration, detected above approximately 100 keV, systematically show subsecond time variations; (ii) longer duration events (>5 min) do not exhibit fast time variations and generally consist of 1-min peaks superimposed on a less intense, sometimes harder, slowly varying component. In addition to these general trends we discuss in more detail three events for which significant count-rates have been detected above 10 MeV