The Low Energy Proton Experiment on ISEE-C

There are still many outstanding problems concerning the origin and propagation of sub-MeV protons in the heliosphere. Proton fluxes from solar flares are modulated by coronal propagation, as well as by large and small scale structures in the solar wind. Acceleration processes in interplanetary space, as well as at the sun, complicate the picture, as does the presence, in interplanetary space close to the earth of protons of magnetospheric origin. Experiment DFH was designed to investigate the problems of low energy proton behavior in the energy range 35-1600 keV. The orbit of ISEE-C is free from magnetospheric effects but permits a high data rate and DFH provides three-dimensional proton flux distribution measurements with good energy resolution at a time resolution of 16 s. Thus it provides 180 independent points on the proton phase space distribution function every 16 s. The instrument was designed and built by the Space Research Laboratory, Utrecht, The Netherlands, the Space Science Department of ESA, and the Cosmic Rays and Space Physics Group of Imperial College, London, England. Section I of this paper reviews briefly the scientific objectives of the experiment. Details of the instrument are given in Section II.