Fluxes of relativistic electrons in low-Earth orbit during the decline of solar cycle 22

Highly relativistic electron events (HREs) contain some of the longest lasting and most energetically intense electron fluxes measured in low-Earth orbit. Their short-term effects on astronauts and space-based electronic systems as well as their long-term influence on the terrestrial climate must be evaluated using measured electron fluxes spanning the ranges of solar inputs. We have used measurements from the High Energy Particle Spectrometer (HEPS) on Upper Atmosphere Research Satellite (UARS) to develop a database of energy- and pitch-angle-resolved electron fluxes. The HEPS data were acquired during the late part of the maximum and the declining phase of solar cycle 22, the latter phase being when HREs are thought to peak in frequency and intensity. We show average differential energy spectra and the limits of variation as a function of energy. The HEPS data shows that the flux of electrons with energies around 1 MeV lasts longer than does the flux of lower energy, but still relativistic, electrons. The data are used to estimate the probability of encountering a selected flux of electrons during similar phases of the solar cycle.