Fine-scale spatial structure in the pitch angle distributions of energetic particles near the midnight trapping boundary

Near the trapping boundary at local midnight the ratio of precipitating to trapped fluxes of electrons (0.16--2.4 MeV) and of protons (0.04--0.95 MeV) observed at low satellite altitudes (approx.750 km) is a strong function of both particle energy and L value. On a particular L shell, isotropy is often observed for particles above a given energy, whereas below that energy the precipitating fluxes are much lower than the trapped fluxes. The onset of isotropy typically occurs at a sharply defined energy threshold which decreases rapidly with increasing L value. The occurrence of isotropy for electrons and protons together show a consistent variation with L when they are consideration in terms of magnetic rigidity. For the onset of isotropy the equivalent gyroradius at the equator of particles with the same energy and a 900 pitch angle spans the range approx.5 km to approx.500 km and varies approximately as (L: L0: 0.1)/sup 1.8/, where L0 is the L shell at which isotropy occurs for electrons with an equatorial gyroradius of 10 km. The observed magnetic rigidity thresholds for isotropy may provide a measure of the effective scale length of the inhomogeneities of the geomagnetic field