Low‐altitude trapped protons at the geomagnetic equator

Geomagnetically trapped protons in the 0.6‐ to 9‐MeV energy range were measured at latitudes near the geomagnetic equator by the Phoenix 1 experiment on board the S81‐1 mission from May to November 1982. The protons show a distribution in latitude along the line of minimum magnetic field strength with a full width at half maximum of ∼10° but with no appreciable longitudinal variation. Between 170 and 290 km the peak proton flux shows a fifth‐power altitude dependence, in contrast to previous measurements at higher altitudes, possibly demonstrating source attenuation. The efficiency of the telescope is calculated as a function of particle pitch angle and used to investigate the time dependence (1969‐1982) of the intensity.