Observation of Energetic Particles at Very Low Altitudes near the Geomagnetic Equator

A considerable flux on the order of 7 particles/${\mathrm{cm}}^{2}$ sec sr (probably protons) in the energy range above 500 keV is observed in the vicinity of the magnetic equator. The particles show a narrow distribution around 90\ifmmode^\circ\else\textdegree\fi{} pitch angle. The low minimum altitudes of the drift paths of the particles in the $L$ region between 1.0 and 1.15 prevent a full drift around Earth. The particles are probably injected at several hundred kilometers altitude by a yet unknown mechanism.