Rocket observation of soft energetic particles at the magnetic equator

Results from a rocket‐borne ion mass spectrometer flown near the magnetic equator at 0108 LMT (local mean time) on March 10, 1970, exhibit an unusual background current above 200 km. This current is observed to increase by 3.5 orders of magnitude between 200 and 260 km before maximizing to a fixed value from 260 km to the 295‐km apogee of the flight. Properties of the background combined with laboratory measurements have permitted probable identification of the background source as 2‐ to 20‐keV electrons or protons. Maximum electron fluxes have been estimated to be of the order of 10 10 particles/cm² s sr in accord with Isis 1 satellite measurements at higher altitudes. The background was not observed on an earlier flight at 1938 LMT, suggesting the particles to be quasi‐trapped in a belt that drifted below 300 km between the two flights. The low altitude penetration of these fluxes may have been related to the great magnetic storm of March 8. Simultaneous measurements of the thermal ion distribution are compared with these results and qualitatively suggest that the soft energetic particles are responsible for an observed O 2 + and NO + enhancement.