Suprathermal electrons near the lower edge of the radiation belt

Suprathermal electrons of energies between 5keV and 10keV were found to exist in the altitude range 200-800km at geomagnetic latitude 20°N, the L-value ranging from 1.17 to 1.25. The electrons were detected with proportional counters of thin plastic windows on board a sounding rocket L-3H-7 launched at 21:00 Japanese Standard Time on September 3, 1971, when solar and geomagnetic activities were quiescent. The angular distribution of the electrons was peaked in the direction perpendicular to the magnetic line of force at altitudes above 500km, indicating that the electrons are trapped in the geomagnetic field. The altitude dependence is in essential agreement with that expected from the collision loss of electrons in the upper atmosphere. The extrapolation of the measured altitude dependence to the radiation belt gives an electron flux much lower than that observed inside the belt.