Observation of the energy distribution of thermal electrons in the midlatitude ionosphere

Results of measurements of the energy distribution of thermal electrons below 1 eV in a midlatitude upper atmosphere are presented and compared with some recent measurements at other places. Measurements are based on the Druyvesteyn method using Langmuir probes. In the periods without solar light, distribution does not depart much from Maxwellian above 0.3 eV. Below 0.2 eV, depletion and sometimes double humps are seen. In the periods with solar light, bumps are sometimes observed on the high energy tail at altitudes between 100 and 160 km. Energy distribution in the F layer above 180 km fits the Maxwellian distribution rather well. The reason for the appearance of such non-thermal electrons at lower altitudes may be due to super-elastic collisions with vibrationally excited nitrogens.