Velocity distribution of ionospheric low-energy electrons

The velocity distribution function of ionospheric electrons is calculated in the low-energy (< 8·7 eV) region. This is the first realistic model calculation for studying the transition of the distribution function from the nonthermal to the thermal part without assuming the Maxwellian distribution for the latter. The calculation is made for four altitudes between 120 and 250 km. All the relevant atomic collision processes are taken into account. Contributions of each individual process to determining the distribution function are discussed. The calculation shows that the deviation of the thermal energy part of the distribution function from Maxwellian in the region around the mean kinetic energy is within several percent. A much larger deviation is found at other portions of the function (still within the thermal part) especially at lower altitudes. In the Appendix, some features of the distribution function at higher energy regions are discussed.