HF radio wave acceleration of ionospheric electrons: Analysis of HF‐induced optical enhancements

The shape of the HF‐pump modified electron energy distribution has long been a central question in the field of ionospheric high‐frequency radio wave modification experiments. Here we present estimates of the enhanced differential electron flux, from 1.9 to 100 eV based on optical multiwavelength (6300, 5577, 8446, and 4278 Å) data and 930 MHz incoherent scatter radar measurements of ion temperature, electron temperature and concentration. According to our estimate, the electron energy distribution is depleted in the energy range of 2 to approximately 4 eV, probably caused by electron excitation of vibrational states in N 2 . At the higher energies the electron energy distribution has a nonthermal tail. Further we show that the altitude variations of the four optical emissions should differ both in altitude coverage and center altitude.