Flickering aurora is characterized by optical emissions varying in intensity with frequencies typically between 5 and 15 Hz. In this report we use high-speed narrow field-of-view imaging in white light to determine the intensity variation and the apparent motion of the flickering spots. The characteristic patterns we find are compared with the spatial patterns derived from interference between electromagnetic ion-cyclotron waves. It is found that good agreement is reached, which we argue should make a strong rejection criterion for theories of flickering aurora.

We present bi-static observations of radio-wave induced optical emissions at 6300 and 5577 Å from a night-time radio-induced optical emission ionospheric pumping experiment at the HIPAS (Fairbanks) facility in Alaska. The optical observations were made at HIPAS and from HAARP located 285 km south-east. From these observations the altitude distribution of the emissions is estimated with tomography-like methods. These estimates are compared with theoretical models. Other diagnostics used to support the theoretical calculations include the new Poker Flat AMISR UHF radar near HIPAS. We find that the altitude distribution of the emissions agree with electron transport modeling with a source of accelerated electrons located 20 km below the upper-hybrid altitude.

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 N₂. 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.

This report presents Monte Carlo simulations of the electron energy distribution for a low ionized plasma interacting with the F-region neutral gas. The results show a depletion in the electron distribution above 2 eV between 10 and 80 %, decreasing with altitude. The depletion is mainly due to electron energy loss to N₂. This micro-physical energy transfer model gives good agreement with optical observations of enhanced emissions from O(¹D) at 6300 Å and EISCAT UHF measurements of electron cooling during HF radio wave heating experiments. Some implications for incoherent scatter spectra are derived. The results suggest that a weak (approximately 1000 times weaker than the ion-line) and wide (2 MHz) peak around 1 MHz from the ion-line in the EISCAT VHF incoherent scatter spectrum should be a consequence of the electron-neutral interaction.

Keywords: Electron distribution, Incoherent scatter spectra, active experiments

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