We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3 4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3 4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called

Black auroras are recognized as spatially well-defined regions within a uniform diffuse auroral background where the optical emission is significantly reduced. Black auroras typically appear post-magnetic midnight and during the substorm recovery phase, but not exclusively so. We report on the first combined multimonochromatic optical imaging, bistatic white-light TV recordings and incoherent scatter radar observations of black aurora by EISCAT of the phenomenon. From the relatively larger reduction in luminosity at 4278 Å than at 8446 Å we show that nonsheared black auroras are most probably not caused by downward directed electrical fields at low altitude. From the observations, we determine this by relating the height and intensity of the black aurora to precipitating particle energy within the surrounding background diffuse aurora. The observations are more consistent with an energy selective loss cone. Hence the mechanism causing black aurora is most probably active in the magnetosphere rather than close to Earth.

Keywords: Atmospheric Composition and Structure: Airglow and aurora, Magnetospheric Physics: Energetic particles: precipitating, Magnetospheric Physics: Electric fields (2411), Ionosphere: Ionosphere/magnetosphere interactions (2736)

The shape of the electron energy distribution has long been a central question in the field of high-frequency radio-induced optical emission experiments. This report presents estimates of the electron energy distribution function, f_e(E), from 0 to 60 eV based on optical multi-wavelength (6300, 5577, 8446, 4278 Å) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron temperature and electron concentration. According to our estimate, the electron energy distribution has a depression at around 2 eV, probably caused by electron excitation of vibrational states in N₂, and a high energy tail that is clearly supra-thermal. The temporal evolution of the emissions indicates that the electron temperature still plays an important role in providing electrons with energies close to 2 eV. At the higher energies the electron energy distribution has a non-thermal tail.

Keywords: electron distribution, artificial airglow, active experiments

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