''Non-Maxwellian'' studies in the auroral F region: A new analysis of incoherent scatter spectra.

When large electric fields occur in the auroral F region, the ion-velocity distribution-function becomes non-Maxwellian. Therefore the incoherent spectra are distorted from their normal shape. Up to now, analyses of the distorted spectra have been done with Raman's model of the distribution function (Winser el al., 1989 Geophys. Res., Lett. 94, 1439-1449; Lockwood and Winser, 1988 Planet. Space Sci. 36, 1295-1384; Suvanto et al., 1989 J. atmos. terr. Phys. 51, 483-495). We have chosen here to use the distribution function calculation based on the polynomial series expansion approximation of Hubert (1983). This needs a model of ion-neutral interactions: the model B of St Maurice and Schunk (1977) Planet. Space Sci. 25, 243-260 is used. This new analysis procedure works with a mixture of ions (O+ and NO+ ions) in a neutral atmosphere composed of O, N-2 and O-2 (Hubert and Kinzelin, 1992 J, geophys. Res. 97, 4053-4059). Simulations using noise perturbed correlation functions have been performed to test our new non-Maxwellian analysis. Results are presented that include three and four-parameter regressions. These parameters have been selected from electron density, neutral and electron temperatures, ion composition, ion-neutral differential velocity and neutral composition. These simulations allows us to propose a strategy for the analysis of real data. We have studied the well-known 27 August 1986 EISCAT CP-3-E experiment that shows a large electric field (100 mV m(-1)) for the scan between 13h00 and 13h30 UT (Winser et al., 1987 Geophys. Res., Lett. 14, 957-960). Assuming Tn, we are able to deduce ionospheric plasma parameters including the ion composition, which is mainly molecular around 275 km altitude.