TIME-DEPENDENT MODELS OF THE AURORAL IONOSPHERE ABOVE EISCAT

Two numerical one-dimensional and lime-dependent models of the topside auroral ionosphere in the altitude range 200-3000 km are presented: t hey are based on two different numerical schemes to solve the eight-mo ment approximation of Boltzmann's equation, namely the Flux Corrected Transport (FCT) and the Method of Lines (ML). The transport equations for densities, velocities, temperatures and heal fluxes are simultaneo usly solved along the magnetic field lines for each constituent of the ionospheric plasma assumed to be composed of electrons and of O+ and H+ ions. These models, using the MSIS-86 neutral atmosphere model, inc lude solar EUV photoionization, chemical and collisional processes bet ween the various charged and neutral species. Steady-state results for both near-summer and winter conditions as well as for diurnal evoluti on are presented and compared to experimental data from the European I ncoherent SCATter (EISCAT) VHF radar. It is shown that independently o f the numerical scheme, the ionospheric structure is very well reprodu ced, given realistic external sources (solar ionization and heating, m agnetospheric energy input). On the basis of the comparisons, the eigh t-moment approximation is validated up to 3000 km altitude. Furthermor e, simulation of a diurnal evolution shows terminator effects on the e nhancement of the downward F2 electron heat flow.