HYPERBOLICITY ANALYSIS OF MULTIMOMENT PLASMA MODELS - APPLICATION TO AURORAL PLASMA OUTFLOWS ALONG MAGNETIC-FIELD

As Evidence of large field-aligned ion outflows from the high latitude ionosphere has been accumulated both above the polar cusp and auroral regions, numerous models were developed in the recent years to invest igate the transport of plasma along magnetic field lines between the i onosphere and the magnetosphere. The description of such large scale d ynamic phenomena usually requires the implementation of fluid type num erical models, although the basic assumptions for such fluid models ar e not appropriate at high altitudes in the collisionless magnetosphere . One-dimensional modelling was based on multifluid and multimoment fl uid models with higher order moments involved to account for the multi ple transitions occurring with altitude and particularly, the transiti on from collision dominated to collisionless plasma. In such models, t he plasma dynamics if described by an appropriate set of transport equ ations involving density number, bulk velocity, temperatures and heat fluxes. The stability of the solutions of linearized systems of such o ne-dimensional transport equations requires the hyperbolicity conditio n. This condition leads to a heat flux limitation for the third-order approximation systems (i.e. the systems that solve heat flux equations self consistently). This criterion fails in auroral zones at high alt itudes and this could explain some numerical instabilities. The case o f multifluid models is also investigated. The hyperbolicity analysis l eads in this case to a limitation of the relative velocity. These cond itions are discussed from a kinetic point of view. These criteria are applied to polar wind models and lead to an explicit and qualitative l imitation of the altitude range for the validity of the models which i s compared with the altitude range already reported from phenomenologi cal considerations.