The interaction of high-m guided poloidal alfven waves with magnetospheric electrons and the ionosphere

Abstract:

Poloidal mode Alfven waves are often generated in Earth’s magnetosphere following interplanetary shocks and/or pressure pulses acting on the magnetopause. These disturbances can excite resonant field line oscillations with frequencies in the mHz range by launching fast mode waves that couple energy to field line resonances. This direct action of the solar wind on the magnetosphere can perhaps explain waves with relatively small azimuthal wavenumbers (m), but not the observed range of waves with m~40-50 on L-shells around 5 or 6. These waves are strongly guided along geomagnetic field lines to the ionosphere, and are generally thought to particle driven, e.g., as a result of bounce-resonance wave-particle interactions following activation of the ring current. This is not the only possible source mechanism as there is evidence of wave generation before the ring current has reacted significantly to shock passage. Putting aside the source mechanism, high-m poloidal modes with strong east west directed electric fields are important primarily because they can elevate differential energy flux for electron energies in the range of 100’s of keV to several MeV. In this paper we use observations of guided poloidal mode Alfven waves to constrain a ULF wave model that describes not only how waves evolve on geomagnetic field lines, but also their interaction with a dynamic height-resolved ionosphere. The ionosphere and neutral atmosphere are specified in the model using the IRI and MSIS models. These regions react to waves and precipitation through heating and cooling, ionization, recombination, and chemical reactions. We present detailed results of the interaction of a poloidal wave observed by the Eiscat radar, and demonstrate that the model used can reproduce all aspects of the radar observations. We consider mechanisms for pulsed precipitation accompanying this wave, which causes a phase difference of ~90 degrees between observed temperature and density spikes. We also demonstrate that increased production of heavy NOx molecular ions is necessary to explain rapid rates of decay of density perturbations in the ionosphere.