Magnetospheric electric fields and currents and their influence on large scale thermospheric circulation and composition

Abstract

The magnetospheric electric convection field and the polar cap field are generated by the interaction between the solar wind and the geomagnetic field. These electric fields map into the thermosphere, mainly within auroral and polar regions, and drive electric currents there. The electric currents transfer heat energy to the neutral gas via Joule heating j·E ( j = electric current density; E = electric field). In addition, they move the neutral wind via momentum transfer by the Ampère force j× B ( B = geomagnetic field). A semiquantitative study of the configurations of Joule heating and momentum force is performed in terms of series of spherical functions. Each individual term drives corresponding wind systems which depend on latitude, longitude, local time and universal time. The role of these various wind systems in causing the observed disturbances in total gas density, composition and F-layer electron density during persistent geomagnetic activity is discussed. Joule heating is mainly responsible for a divergent irrotational wind field accompanied by significant pressure amplitudes. The momentum force primarily generates a rotational non-divergent wind and only small pressure variations. The various constituents of the neutral gas react in a quite different manner to these two wind systems. It is primarily the divergent wind due to Joule heating which causes the observed depletion of oxygen and helium in the higher latitude regions during magnetic storms, while the influence of the rotational wind due to the momentum force on that deviation from diffusive equilibrium is small. Likewise, the ionospheric storms are primarily caused by the divergent winds.