IONOSPHERIC CONDUCTANCE DISTRIBUTION AND MHD WAVE STRUCTURE - OBSERVATION AND MODEL

The ionosphere influences magnetohydrodynamic waves in the magnetosphe re by damping because of Joule heating and by varying the wave structu re itself. There are different eigenvalues and eigensolutions of the t hree dimensional toroidal wave equation if the height integrated Peder sen conductivity exceeds a critical value, namely the wave conductance of the magnetosphere. As a result a jump in frequency can be observed in ULF pulsation records. This effect mainly occurs in regions with g radients in the Pedersen conductances, as in the auroral oval or the d awn and dusk areas. A pulsation event recorded by the geostationary GO ES-6 satellite is presented. We explain the observed change in frequen cy as a change in the wave structure while crossing the terminator. Fu rthermore, selected results of numerical simulations in a dipole magne tosphere with realistic ionospheric conditions are discussed. These ar e in good agreement with the observational data.