Abstract. The ionosphere in¯uences magnetohydrodynamic waves in the magnetosphere by damping because of Joule heating and by varying the wave structure itself. There are di erent eigenvalues and eigensolutions of the three dimensional toroidal wave equation if the height integrated Pedersen 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 e ect mainly occurs in regions with gradients in the Pedersen conductances, as in the auroral oval or the dawn and dusk areas. A pulsation event recorded by the geostationary GOES-6 satellite is presented. We explain the observed change in frequency as a change in the wave structure while crossing the terminator. Furthermore, selected results of numerical simulations in a dipole magnetosphere with realistic ionospheric conditions are discussed. These are in good agreement with the observational data.Key words Ionosphere á (Ionosphere±magnetosphere interactions) á Magnetospheric physics á Magnetosphere ± ionosphere interactions á MHD waves and instabilities.
Abstract. The ionosphere in¯uences magnetohydrodynamic waves in the magnetosphere by damping because of Joule heating and by varying the wave structure itself. There are dierent eigenvalues and eigensolutions of the three dimensional toroidal wave equation if the height integrated Pedersen 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 eect mainly occurs in regions with gradients in the Pedersen conductances, as in the auroral oval or the dawn and dusk areas. A pulsation event recorded by the geostationary GOES-6 satellite is presented. We explain the observed change in frequency as a change in the wave structure while crossing the terminator. Furthermore, selected results of numerical simulations in a dipole magnetosphere with realistic ionospheric conditions are discussed. These are in good agreement with the observational data.Key words Ionosphere á (Ionosphere±magnetosphere interactions) á Magnetospheric physics á Magnetosphere ± ionosphere interactions á MHD waves and instabilities.
Abstract.In a horizontally homogeneous high-latitude ionosphere a steady state Hall current closes within the E region and does not contribute to the interaction between the ionosphere and magnetosphere. When the steady state assumption is given up, Hall currents driven by a non-zero OB/Ot become divergent and must transiently close via the magnetosphere. It is shown, that this process is of non-local nature and can redistribute the field-aligned current in space and time. At scales of tens of kilometers the effect becomes significant for frequencies at about I Hz and above, at larger scales even for lower frequencies. The rearrangement of the field-aligned current is such that elongated structures tend to warp, and thus it resembles some of the typical dynamic evolution of auroral forms. Even a transient local amplification of the current density is possible.
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