Abstract
Abstract. The ionosphere influences magnetohydrodynamic waves in the magnetosphere by damping because of Joule heating and by varying the wave structure itself. There are different 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 effect 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 · (Ionosphere · magnetosphere interactions) · Magnetospheric physics · Magnetosphere · ionosphere interactions · MHD waves and instabilities.
Highlights
ULF pulsations have been observed for more than a century and have interpreted following Dungey's work (1954) as magnetohydrodynamic (MHD) waves in the magnetosphere
We presented a pulsation event, observed by Geostationary Operational Environmental Satellite 6 (GOES-6) during dusk in spring, where the frequency suddenly changes
The ionosphere in the Northern Hemisphere has a negative value in whereas in the Southern Hemisphere it changes from a negative value in the day-side to a positive value when crossing the day-night terminator
Summary
ULF pulsations have been observed for more than a century and have interpreted following Dungey's work (1954) as magnetohydrodynamic (MHD) waves in the magnetosphere. Under these conditions the ionosphere is one of the main damping mechanisms for Alfve n waves in the magnetosphere (Newton et al, 1978; Glassmeier et al, 1984) It aects the mode structure of the waves, because with changing boundary conditions the 3D wave equation has dierent eigenvalues and dierent eigensolutions for a particular ®eld line. The purpose of this study is to apply the well-known ionospheric reection theory to an observed ULF pulsation event in the dusk sector when crossing the terminator, i.e. when crossing geomagnetic ®eld lines exhibiting dierent ionospheric reection conditions We propose that this observation shows a transformation of a half wave into a quarter wave harmonic toroidal mode. It can be seen that around the time where the frequency step occurs, which is indicated by the dotted line, the Pedersen conductance in the Southern Hemisphere reaches relatively low values.
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