Abstract. We have calculated the integrated ULF wave power in the Pc5 band at two stations, Kevo (part of the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer array in Scandinavia, at auroral zone latitudes), and Cape Dorset (part of the Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada, at cusp latitudes), and compared this power against the solar wind velocity for the last six months of 1993, a period characterized by two persistent high-speed solar wind streams. We find for both local noon at Cape Dorset, and for local morning at Kevo, the Pc5 band power (0.002 -0.010 Hz) integrated over a six-hour period exhibits a clear power-law dependence on the solar wind velocity. At Cape Dorset we found power c• Vsw 4, with a correlation coefficient r = 0.73, and at Kevo we found power c• Vsw 6'5, with r = 0.74. Much of the remaining variation in Pc5 power is due to temporal patterns evident at both stations in response to recurrent high speed streams. Power was strongest at the leading edge of each high speed stream and subsequently decreased more quickly than Vsw. Our observations suggest that it is insufficient to make estimates of Pc5-range ULF wave power on the basis of Vsw alone' one must consider other physical factors, either intrinsic to the solar wind or related to its interaction with Earth's magnetosphere. The Kelvin-Helmholtz instability is often considered to play a dominant role in this interaction, and the level of instability depends on both velocity and density. By means of a simple simulation using typical density and velocity values during the passage of a high speed stream, we were able to obtain good agreement with the t.emporal variations we observed. Finally, this study indicates that ground-based pulsation observations can provide reliable proxies of the initial passage of high speed solar wind streams past Earth.

Abstract. A statistical analysis of transverse wave activity in the Pc 5 frequency band using satellite magnetic field data from a 6 month interval shows a symmetric distribution along the geostationary orbit in the magnetosphere, whereas observations on the ground display an asymmetric distribution with regard to the noon sector with maximum wave activity on the morningside. As a possible cause of these different local time variations of the wave activity, the ionospheric screening effect is discussed, which depends on the radial scale lengths or resonance width of the waves as well as on their azimuthal wave number. It is suggested in this study that a local time variation of the resonance width of the waves is caused by a variation of the radial gradient of the plasma density along the geostationary orbit, leading to a slowly varying efficiency of the ionospheric screening effect with local time. Local time variations of the resonance width thus will be of equal importance to explain the observed asymmetry as, for example, local time variations of wave sources causing local time variations of the azimuthal wave number.

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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.

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Abstract. For more than two decades numerical models of the Earth's magnetosphere have been used successfully to study magnetospheric dynamic features such as the excitation of ULF pulsations and the mechanism of ®eld line resonance. However, numerical formulations simplify important properties of the real system. For instance the Alfve n continuum becomes discrete because of a ®nite grid size. This discretization can be a possible source of numerical artifacts. Therefore a careful interpretation of any observed features is required. Examples of such artifacts are presented using results from a three dimensional dipole model of the magnetosphere, including an inhomogeneous distribution of the Alfve n velocity.