A comparative analysis of low-latitude Pi2 pulsations observed by Orsted and ground stations

[1] Orsted is a low-altitude polar orbiting satellite, which is advantageous for investigating the spatial structure of low-frequency events like Pi2 pulsations. Using 1-s vector magnetic field data from April 1999 to May 2002 obtained from Orsted and Kakioka magnetic observatories, we found that the field aligned component (Bparallel to) of Pi2 pulsations at the satellite height is highly correlated with the H component on the ground, but the other two components, i.e., eastward component (B-E) and radial component (B-R), do not show a clear correlation with the ground observations. The observed results for nightside events provide evidence that nightside Pi2s at low latitude are generated from cavity resonance. Two cases observed by Orsted when it was located on the dawnside or duskside also show cavity resonance properties. However, when Orsted was on the dayside, the oscillations observed by the satellite are out of phase with that observed on the ground, suggesting that the dayside Pi2s are more likely related to some dayside ionospheric current systems rather than that caused by a global cavity resonance mode. The amplitude variation recorded by Orsted shows a peak in the equatorial region, which, for the first time, gives clear observational support for earlier model calculations. We also estimated the screening effect of the ionosphere on MHD waves and suggest that when the cavity resonance mode is valid for generation of Pi2 pulsations at low latitudes, the screening effect is negligible; that is, the compressional waves are seen directly as Pi2 pulsations at the ground.