Sudden impulses at geosynchronous orbit and at ground

Abstract

Sudden impulses of the magnetospheric and geomagnetic field are caused by sudden increases in the dynamic pressure of the solar wind, generally associated with the Earth’s arrival of interplanetary shock waves or discontinuities. We compared the observed field jumps, at geosynchronous orbit and at ground, with those predicted by a theoretical model (T04, Tsyganenko and Sitnov, 2005) for transitions between two steady states representations of the magnetosphere under different solar wind conditions. The geosynchronous response (mostly along the northward component, B z) in the dayside hemisphere is basically consistent with the magnetic field jump expected for changes of the magnetopause current alone. A similar conclusion holds for the positive changes observed in the nightside hemisphere; by contrast, the competing contributions of several current systems (from the magnetopause, cross-tail current, ring current, field aligned currents) determine in this time sector a large variety of small amplitude and negative responses that cannot be univocally interpreted. A different situation emerges in ground observations, at low latitudes. Here the observed responses along the northward component, H, in the dawn sector are consistent with changes of the magnetopause current; by contrast, in the entire postnoon hemisphere, they are greater than expected. The H variations are also typically accompanied by a significant change of the eastward component, D. These aspects reveal additional ionospheric contributions which determine a negative change of the D component through the entire day and a remarkable enhancement (≈50%) of the H component in the postnoon sector. The comparison between observations and model representations in several cases allows to discriminate between the roles of the current systems which compete in determining the field variations. Occasionally, SI manifestations are accompanied by trains of ULF fluctuations (f≈1–100 mHz); we present a case in which the experimental observations suggest evidence for a cavity/waveguide oscillation at f≈3.2 mHz.