The purely compressional effect of the magnetopause current is usually not noticeable at Halley.
One exception to this is the Sudden Impulse signature which may sometimes be discerned because of
its clear, step-like appearance. It is caused by a solar wind shock wave travelling past the Earth.
Behind the shock the pressure applied to the magnetopause is greater causing a sudden compression
of the magnetospheric magnetic field. An (extreme) example is shown in the figure. The figure
shows the H, D and Z components of the magnetic field at Halley over a one hour interval. The Sudden
Impulse signature is seen at ~23:30 UT.
A sudden magnetospheric compression often drives transient ionospheric currents too (Southwood and Kivelson, 1990; and see Travelling Convection Vortices in section 184.108.40.206). These give an oscillatory ground magnetic perturbation that is in addition to the compressional perturbation due to the magnetopause currents (Freeman et al., 1990). The ionospheric currents and associated ground magnetic perturbations maximise in the equatorial and auroral zones (Russell et al., 1992; Le et al., 1993). Hence at Halley the effect from the ionospheric currents may swamp that due to the magnetopause currents.
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