As the solar wind [solar wind] dynamic pressure increases, the magnetosphere
is squashed inwards and the magnetopause currents increase in order to confine the geomagnetic field
within this smaller volume. For this reason, the size of the magnetic field perturbation is proportional
to the square root of the solar wind dynamic pressure. This is shown in the figure where the hourly
averaged horizontal magnetic field strength measured on the Earth's surface at the equator is plotted
as a function of the solar wind dynamic pressure measured by a satellite (Araki et al., 1993).
The upper envelope is the compression due to the magnetopause currents. For typical solar wind pressures the compressional magnetic perturbation is less than 30 nT. Due to the current sheet geometry, the horizontal component of the compressional magnetic perturbation decreases with latitude ( Le et al., 1993). Over the range of observed solar wind dynamic pressures the compressional horizontal perturbation at Halley is unlikely to exceed 10 nT.
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