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At higher latitudes on the Earth's surface the magnetic perturbation due to an equatorial
ring current is directed more radially. Hence, at Halley,
we can expect the effect of the ring current on the horizontal magnetic field component to be
much less than that of the DST index measured at the equator. Without knowing the DST index, it is very
difficult to separate out the ring current contribution. One possible method is to make use of the fact that
whilst the ring current may increase on short (< 1 h) time scales, it decays on much longer (> 1
day) time scales. By subtracting the quiet-time field just before an interval of interest from
that just after, the change in the magnetic perturbation due to any ring current intensification
may be crudely estimated. This can be considered as a systematic error to the magnetometer analysis
of the ionospheric currents.
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