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A circular current loop approximates the ring current of the Earth. Since the ring
current flows westward the magnetic perturbation at the Earth's equator is opposite to the direction
of the horizontal main geomagnetic field. The total strength of the ring current is gauged by the
decrease in the horizontal magnetic field averaged over several ground magnetometers distributed
around the equator. Hourly averages of this quantity are referred to as the DST index. DST index
measurements are shown in the figure and have already been discussed in relation to the
magnetopause currents. Statistically, the maximum positive values indicated the strength of the
magnetopause current. The large negative deviations of up to -200 nT below the upper envelope indicate
the strength of the ring current. The strength of the ring current is seen to be strongly
correlated with the north-south component (Bz) of the interplanetary magnetic field.
When Bz is negative coupling between the solar wind and the magnetosphere
is facilitated and energy is transferred into the magnetosphere, part of which ends up in the
ring current.
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