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As a ground station rotates under the DP2 current system that is fixed in the Sun-Earth frame it
should see a diurnal variation in the magnetic field perturbation. An example of this is shown in the figure.
This shows the H, D and Z components of the ground magnetic field at Halley over a 24 hour interval from 15 UT
on January 14, 1988. The interval begins and ends with a quiet-time field. Shortly after the start of the
interval when Halley is in the afternoon sector a positive H perturbation is seen to grow, consistent with
the development of an eastward electrojet. Around 22 UT in the evening sector the H perturbation reverses
sign consistent with the station moving under a westward electrojet which persists throughout the morning
sector until the magnetic perturbation dies away. As the H component perturbation reverses sign the D
component perturbation becomes a maximum. This shows that the equivalent current rotates from north-eastward
through southward to south-westward over the interval. This is consistent with the DP2 current
variation. Note the variation in the Z component too which indicates strong horizontal
current gradients and/or changes in the electrojet position relative to the station. Such a strong, sustained DP2 variation is rarely seen at Halley because typically it is located equatorward of the main DP2 electrojets, especially on the dayside. The variability of the IMF direction continuously changes Halley's position with respect to the main electrojet currents. The example shown in the figure occurred during an interval of continuously southward IMF when the DP2 current system is statistically more equatorward. Such an interval is known as a magnetic storm. Note that during this storm there is alot of variation on time scales of an hour or less. This is attributable to variations in the conductivity and electric fields associated with substorms and the associated DP1 current system. |
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