STORM-TIME MAGNETOSPHERIC CURRENTS INFERRED FROM MIDLATITUDE GEOMAGNETIC-FIELD VARIATIONS

Both D and H components of geomagnetic field data from 10 mid-latitude stations are used to examine the storm-time magnetospheric current system by deriving the longitudinally symmetric and asymmetric components of the fields. The asymmetry in D component suggests that net downward and upward field-aligned currents exist in the morning sector and from the evening to the early morning sector, respectively, when the IMF is southward. It also suggests the appearance of a net downward current in the afternoon sector at the developing stage of the storm-time ring current. These suggestions are confirmed by Magsat observation over the ionosphere since the direction of disturbances over the ionosphere is not the opposite but the same as that on the ground. This downward current signature co-exists with that of the morning sector during the southward IMF, suggesting that two different current systems co-exist at the developing phase of the ring current. One is the so-called partial ring current system whereas the other is the current system controlled by the IMF-B(z). A systematic phase shift of asymmetry in local time is seen in the dusk-side H but not in the D component, indicating that the contribution from westward drifting ring current particles can not be neglected as a cause for the H component asymmetry.