The behavior of the electric field within the substorm current wedge

We use the Goose Bay HF radar to investigate the behavior of the electric field within the substorm current wedge for a small substorm (< 200 nT) which occurred on February 15, 1992. Mid- and high-latitude magnetometer data helped to locate the radar backscatter with respect to the longitudes and latitudes of the substorm currents, as well as describing the time development of the substorm itself. Velocities in the portion of the field of view of the Goose Bay HF radar closest to the Frederikshab magnetometer in Greenland were compared directly with the ionospheric currents inferred from the magnetometer. During the early growth phase, the plasma flow and current gradually increased in response to the DP2 electric field and are related by an estimated effective height-integrated conductivity, Σ*, ∼ 1 S. The plasma flow and current continued to increase in the same proportion (i.e., Σ* was still ∼ 1 S) as the electrojet intensified in stages during the late growth/early expansion phase. In this interval there was probably a pseudo-breakup which established a longitudinally and latitudinally narrow substorm current wedge. The increased plasma flow and current measured at Frederikshab are attributed to the superposition of the pseudo-breakup/current wedge electric field on the preexisting DP2 electric field. Evidently, the enhanced ionospheric conductivity strip usually associated with the current wedge is initially located equatorward of Frederikshab, since its effect is not apparent at that location. During the expansion phase proper, a clearly resolved current wedge expanded so that the radar backscatter lay within it. The expansion led to an increased current over Frederikshab, but a slightly suppressed plasma flow, related by an increased Σ* ∼ 4 S: the effect of the precipitation induced conductivity strip now apparent. It is suggested that near-Earth current disruption may play a significant role in the onset of this particular substorm, which occurred during a nonstorm time interval.

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