Temporal and spatial aspects of the cusp inferred from local and global ground- and space-based observations in a case study


Global and local observations, from both ground and space facilities, are brought to bear to understand the origin of a stepped cusp ion dispersion observed 3 hours prenoon under an interplanetary magnetic field characterized by strongly positive By, conditions under which the cusp is believed to be shifted postnoon. Our main aim is to distinguish the temporal from the spatial cusp features. The local observations from Svalbard are from approximately the 0900–1200 MLT sector, while the in situ observations are from Polar at 0900 MLT. The global auroral observations were acquired by the Earth camera on Polar. The aurora from the ground perspective appeared as a periodic sequence of brightening events located on either side of a “midday gap aurora” centered at about 1030 MLT. At noon the aurora appeared as a semiperiodic sequence of brightening events. These were concurrent with enhancements of the overhead ionospheric Hall current. Polar observed two steps of a staircase ion energy-latitude dispersion. These two steps were, in turn, concurrent with two consecutive auroral brightening events. We argue from this complement of observations that the stepped cusp was temporal in nature and resulted from a time-varying reconnection process. Global UV imagery was employed to complement the ground observations for the purpose of separating the temporal from the spatial aspects. The imager captured, namely, two bands of strong UV emission in the prenoon and postnoon sectors, between which there appeared a persistent, weak UV aurora. We interpret this to be an underlying spatial substratum. Patchy and bursty UV enhancements coincided with the transients seen in the local ground and space measurements, corroborating further our distinction between space and time in the cusp. Comparing these inferences with recent work on the response of the dayside aurora to reconnection at the magnetopause, we find a close correspondence. In addition, we document the field-aligned currents associated with the stepped cusp, which, in turn, compare favorably with recent theoretical ideas. This work is a contribution to the current debate on the nature of the stepped cusp and suggests that multiple data sets are essential to study this problem.

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JGR: Space Physics



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