Temporal variations in a four-sheet field-aligned current system and associated aurorae as observed during a Polar-ground magnetic conjunction in the midmorning sector


We relate measurements of temporarily varying field-aligned current systems (FACs) and their associated plasmas made by the Polar spacecraft at midmorning local times and likewise temporarily varying aurorae observed from Svalbard, Norway, when the magnetic footprint of the spacecraft passed across the field-of-view of the ground instruments on 3 December 1997. We combine the in situ observations of plasma and magnetic and electric fields with meridian scanning photometry and all-sky imagery from the ground site. The interplanetary magnetic field (IMF) pointed strongly east (By ≫ 0) and generally south. Descending from ∼7.5 to ∼5.5 RE and heading south, Polar traversed a four-sheet current system: a twin-sheet cusp current system C1-C2, spanning 80.6°–77.7° invariant latitudes (ILT), and then the traditional regions 1 (R1) and 2 (R2) currents, extending from 77.7° to 73.3° ILT. A convection reversal separated the C1-C2 from the R1-R2 FACs. Currents C1 and R2 flow out of the ionosphere, while C2 and R1 flow into the ionosphere. Within C1, Polar observed six bursts of ions of typically magnetosheath energies (≤2 keV) repeating every ∼5 min, accompanied by intensified field-aligned electron beams and magnetic field depressions. Auroral data, acquired later but under very similar IMF conditions and at the same latitudes as when Polar was within C1, suggest auroral forms which are pulsed in both red and green lines with a similar period to the plasma bursts observed earlier at Polar. Within C2, at Polar, the pulsing ceased, but magnetosheath plasma was still present, albeit at diminished intensity. This was in part also a temporal change as the IMF clock angle decreased to ∼60°. Equatorward of the cusp aurora we find a mixing region of magnetosheath and magnetospheric plasmas with spectral characteristics of the boundary plasma sheet (BPS) forming the source of the R1 current. Plasma inhomogeneities and bipolar current elements embedded therein were related to discrete east-west aligned arcs located equatorward of the cusp aurora. Within R2, when Polar was sampling the central plasma sheet, a field line resonance was observed on Polar and the underlying ground station HOP. A steady southward IMF rotation elicited a continued equatorward migration of all aurorae. Observations within C1-C2 are discussed within the conceptual model of Taguchi et al. [1993] but extended to include the time-dependent aspect so prominent in our observations. We thus hypothesize that the observations are due to time-varying reconnection under positive By conditions. The passage of the resulting reconnected flux tubes are registered at Polar as ion bursts whose lack of significant energy-latitude dispersion is likely due to the fact that Polar descends by only 1.1 degrees as it traverses the C1 current.

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JGR: Space Physics



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