An instrumented rocket payload was launched into a polar cap F layer aurora to investigate the energetic particle, plasma, and electric circuit parameters of a Sun-aligned arc. The rocket was launched from Sondrestrom, Greenland, on March 15, 1985, at 0205:52 UT (approximately midnight corrected geomagnetic local time). On-board instruments measured energetic electron flux, ion composition and density fluctuations, electron density and temperature, electron density fluctuations, and ac and dc electric fields. The payload traversed a rapidly moving Sun-aligned, F layer arc near apogee (429 km altitude). Real-time all-sky imaging photometer measurements of the location and motion of the aurora, conducted from an aircraft in the vicinity of the trajectory, were used to determine the proper geophysical situation for launch. Comparison of the in situ measurements with remote optical measurements shows that the arc was produced by fluxes of low-energy (<1 keV) electrons. Field-aligned potentials in the arc inferred from the electron spectra had a maximum value of approximately 300 V, and from the spectral shape a parent population of preaccelerated electrons characteristic of the boundary plasma sheet or magnetosheath was inferred. Electric field components along and across the arc show sunward flow within the arc and duskward drift of the arc consistent with the drift direction and speed determined from optical imaging. Thus this arc is drifting duskward under the influence of the convection electric field. Within the arc the thermal plasma density shows a moderate decrease, even in the presence of precipitating electron fluxes. Three possible explanations for this (field-aligned currents, chemistry, and transport) are considered. The field-aligned currents associated with the arc are calculated from measured electric fields and computed conductivities. The calculated upward field-aligned current agrees very well in magnitude and location with that carried by the energetic electron flux. The ac electric field and ion mass spectrometer measurements allowed the first direct comparison of the low-frequency cutoff in the wave spectra with the local lower hybrid frequency. There is good agreement at high altitudes, and the disagreement at low altitudes can be explained by propagation effects. Finally, ionospheric irregularity and electric field fluctuations indicate two different generation mechanisms on the dawnside and duskside of the arc. On the duskside, parameters are suggestive of an interchange process, while on the dawnside, fluctuation parameters are consistent with a velocity shear instability.
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