Multipoint measurements of large electric fields and shears in the Auroral zone

This work presents results from the Auroral Turbulence II (AT II) sounding rocket launched from the Poker Flat Research Range, near Fairbanks, Alaska, on February 11, 1997. The rocket consisted of three identically instrumented payloads that reached an apogee of 550 km and flew through several arc structures in a pre-midnight auroral breakup. Three payload measurements were desired to separate the temporal from spatial aspects of auroral forms, and to investigate three dimensional flows and fields that cannot be resolved using traditional single point measurements. The focus of this study has been in the structure and changes in the dc electric field. Although the AT II payloads traversed many auroral arc structures, most of the forms had negligible electric fields associated with them. The one exception was when the payloads crossed a large, stable arc at nearly 500 km altitude. In the middle region of the arc, and near the poleward boundary, the payloads measured significant electric fields, as much as 450 mV/m in the poleward region of the arc. The payloads also measured different electric field structures, while approximately three kilometers apart, indicating a gradient in the field between measurements. Spectral analysis of the ac electric field data show broadband electrostatic waves in the regions of enhanced dc electric field. The multiple measurements indicate a region of spatially localized electric field, electric field shear and wave activity that both drifts in space and changes amplitude temporally. In-situ changes in the electric field are examined in parallel with all-sky imagery obtained near the footpoint of the payload trajectories. We have determined that there are errors in the dc electric field measurements that are most likely explained by a shadowing of the potential spheres by the payload. Despite the uncertainty in absolute, magnitudes of the dc field, there are clear differences in the field signatures between payload measurements that can be shown to be spatial shears. The shear in the localized electric field, the observed broadband electrostatic waves, and the low field aligned currents measured by the magnetometers indicate an instability mechanism responsible for the wave growth. The most likely candidate, given the environment, is the inhomogeneous energy density driven instability (IEDDI), which we believe to be responsible for the observed electrostatic waves observed near the oxygen cyclotron frequency