Observation and interpretation of particle and electric field measurements inside and adjacent to an active auroral arc

Abstract

A Javelin sounding rocket instrumented to measure electric fields, energetic particles, and suprathermal electrons was flown across an auroral display during the late expansion phase of a substorm. Four distinct regions of fields and particles were delineated and are interpreted here in light of our present understanding of auroral zone dynamics. Poleward of the display the particle fluxes were very low, and the convection velocity was antisunward and very smooth, apparently unaffected by the substorm in progress at lower latitude. The pitch angle distributions of the energetic electrons detected in this region were peaked at 90°, providing evidence that the magnetic field lines were closed. Just poleward of the visible aurora, intense fluxes (109 cm−2 s−1) of soft electrons (energy peak at ∼2 keV) were detected. The boundary of this region was quite sharp and was also characterized by a change in the convection velocity and direction. Between this boundary and the visible aurora the E field and the electron flux magnitudes were anticorrelated and quite variable. Inside the visible arc the electric field was large, was correlated with the electron fluxes, and had a large equatorward component. These observations are consistent with a westward electrojet flowing along the visible arc enhanced by both the increased conductivity and the large field. The electron flux was intense and extended to higher energy in the visible arc region. At the boundary between the arc and diffuse aurora to the south a negative charge sheet separated equatorward and poleward directed electric fields, an observation which can be explained by polarization caused by conductivity gradients at the arc boundary. Wave experiments show this region to be the source of intense electrostatic waves, discussed in the companion paper. In the region of poleward electric field the electron flux was much reduced and had a hard spectrum. These electric field and particle flux patterns are consistent with an auroral arc model which includes a westward electrojet driven by partial polarization of a highly conducting region, downward field-aligned currents in the region of poleward electric field probably carried by thermal particles, and upward current carried by precipitating electrons in the discrete visible arc. Analysis of the particle fluxes shows that the plasma sheet can act as a source but that acceleration mechanisms are required to explain the detailed spectra. In the soft precipitation zone (northern edge of disturbed region) the fluxes are consistent with a factor of 2 energization of undisturbed plasma sheet particles such as would occur in a collapse of a distended magnetic field. The discrete arc fluxes require further energization by a factor of 10 and resemble fluxes measured in the equatorial plane during the expansion phase. The hard fluxes in the equatorward zone are further energized and may act as a source for the outer radiation belt as inward convection further energizes them.