Sounding rocket observations of particle precipitation in a polar-cap electron aurora

Abstract

Results of low-energy particle measurements made from a sounding rocket launched from Resolute Bay, Northwest Territories (invariant latitude 84.5°), into a polar-cap aurora are presented. Soft electron fluxes were detected above a relatively stable 1-kR auroral arc that was aligned in the usual earth-sun direction. The electron precipitation, which was only slowly varying in time, was found to have an isotropic pitch-angle distribution and a differential energy spectrum that peaked in the 1.5- to 2.0-kev energy range with a peak intensity of ∼107 cm−2 sec−1 ster−1 kev−1. Outside the arc, the electron energy flux fell below 10−2 erg cm−2 sec−1, as compared with 0.15 erg cm−2 sec−1 inside the arc. No positive-ion precipitation was detected, except that due to the remnant of a solar proton event. Upper limits of low-energy auroral proton fluxes were set near 105 cm−2 sec−1 ster−1 kev−1 in the 0.6- to 6.0-kev energy interval. These data and previous measurements in the nightside and dayside auroral zones are compared, and current magnetospheric models, in particular the relationship between polar-cap auroras and such concepts as direct neutral-point entry and neutral-point acceleration mechanisms, are discussed.