DMSP optical and electron measurements in the vicinity of polar cap arcs

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Abstract
We have completed an extensive analysis of the electron and optical data from the DMSP satellites for an extended period of polar cap arc occurrences on December 12, 1977. The polar cap arcs are observed in three distinct intervals in a period of quieting after a time of intense substorm activity. Each interval occurs when the IMF Bx is less than 0 and follows a northward turning of the IMF Bz. The intervals are separated by periods of southward Bz and weak substorm activity during which no enhanced precipitation is seen in the polar cap. The polar cap arcs persist for periods of more than an hour with only gradual variation in position and intensity. At the time of our study the highest‐energy flux and average energy for electrons precipitating into the polar cap are seen in the northern polar cap. The observation of polar cap arcs is associated with the admittance of large and variable fluxes of low‐energy electrons into a major portion of both the northern and southern hemisphere polar caps. These fluxes fall into the following categories: First, nearly Maxwellian distributions of electrons with temperatures between 50 eV and 200 eV and number densities varying from 0.03/cm³ to 4/cm³. The highest densities are found at the poleward boundary of the diffuse aurorae and near the visible polar cap arcs. The lowest densities are associated with the polar rain. Second, distributions of electrons peaked between 50 eV and 200 eV. These distributions result from acceleration of the cold Maxwellian distribution through a potential of 50 to 200 V without any heating of the electrons. Third, distributions of electrons displaying two populations; an intense low‐energy component with a temperature of ∼20 eV and a much weaker high‐energy component with a temperature of 180 eV. We interpret such distributions as evidence of direct admittance of magnetosheath electrons into the polar cap. Fourth, distributions of electrons peaked at ∼1 keV. These distributions produce the visible arcs. They result from the acceleration of a two‐component electron population with temperatures of 100 and 350 eV through a potential drop of ∼750 V.

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