Evidence for parallel electric field particle acceleration in the dayside auroral oval

Electron and ion energy spectra and electron pitch angle distributions are presented for two sounding rocket flights in the dayside auroral zone. At times, effects of dc electric fields parallel to the magnetic field are evident in that: (1) within precipitation features, protons are decelerated by an amount of energy consistent with that which electrons gain and (2) electrons are sometimes aligned to within 3 deg (full width at half maximum) of the magnetic field. A maximum altitude for the accelerating region of several thousand kilometers is deduced from the narrow width of the pitch angle distribution and also from time-of-flight delays between the observation of accelerated electrons and decelerated protons.