Simultaneous optical and particle data from the ISIS-2 satellite are used
to characterize polar cap arcs. Polar cap arcs are identified from
two-dimensional geomagnetic transforms of the optical data along with
precipitating electrons data for the time at which the satellite is on the
field line intersecting the arc. No precipitating protons were detected for
any of the arc crossings. The pitch angle distribution of the precipitating
electrons is generally isotropic and the differential electron spectra show
enhancements in the flux in the 300 - 750 eV energy range. The average energy
of the precipitating electrons for the different arcs ranges from about 300
to 600 eV. A possible explanation ot the observed precipitating particle
characteristics is that parallel electric fields are accelerating polar rain
type spectra at an altitude of several thousand km. For the arc crossings
reported here the equivalent 4278 Å emission rate per unit energy
deposition rate has a mean value of
162 R/(erg cm -2 s -1).
Average 3914 Å intensities are about 0.8 kR while 6300 Å intensities
range from 0.5 to 3 kR. Model calculations indicate that direct impact
excitation is a minor source for the 5577 Å emission rate, but supplies
approx. 40% of the 6300 Å emission.
