DISTRIBUTION OF ENERGETIC POSITIVE-ION SPECIES ABOVE A DIFFUSE MIDNIGHT AURORA

Observational approaches to the problem of magnetospheric plasma origin usually depend upon mass-discriminating measurements of ion distribution functions. A new instrument for this purpose has been developed and flown via sounding rockets above a diffuse midnight aurora during geomagnetically quiet conditions. The instrument combines wide mass and energy ranges (2. 5 less than equivalent to ME less than equivalent to 160 amu multiplied by (times) keV), high mass resolution with good sensitivity, and superior rejection of undesired particles. The rocket payload (29. 008) was launched on February 2, 1978, from Chatanika, Alaska, and reached an apogee of 615 km at approximately 0900 UT (2300 LT), traversing invariant latitudes from 65. 5 degree to 67. 4 degree . Only ions identified as H** plus and He** plus ** plus were detected, and the He** plus ** plus was not clearly above background, having an upper limit of 2 - 4% of the H** plus intensity. All other ions, including the ionospheric species He** plus and O** plus , had upper limits at a similar or smaller level. The ion observations, taken together with electron observations reported in a companion paper, appear to be consistent with the model of diffuse auroras in which trapped plasma precipitates owing to strong pitch angle diffusion on auroral field lines linking the near-earth plasma sheet.