Abstract
We have studied the detailed electron distributions measured in a westward travelling surge. The particle data, obtained by a sounding rocket, have been related to the auroral structures as recorded by a low light level TV. The event took place close to magnetic midnight on January 27, 1979.
Two different types of particle spectra were detected. The first type consisted of two populations; a low energy population of secondary electrons with j ∼ E−2 and a plasma sheet population. The second type of spectra contained a Maxwellian population with a characteristic energy of a few hundred eV in addition to the previous two populations. The few hundred eV Maxwellian and the plasma sheet population had been accelerated in a potential drop parallel to the magnetic field lines.
The auroral luminosity depends on the integrated energy flux. In regions with medium or low luminosity the spectra could be of either type, while the brightest auroral structures were connected to spectra of the second type, that is accelerated spectra with a low temperature population. It is suggested that the parallel potential drops are created on field lines magnetically connected to regions in the distant magnetosphere where the few hundred eV population is present.
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