Auroral electron distributions within and close to the Saturn kilometric radiation source region

摘要

On 17 October 2008, Cassini observed for the first time the electron populations associated with the crossing of a Saturn kilometric radiation source region and its surroundings. These observations allow for the first time the constraint and quantification of the high-latitude acceleration processes, the current systems, and the origin of the low-frequency electromagnetic waves. Enhanced fluxes of field-aligned energetic electrons were measured by the Cassini electron plasma spectrometer in conjunction with unusual intense field-aligned current systems identified using the magnetometer instrument. In the region where downward field-aligned currents were measured, electron data show evidence of two types of upward accelerated electron beams: a broadband energetic (1-100 keV) electron population that is observed throughout the region and a narrow-banded (0.1-1.keV) electron population that is observed sporadically. In the regions where the magnetic field signatures showed evidence for upward field-aligned currents, we observe electron loss cone distributions and some evidence of shell-like distributions. Such nonthermal electron populations are commonly known as a potential free energy source to drive plasma instabilities. In the downward current region, the low-energy and energetic beams are likely the source of the very low frequency emissions. In the upward current region, the shell distribution is identified as a potential source for Saturn kilometric radiation generation via the cyclotron maser instability.