Auroral precipitation flux of ions and electrons in saturn's outer magnetosphere

This paper examines particles and fields data obtained by the Voyager spacecraft in Saturn's outer magnetosphere with a view towards assessing the role that medium-energy ions and electrons have in stimulating the u.v. aurora. The magnetic field displays a high level of fluctuation of two characteristic types: large-scale, coherent depressions in the field strength associated with the plumes of Titan and a small-scale, incoherent turbulence presumed to be a consequence of the high-β plasma environment. The fluctuation amplitude is sufficiently large that ions which resonantly interact with the noise should be rapidly scattered in pitch angle to achieve a condition of isotropy. Electrons are assumed to interact strongly with lower hybrid waves and are accelerated to energies of a few kiloelectron volts although direct evidence for the waves is not obtainable. The available energy input to the aurora by protons is 5 × 10 ⁹ W while an upper bound to that of N ⁺ ions is ∼2 × 10 ¹⁰ W. Electrons in the range 1-10 keV can contribute upwards of 5 × 10 ¹⁰ W (just short of the 2 × 10 ¹¹ W requirement set by the u.v. spectrometer measurements) and perhaps more if a field-aligned potential drop above the aurora is present. It is concluded that electrons are most likely the primary precipitation energy source for the aurora as a result of energy transfer from Titanogenic N ⁺ pickup ions in a corotation-dominated magnetosphere.