Saturn's low-latitude boundary layer: 1. Properties and variability

Abstract

[1] Transport of solar wind plasma into a planetary magnetosphere produces an internal boundary layer adjacent to the magnetopause that contains a mixture of solar wind and magnetospheric plasma. The potential processes responsible for this transport are varied, and their relative importance remains unclear, as well as how their competition varies between the magnetized planets. In this paper we examine the properties and variability of Saturn's low-latitude boundary layer (LLBL) using observations made by the Cassini spacecraft; the electron structure of Saturn's LLBL is examined in a companion paper. The duration of spacecraft excursions into the LLBL was generally between ∼3 and ∼23 min, and the electron environment of Saturn's LLBL was intermediate between that of the magnetosheath and that of the magnetosphere, as expected. Estimates of the speed of the magnetopause current layer are of the order of 100 km s−1, and the estimated thickness of the LLBL is of the order of 1 Saturn radius. Our results do not reveal a strong influence of the orientation of the interplanetary magnetic field on the thickness of Saturn's LLBL, which is unlike its terrestrial counterpart. There is also no clear dawn-dusk asymmetry in the thickness of Saturn's LLBL, which might be expected on the basis of our understanding of the growth of the Kelvin-Helmholtz instability at Saturn's magnetopause. We discuss the implications of these findings for the physics of Saturn's magnetospheric boundary and what this could mean for our understanding of how boundary layers are formed in planetary magnetospheres.