Waves and Instabilities in Saturn's Magnetosheath: 1 Mirror Mode Waves and Their Impact on Magnetopause Reconnection

Abstract

A comprehensive catalog of 1,589 Saturn magnetosheath traversals by Cassini between 2004 and 2012 was used to perform a statistical study of mirror mode (MM) waves and assess their role in influencing magnetic reconnection at the magnetopause (MP). MM waves have been observed in many planetary magnetospheres and magnetosheaths, comets and the solar wind. Understanding the conditions under which they grow and dominate can reveal their role in influencing plasma dynamics. Using a thresholding method on both magnetic field and plasma data, MM wave candidates can be identified. The magnetic field characteristics and occurrence distributions of these waves against different locations and conditions were found. MM waves were found from 4 to 19 hr local time (partly due to data coverage), and distances of 0–12 $R_S$ from the magnetopause (MP). The occurrence of MM dips was more frequent near the MP and magnetosheath flanks, analogous to the Jovian system. MM dips exhibited a minimum field strength saturation $\sim$ 0.5 nT, with the largest dip inferred to be in mirror-stable plasma. Notably, larger amplitude MM dips were typically found nearer the MP boundary which increases $\Delta \beta$ across the boundary thus increasing the magnetic shear necessary for the onset of MP reconnection. Thus, MM waves may be important in plasma dynamics near Saturn's magnetopause.