The role of magnetic shear on the evolution of mirror mode waves and their influence on magnetic reconnection at Saturn’s magnetopause

Abstract

Mirror modes are large amplitude, non-propagating compressive structures often observed in the magnetosheath. They appear in the form of quasi-sinusoidal oscillations in the magnetic field, with clear magnetic dropouts (‘dips’) or enhancements (‘peaks’). Mirror modes are usually accompanied by a corresponding, anticorrelated signature in plasma density. Typically, the growth of mirror mode fluctuations is triggered when magnetized plasma traverses the bow shock (BS) or draping of the magnetic field around the magnetopause (MP), producing anisotropic ion distribution functions in a high plasma β environment.In this work, nine years of in-situ Cassini data and the latest published catalogue of BS and MP crossings at Saturn were used to perform a detailed statistical analysis of magnetic fluctuations associated with mirror mode structures in Saturn’s magnetosheath. 182 single traversals through the magnetosheath between the bow shock and the magnetopause were used to study the evolution of mirror mode structures across Saturn’s outer and inner magnetosheath and to assess the effects of magnetic shear at the MP on the mirror mode structures near the MP. Violante et al. (1995) analysed two MP crossings at Saturn by Voyagers 1 and 2 and found that in the high magnetic shear MP, mirror waves appear with increasing amplitude and decreasing frequency until the MP, whilst in the other low magnetic shear case, the mirror waves ceased to grow in the presence of a plasma depletion layer but are still present, albeit with smaller amplitude.The amplitude and frequency of mirror waves near the magnetopause are also examined against signs of magnetic reconnection. Tsurutani et al. (1982) suggested that the amplitude of mirror wave structures could impact magnetic reconnection at the MP boundary as alternating high and low regions and the plasma temperature anisotropies may lead to patchy and sporadic reconnection.