Mirror Mode Structures in the Kronian Magnetosheath – Instability Criterion
Date:
Abstract: We performed statistical characterisation of mirror mode waves (MMW) in Saturn’s magnetosheath based on Cassini observations.
Mirror modes are large amplitude, non-propagating compressive structures often observed in the magnetosheath which appear in the form of quasi-sinusoidal oscillations in the magnetic field, with clear magnetic dropouts (‘dips’) or enhancements (‘peaks’), they are usually accompanied by a corresponding, anticorrelated signature in plasma density. Typically, the growth of mirror mode fluctuations is triggered when magnetized plasma traverses a weak shock, producing anisotropic ion distribution functions in a high plasma environment.
Plots of distributions of MMW in time and distance from the nearest BS and MP crossing, distributions of magnetic field conditions, and distribution in local time, were produced. The results were: 1) At Saturn, MM ‘dips’ occur less frequently than at Jupiter. Their presence appears uncorrelated with local time unlike the Jovian system. This could be due to different magnetosphere sizes affecting the effective path length within the plasma flow over which MM grow, saturate and collapse. 2) At both planets, dips are generally more frequently observed closer in time and distance to the MP. This is consistent with the dips being a signature of later (collapse) stage of the evolution of MMW.
Further work will include additional data analysis, along with the use of dispersion solvers such as WHAMP will help address the questions about which locations in the magnetosheath flow are most likely to promote the appearance of MMW structures in different stages of their evolution.
Cheng, I K., Achilleos, N. “Mirror Mode Structures in the Kronian Magnetosheath – Instability Criterion”, AGU. December 12-16, 2022. Chicago, IL, US.
The poster being presented can be found here.