Waves and Instabilities in Saturn's Magnetosheath: 2. Dispersion Relation Analysis

Abstract

The WHAMP (Rönnmark, 1982, https://inis.iaea.org/search/search.aspx?orig_q=RN:14744092)and LEOPARD (Astfalk & Jenko, 2017, https://doi.org/10.1002/2016ja023522) dispersion relation solverswere used to evaluate the growth rate and scale size for mirror mode (MM) and ion cyclotron (IC) instabilitiesunder plasma conditions resembling Saturn's magnetosheath in order to compare observations to predictionsfrom linear kinetic theory. Instabilities and waves are prevalent in planetary magnetosheaths. Understanding theorigin and conditions under which different instabilities grow and dominate can help shed light on the role eachinstability plays in influencing the plasma dynamics of the region. For anisotropic plasmas modeled with bi‐Maxwellian particle distribution, the dispersion, growth rate, and scale size of MM and IC were studied asfunctions of proton temperature anisotropy, proton plasma beta, and oxygen ion abundance. The dispersionsolvers showed that the IC mode dominated over MM under typical conditions in Saturn's magnetosheath, butthat MM could dominate for high enough O+ abundance (>40% ne). These water ion‐rich plasma conditionsare occasionally found in Saturn's magnetosheath (Sergis et al., 2013, https://doi.org/10.1002/jgra.50164). Themaximum linear growth rates (γm/Ωp)for MM ranged from 0.02 to 0.2, larger than expected from observations.The scale size at maximum growth rate ranged from 4 to 12 ρp, smaller than expected from observations. Theseinconsistencies could potentially be attributed to diffusion and non‐linear growth processes.