Kinetic Generation of Whistler Waves in the Turbulent Magnetosheath

Abstract

The Earth's magnetosheath (MSH) is the space plasma region downstream of the bow shock where solar wind plasma is heated, slowed down, and deflected around the magnetosphere. Numerous physical processes take place in the MSH, among which are pitch-angle scattering from wave-particle interactions (He et al., 2019), and particle trapping (Ahmadi et al., 2018;Yao et al., 2018), which shape the velocity distributions of the electrons and ions and contribute to the heating of the plasma. Understanding these processes and the conditions in which they arise is an active research topic.The conditions in the MSH depend on the geometry of the bow shock, where it is common to differentiate between two different cases: quasi-parallel (Q ‖ ) and quasi-perpendicular (Q ⊥ ). In the Q ⊥ case, the shock normal angle θ bn (the angle between the bow shock normal direction and the interplanetary magnetic field (IMF)) is larger than 45° (Balogh et al., 2005). Typical for this region is ion pressure anisotropy which favors the mirror mode instability (Dimmock et al., 2015), and particle energization is mainly caused by compression. In the Q ‖ case, θ bn < 45°. Since the MSH is magnetically connected to the IMF, it strongly interacts with the upstream transients and discontinuities hitting the bow shock. The Q ‖ MSH is characterized by significant variations in the magnetic field, particle velocity, density, and temperature. The fluctuations of the plasma parameters have larger amplitude than in the Q ⊥ case. Current sheets (Vörös et al., 2016;Yordanova et al., 2020), high speed jets (Hietala Abstract The Earth's magnetosheath (MSH) is governed by numerous physical processes which shape the particle velocity distributions and contribute to the heating of the plasma. Among them are whistler waves which can interact with electrons. We investigate whistler waves detected in the quasi-parallel MSH by NASA's Magnetospheric Multiscale mission. We find that the whistler waves occur even in regions that are predicted stable to wave growth by electron temperature anisotropy. Whistlers are observed in ion-scale magnetic minima and are associated with electrons having butterfly-shaped pitch-angle distributions. We investigate in detail one example and, with the support of modeling by the linear numerical dispersion solver Waves in Homogeneous, Anisotropic, Multicomponent Plasmas, we demonstrate that the butterfly distribution is unstable to the observed whistler waves. We conclude that the observed waves are generated locally. The result emphasizes the importance of considering complete 3D particle distribution functions, and not only the temperature anisotropy, when studying plasma wave instabilities. Plain Language SummaryThe magnetosheath (MSH) is the region downstream of the Earth's bow shock where solar wind plasma is slowed down, heated, and deflected around the magnetosphere. The angle between the interplanetary magnetic field and the bow shock normal direction determines the properties of the MSH leading to the formation of two distinct c...

Waves in Magnetosheath Jets—Classification and the Search for Generation Mechanisms Using MMS Burst Mode Data

“…They suggested that a perpendicular temperature anisotropy (T e‖ < T e⊥ ) in a particular energy range can generate whistler waves. Svenningsson et al (2022) found whistler waves in the magnetosheath in regions stable to the previous described instability. These whistler waves are found inside ion-scaled local magnetic minima and are associated with butterfly-shaped electron pitch-angle distributions.…”

“…Svenningsson et al. (2022) found whistler waves in the magnetosheath in regions stable to the previous described instability. These whistler waves are found inside ion‐scaled local magnetic minima and are associated with butterfly‐shaped electron pitch‐angle distributions.…”

“…Yao et al (2018) found that butterfly pitch-angle distribution were created by betatron cooling inside mirror modes. Svenningsson et al (2022) suggested that butterfly pitch-angle distribution can have other sources such as discontinuities or current sheets. Since we were not able to classify the 1 Hz waves we cannot determine which of the named processes are important for forming a butterfly-shaped pitch-angle distributions inside of jets.…”

“…Thorne and Tsurutani (1981) suggested that whistler mode waves in the magnetosheath are generated from the cyclotron resonance instability for anisotropic electron temperatures with T e ‖ < T e ⊥ . However, whistler waves in the magnetosheath have also been observed for butterfly‐shaped pitch‐angle distribution even though T e ‖ > T e ⊥ (Svenningsson et al., 2022). These whistler waves were also associated with magnetic field minima.…”

Statistical Distribution of Whistler Mode Waves in the Martian Induced Magnetosphere Based on MAVEN Observations

“…The occurrence rates of both left‐hand and right‐hand whistler mode waves are the highest in the subsolar region, followed by the magnetic pileup region, and the lowest in the magnetotail region. The magnetosheath, an unstable region where solar wind plasma is decelerated, heated and deflected, is rich in the anisotropic electrons, which may explain the highest occurrence rates of whistler mode waves in the subsolar region (Svenningsson et al., 2022). This also indicates that the generation mechanisms for left‐hand and right‐hand waves may be the same.…”