No 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...

Gyrokinetic investigation of the nonlinear interaction of Alfvén instabilities and energetic particle-driven geodesic acoustic modes

&lt;p&gt;Whistler waves, right-hand polarized waves with frequencies below the electron cyclotron frequency, are common in many space plasma regions such as the Earth&amp;#8217;s magnetosheath. They can be generated by electron temperature anisotropy, in which case the instability grows through cyclotron resonance. A common way to determine the stability of an electron distribution function is to compare the parallel and perpendicular temperature (with respect to the background magnetic field) to stability thresholds. However, such an approach based on the moments of the distribution function can potentially leave out some properties of the distribution which are important for wave generation.&lt;/p&gt;&lt;p&gt;In this work, we investigate the features of the electron distribution functions measured by MMS in the turbulent magnetosheath downstream of a quasi-parallel shock. We show that even though statistically whistler waves tend to occur close to the regions where the stability threshold is exceeded, they are also observed in regions predicted to be stable to wave generation. For such waves we observe that the electron pitch angle distribution often has the so-called butterfly shape (with minima in both the parallel and perpendicular directions) and is located in magnetic field minima. Using a linear numerical dispersion solver (WHAMP), we show that the butterfly distribution is unstable to whistler wave generation even though the instability threshold based on the associated moments is not exceeded. Comparison between the numerical results and waves measured by the MMS spacecraft indicate that the observed whistler waves are generated by the butterfly distribution. This phenomenon has previously been observed in mirror modes and large scale magnetic holes. Our findings show that it also occurs on smaller scales (~1 ion inertial length) in more turbulent environments, such as the quasi-parallel magnetosheath.&lt;/p&gt;