Ion Scattering and Energization in Filamentary Structures Through Earth's Magnetosheath

Abstract

The action of microphysical processes on shocked solar wind plasmas through the magnetosheath take a variety of forms (e.g., Vörös et al., 2019). Notably, statistical observations (Fuselier et al., 1994;Maruca et al., 2018) have revealed an inverse relationship between the ion temperature anisotropy and value of plasma beta. This relationship has been interpreted in terms of the adiabatic evolution of the distribution, under-going compression or expansion, and the consequent driving of the temperature anisotropy toward threshold conditions of various ion velocity space instabilities (Trávnícek et al., 2007). Diffusion in velocity space, once these thresholds are met, limits the observed anisotropy (Gary & Lee, 1994;Gary et al., 1997).In a system continuously driven by macroscale variations, such as the magnetosheath, the ion distribution may be maintained in a marginally unstable state set by the balance between the driven adiabatic response and diffusion due to wave emission.While the adiabatic response may push the ion distribution toward instability thresholds it is not the only process that may drive ion distribution evolution. The magnetosheath is pervaded by turbulent fluctuations on scales that extend well into the ion kinetic range (e.g., Chaston et al., 2013;Roberts et al., 2018;Sahraoui et al., 2006). These field structures will scatter the ion plasma leading to non-adiabatic changes in the ion distributions (Johnson & Cheng, 2001). Statistical observations suggest that this process may drive heating of magnetosheath ion distributions at a rate of ∼1 eV/s corresponding to significant enhancements in

Ion Kinetics of Plasma Flows: Earth's Magnetosheath versus Solar Wind

“…Magnetosheath magnetic field fluctuations in the ionkinetic frequency range share many properties of solar wind fluctuations dominated by kinetic Alfvén waves (Alexandrova et al. 2004, 2006Chen & Boldyrev 2017;Roberts et al 2018) and coherent structures (current sheets and discontinuities; see, e.g., Chasapis et al 2015;Chaston & Travnicek 2021) that heat and scatter ions (Chasapis et al 2015;Parashar et al 2018;Chen et al 2019;Chaston & Travnicek 2021) in analogy with the solar wind (e.g., Arzamasskiy et al 2019;Qudsi et al 2020). As in the solar wind, magnetosheath pdfs are seen to fall within the anisotropy constraints defined by kinetic instabilities (e.g., Maruca et al 2018, and references therein).…”

On the Role of Kinetic Alfven Waves in the Magnetosheath Ion Thermalization Around the Night‐Side Magnetopause

“…The ion distribution dynamics driven by ion resonant interactions with KAWs have been observed and well documented for multiple events (see C. Chaston et al., 2005; C. C. Chaston et al., 2012, 2013; C. Chaston et al., 2020; C. H. K. Chen et al., 2019; Gershman et al., 2017), whereas series of theoretical models have confirmed the key role of KAWs in magnetosheath ion scattering (C. C. Chaston & Travnicek, 2021; Cheng et al., 2020; Izutsu et al., 2012; Johnson & Cheng, 1997). Therefore, investigation of magnetosheath ion dynamics around the flank magnetopause should include effects of wave‐particle interactions.…”

“…This diffusive spreading of ion distribution is mostly due to thermalization of plasma flow: wave‐particle resonant interactions and ion demagnetization by strong gradients of wave electric fields result in pitch‐angle scattering and energy exchange between parallel and transverse degrees of freedom. Moreover, resonant interactions may also accelerate ions: Landau resonance should lead to parallel acceleration (C. H. K. Chen et al., 2019), whereas cyclotron resonances should provides cross‐field heating (Arzamasskiy et al., 2019; C. C. Chaston et al., 2013; C. C. Chaston & Travnicek, 2021; González et al., 2020). Ion temperature components, which are set as $${T}_{{\Vert }_{0}}={T}_{{\perp }_{0}}=300$$ eV initially, grow to $${\sim} 3\times {T}_{{\Vert }_{0}}\sim 1$$ keV and $$4\times {T}_{{\perp }_{0}}\sim 1.2$$ keV close to the magnetopause ( x / L ∼ −3) within the simulation time.…”

“…, five different propagation angles with respect to the ambient magnetic field θ ∈ [85°, 89°] (such very oblique propagation is typical for KAWs, see C. Chaston et al, 2008;C. C. Chaston & Travnicek, 2021) and five different phase angles ϕ ∈ [45°, 85°] (i.e., for each frequency we have 25 plane waves with different θ and ϕ).…”

“…Chen et al., 2015; Peroomian, 2003) and essentially controlled by the magnetopause current sheet configuration (see discussion in H. Hasegawa, 2012; H. Hasegawa et al., 2002; Wing et al., 2014). Moreover, the simulation of ion interactions with KAWs is further complicated by KAWs instability to filamentation (Champeaux et al., 1997; Dreher et al., 2005; Laveder et al., 2002), which leads to the formation of various nonlinear ion‐scale Alfvenic structures (e.g., Alexandrova et al., 2004; C. Chaston et al., 2020) contributing to ion scattering and heating (C. C. Chaston & Travnicek, 2021). Such a mixture of KAWs and Alfvenic structures on ion‐kinetic scales is observed as broadband electromagnetic spectra dominated by cross‐field electric fields at higher frequencies (e.g., C. Chaston et al., 2008; C. C. Chaston et al., 2013).…”