Heliosheath fluctuations near the perpendicular termination shock: Two‐dimensional hybrid simulations

Abstract

[1] The two-dimensional Los Alamos hybrid simulation code is used to study the excitation of fluctuations and the associated ion dynamics at the high Alfvén-Mach heliospheric termination shock and in the near-shock heliosheath. This simulation represents the electrons as a zero-mass fluid, addresses only a perpendicular shock, and considers the upstream ions to consist of a cool solar wind component and a more energetic pickup component. The shock yields two strongly heated downstream components: the more dense thermals and the more tenuous suprathermals, each with strong T ? > T k anisotropies. In the downstream, relatively homogeneous sheath plasma, linear dispersion theory predicts that each component anisotropy drives both the Alfvén-cyclotron instability and the proton mirror instability. The simulation demonstrates that Alfvén-cyclotron modes dominate mirror-like modes in the downstream, in contrast to earlier two-dimensional hybrid simulations of high-Mach quasi-perpendicular shocks without pickup ions, in which the Alfvén-cyclotron and mirror modes were excited to comparable intensities. A hypothesis is presented that the presence of pickup ions implies a relatively low magnetosonic Mach number at termination shocks, thereby favoring the excitation of the Alfvén-cyclotron instability. The primary consequence of wave-particle interactions from this instability is pitch angle scattering, so the energetic part of the ion perpendicular velocity distribution in the heliosheath is depleted by comparison with the distribution computed from a comparable one-dimensional hybrid simulation.

Simulations of pickup ion mediated quasi‐perpendicular shocks: Implications for the heliospheric termination shock

“…They revealed that most of the dissipation at a TS with > 20 % PUIs, which is a reasonable value inferred from the Voyager 2 measurement [ Richardson , ], is provided by the heating of PUIs. The results based on their one‐dimensional simulations have recently been supported by two‐dimensional simulations performed by Liu et al []. Only a few authors discussed initial acceleration processes of particles on the basis of hybrid simulations.…”

Injection Process of Pickup Ion Acceleration at an Oblique Heliospheric Termination Shock

“…Both upstream and downstream large-amplitude waves are generated. Liu et al (2010) showed, by performing a 2D hybrid simulation of a perpendicular shock including 20% PUIs, that Alfvén ion cyclotron (AIC) instability and mirror instability lead to the downstream turbulent field. The AIC waves are visible in B x and B z and mainly propagate along the downstream magnetic field, while the mirror waves have wavenumbers oblique to it and are the most visible in B y .…”

Turbulence in the Outer Heliosphere

“…It should be noted that even without the background turbulence, fluctuations at the ion scale can be self-generated due to temperature anisotropy in the downstream region immediately behind the shock (e.g., Wu et al 2009;Liu et al 2010;Wu et al 2010;Kumar et al 2018;Gedalin et al 2020), by instabilities of the shock front (e.g. Burgess et al 2016), and by proton beams in the upstream region that are associated with protons reflected from the shock if the angle between the shock normal and the magnetic field is not too large (e.g.…”