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

• Published in: Journal of Geophysical Research: Space Physics 2010-12, Vol.115 (A12), p.n/a
• Main Authors: Liu, Kaijun, Gary, S. Peter, Winske, Dan
• Format: Article
• Language: English
• Subjects: Alfven-cyclotron instability; Anisotropy; Astrophysics; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fluctuations; heliosheath; hybrid simulation; Ions; mirror instability; Planetology; Plasma physics; Space; termination shock; Velocity distribution
• ISSN: 0148-0227; 2169-9380; 2156-2202; 2169-9402
• DOI: 10.1029/2010JA015694

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∥ 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.