On the Formation of Super-Alfvénic Flows Downstream of Collisionless Shocks

Abstract

Abstract Super-Alfvénic jets, with kinetic energy densities significantly exceeding that of the solar wind, are commonly generated downstream of Earth’s bow shock under both high- and low-beta plasma conditions. In this study, we present theoretical evidence that these enhanced kinetic energy flows can be driven by firehose-unstable fluctuations and compressive heating within collisionless plasma environments. Using a fluid formalism that incorporates pressure anisotropy, we estimate that the downstream flow of a collisionless plasma shock can be accelerated by a factor of 2–4 following the compression and saturation of firehose instability. By analyzing quasi-parallel magnetosheath jets observed in situ by the Magnetospheric Multiscale (MMS) mission, we find that approximately 11% of plasma measurements within these jets exhibit firehose-unstable fluctuations. Our findings offer an explanation for the distinctive generation of fast downstream flows in both low (β < 1) and high (β > 1) beta plasmas, and provide new evidence that kinetic processes are crucial for accurately describing the formation and evolution of magnetosheath jets.

BibTeX

@article{osmane2024,
  title = {On the Formation of Super-Alfvénic Flows Downstream of Collisionless Shocks},
  author = {A. Osmane, S. Raptis},
  journal = {The Astrophysical Journal},
  year = {2024},
  doi = {10.3847/1538-4357/ad8570}
}

Links

• Publisher Page
• DOI: 10.3847/1538-4357/ad8570
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