Mechanism behind magnetosheath ion field-aligned anisotropy

Establish the physical mechanism responsible for field-aligned anisotropy in magnetosheath ion populations across the magnetosheath and ascertain whether magnetic mirror forces associated with field-line draping and compression near the magnetopause can quantitatively account for the observed secondary thermal population and directional heat flux.

Background

The paper reports that magnetosheath ions exhibit persistent field-aligned anisotropy across the magnetosheath, characterized by significant heat flux along the magnetic field. Case studies and statistical analyses suggest this anisotropy is influenced by upstream parameters (IMF strength, solar wind dynamic pressure/energy flux) and by downstream magnetic field geometry relative to the magnetopause.

The authors propose a scenario where magnetic field draping and compression near the magnetopause create magnetic gradients and mirror forces that reflect magnetosheath ions away from the compression region, producing anisotropy. However, they explicitly note that the underlying cause remains unresolved and call for further investigation, including global hybrid simulations and test-particle studies.