Kinetic structure of intermediate shocks: Implications for the magnetopause
Authors: Karimabadi H. , Krauss‐Varban D. , Omidi N. 
Published: February 2004

A general study of the structure and stability of intermediate shocks (IS) in an Isotropic plasma is presented using a hybrid as well as a resistive Hall MHD code. Special emphasis is put on the question of whether the rotational layers observed at the magnetopause can be intermediate shocks. The shocks are formed dynamically by the interaction between a flowing plasma and a stationary piston. Coplanar ISs (both strong and weak) are found to be stable in a collisionless plasma. The existence of slow shocks in a high beta plasma is also established for the first time. Noncoplanar ISs are found to be time‐dependent, evolving toward a rotational discontinuity (RD) after some characteristic time τ which can be quite long (1000Ω−1, where Ω is the ion gyrofrequency). The value τ is larger the closer the rotation angle is to 180°. Since the jumps in plasma parameters are larger across a strong IS than a weak IS, the time evolution into an RD is usually, but not always, longer for a strong IS. During the course of this evolution, the scale length of the rotational layer remains fixed, but the jumps in the magnetic field, density and temperature across the shock decrease in time. Rotations larger than 180° are found to be unstable, decaying into a state of minimum shear (i.e., rotation angle less than 180°). There are various length scales associated with an IS in the kinetic regime. The shortest scale is found to be the length scale over which rotation of the transverse component of the magnetic field takes place. This scale can have a half width as small as one ion inertial length (c/ωp) for electron sense rotations and 3c/ωp for ion sense rotations, for an upstream ion beta of unity. Both of these scales are consistent with the observed thickness at the magnetopause and identical to the corresponding RD scales. A general feature of ISs is the presence of backstreaming ions consisting of both shock‐reflected ions and plasma leakage from downstream. The highest density of backstreaming ions is typically in the range of 10–20% of the far upstream plasma for strong ISs and 2–6% for the weak ISs. Higher density of backstreaming ions is possible if upstream ion beta is larger than unity and/or there is a change in the anisotropy across the discontinuity. The relative streaming between the backstreaming ions and the incoming plasma can lead to excitation of Alfvén ion cyclotron waves (A/IC). The interaction of these waves with the shock can result in cyclic shock reformation and leads to significant wave turbulence downstream. A detailed study of the mode conversion of the A/IC waves across both slow and intermediate shocks and the resulting downstream wave spectrum are presented. The possibility that the large number of reflected ions observed at the magnetopause may be due to the presence of strong ISs is considered. The identification of strong ISs and their distinction from RDs should be possible in observations due to significant differences that exist between jump conditions and overall structure of the two discontinuities. The jumps in the plasma parameters across a weak IS are typically small. This together with the fact that the weak ISs and RDs have very similar thickness and other overall properties makes the distinction between weak ISs and RDs in the observations largely inconsequential. However, at large noncoplanarity angles the weak IS approaches the RD limit in a relatively short time ≲ 100Ω−1. Thus, magnetopause rotations with large noncoplanarity angles are most likely either RDs or strong ISs. Finally, direct comparisons between fluid (resistive Hall MHD) and kinetic simulations show that fluid theory is not applicable to study of ISs in a collisionless plasma.