The Dynamics of Very High Alfvén Mach Number Shocks in Space Plasmas

Abstract

Astrophysical shocks, such as planetary bow shocks or supernova remnant shocks, are often in the high or veryhigh Mach number regime, and the structure of such shocks is crucial for understanding particle acceleration and plasma heating, as well inherently interesting. Recent magnetic field observations at Saturn's bow shock, for Alfvén Mach numbers greater than about 25, have provided evidence for periodic non-stationarity, although the details of the ion-and electron-scale processes remain unclear due to limited plasma data. High-resolution, multispacecraft data are available for the terrestrial bow shock, but here the very high Mach number regime is only attained on extremely rare occasions. Here we present magnetic field and particle data from three such quasiperpendicular shock crossings observed by the four-spacecraft Cluster mission. Although both ion reflection and the shock profile are modulated at the upstream ion gyroperiod timescale, the dominant wave growth in the foot takes place at sub-proton length scales and is consistent with being driven by the ion Weibel instability. The observed large-scale behavior depends strongly on cross-scale coupling between ion and electron processes, with ion reflection never fully suppressed, and this suggests a model of the shock dynamics that is in conflict with previous models of non-stationarity. Thus, the observations offer insight into the conditions prevalent in many inaccessible astrophysical environments, and provide important constraints for acceleration processes at such shocks.

Nonstationary Quasiperpendicular Shock and Ion Reflection at Mars

“…To subtract the solar wind contribution, we use and interpolate data from the SWIA fine mode designed to track and measure solar wind beam ions at a particular subset of energies and directions (Halekas et al, 2015). Quasiperiodic enhancements in the magnetic field and ion density are seen in panels (a) and (b), with an average period of ∼30 s, comparable to the upstream proton gyroperiod 24.2 s. Similar periodic modulations have been observed upstream of Earth and Saturn, which were attributed to the reformation of the bow shock at high Mach numbers (Sulaiman et al, 2015; Sundberg et al, 2017).…”

“…Near the shock, the solar wind is slowed down and incident ions undergoing reflection have speeds lower than the pristine solar wind, , which could explain why reflected ions are often inside the dashed ellipse. The reflection may also be nonspecular (Sundberg et al, 2017).…”

“…The fundamental physical processes of reformation in collisionless shocks are poorly understood and are far from being settled, in part due to limited in situ measurements of the processes. A few studies have shown evidence of nonstationarity and reformation at the terrestrial (Dimmock et al, 2019; Lefebvre et al, 2009; Lobzin et al, 2007; Mazelle et al, 2010; Sundberg et al, 2017), and planetary bow shocks (Shan et al, 2020; Sulaiman et al, 2015; Tiu et al, 2011). Nonstationarity can also manifest itself in the form of shock ripples formed near the shock overshoot (Johlander et al, 2016).…”

Magnetic Field Amplification by the Weibel Instability at Planetary and Astrophysical Shocks with High Mach Number

“…However, three-dimensional (3D) [29], and some two-dimensional (2D) [30,31], particle-in-cell (PIC) simulations of quasiperpendicular high-M A shocks demonstrate strong amplification of the upstream magnetic field due to the ion-ion filamentation/Weibel instability [32,33], which results from the interaction of upstream and shock-reflected ions. The mediation of high-M A shocks by the Weibel instability is also confirmed by laboratory experiments [34] and in situ measurements of the Earth's bow shock at M A ≃ 39 [35]. In this Letter, we discuss a mechanism of magnetic field amplification that is based on a realistic description of perpendicular nonrelativistic high-M A shocks and can explain the correlation between field strength and M A observed with Cassini at Saturn's bow shock.…”

Emergence of MHD structures in a collisionless PIC simulation plasma

“…Collisionless plasmas support energetic structures that are not captured by a single-fluid MHD theory and that can play a vital role in the thermalization of plasma. Examples are magnetosonic solitons [6,7] and the beams of shock-reflected particles ahead of the bow shock [8], which enforce a non-stationarity of the shock [9][10][11][12]. Single-fluid MHD simulations are nevertheless used to solve problems in collisionless plasma based on the argument that they can describe the plasma dynamics on a large enough scale.…”