Abstract

The Mach number is one of the key parameters of collisionless shocks. Understanding shock physics requires knowledge of the spatial scales in the shock transition layer. The standard methods of determining the Mach number and the spatial scales require simultaneous measurements of the magnetic field and the particle density, velocity, and temperature. While magnetic field measurements are usually of high quality and resolution, particle measurements are often either unavailable or not properly adjusted to the plasma conditions. We show that theoretical arguments can be used to overcome the limitations of observations and determine the Mach number and spatial scales of the low-Mach number shock when only magnetic field data are available.

Highlights

  • Collisionless shocks [1] are one of the most ubiquitous phenomena in space plasmas

  • The Mercury bow shock is typically a low-Mach number shock and the 20 Hz MESSENGER magnetic field measurements are sufficiently good for the application of the proposed methods [39, 40]

  • The analysis in the study is done in the normal incidence frame (NIF), that is, the shock frame in which the upstream plasma flow is along the shock normal

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Summary

INTRODUCTION

Collisionless shocks [1] are one of the most ubiquitous phenomena in space plasmas. The unfading interest in collisionless shocks is related to the fact that they are the most efficient accelerators of charged particles in the known universe [2–14]. Understanding the processes inside the shock requires, first and foremost, knowledge of the fields inside the transition layer together with their dependence on time and space Observational determination of this is not an easy problem. Determination of the Mach number and the spatial scales requires sufficiently good particle measurements. The available magnetic field measurements are typically very good It would be helpful if the Mach number and the spatial scales could be reasonably estimated using magnetic field data alone. The Mercury bow shock is typically a low-Mach number shock and the 20 Hz MESSENGER magnetic field measurements are sufficiently good for the application of the proposed methods [39, 40]. We analyze in detail two selected shock profiles

THEORETICAL BASIS FOR ESTIMATES OF THE SHOCK SCALES AND MACH NUMBER
Phase-Standing Whistler Precursor
Foot Length
Downstream Magnetic Oscillations
Distance From the Overshoot Maximum to the Undershoot Minimum
Noncoplanar Magnetic Field
BASICS OF NUMERICAL ANALYSIS
A LOW-MACH NUMBER SUBCRITICAL SHOCK
A LOW-MACH NUMBER SUPERCRITICAL SHOCK
VERIFICATION WITH MAGNETOSPHERIC MULTISCALE
Findings
DISCUSSION AND CONCLUSION

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