The Discrepancy Between Simulation and Observation of Electric Fields in Collisionless Shocks

Lynn B Wilson,Vadim Roytershteyn,Li-Jen Chen

doi:10.3389/fspas.2020.592634

Abstract

Recent time series observations of electric fields within collisionless shocks have shown that the fluctuating, electrostatic fields can be in excess of one hundred times that of the quasi-static electric fields. That is, the largest amplitude electric fields occur at high frequencies, not low. In contrast, many if not most kinetic simulations show the opposite, where the quasi-static electric fields dominate, unless they are specifically tailored to examine small-scale instabilities. Further, the shock ramp thickness is often observed to fall between the electron and ion scales while many simulations tend to produce ramp thicknesses at least at or above ion scales. This raises numerous questions about the role of small-scale instabilities and about the ability to directly compare simulations with observations.

Highlights

Collisionless shock waves are an ubiquitous phenomenon in heliospheric and astrophysical plasmas
They have been shown to have wavelengths on the order of a few to several Debye lengths7, or 10–100 s of meters near 1 AU (Fuselier and Gurnett, 1984; Breneman et al, 2013; Goodrich et al, 2018; Goodrich et al, 2019). They are thought to be driven unstable by the free energy in currents (Biskamp et al, 1972; Lemons and Gary, 1978), temperature gradients (Allan and Sanderson, 1974), electron heat flux (Dum et al, 1980; Henchen et al, 2019), or ion/ion streaming instabilities (Auer et al, 1971; Akimoto and Winske, 1985; Akimoto et al, 1985b; Goodrich et al, 2019) or they can result from a nonlinear wave-wave process (Cairns and Robinson, 1992; Dyrud and Oppenheim, 2006; Kellogg et al, 2013; Saito et al, 2017). These modes are important for collisionless shock dynamics because they can stochastically accelerate thermal electrons generating self-similar velocity distribution functions (VDFs) or the so called “flattop” distributions (Vedenov, 1963; Sagdeev, 1966; Dum et al, 1974; Dum, 1975; Dyrud and Oppenheim, 2006)
We will present two example observations made by the THEMIS (Angelopoulos, 2008) and MMS (Burch et al, 2016) missions to further illustrate the difference in magnitude between δE and Eo

Summary

INTRODUCTION

Collisionless shock waves are an ubiquitous phenomenon in heliospheric and astrophysical plasmas. They are thought to be driven unstable by the free energy in currents (Biskamp et al, 1972; Lemons and Gary, 1978), temperature gradients (Allan and Sanderson, 1974), electron heat flux (Dum et al, 1980; Henchen et al, 2019), or ion/ion streaming instabilities (Auer et al, 1971; Akimoto and Winske, 1985; Akimoto et al, 1985b; Goodrich et al, 2019) or they can result from a nonlinear wave-wave process (Cairns and Robinson, 1992; Dyrud and Oppenheim, 2006; Kellogg et al, 2013; Saito et al, 2017) These modes are important for collisionless shock dynamics because they can stochastically accelerate thermal electrons (parallel to Bo) generating self-similar velocity distribution functions (VDFs) or the so called “flattop” distributions (Vedenov, 1963; Sagdeev, 1966; Dum et al, 1974; Dum, 1975; Dyrud and Oppenheim, 2006). Part of the reason for the lack of dependence on shock geometry is that the fluctuations in the foreshock upstream of a quasi-parallel shock, for instance, locally rotate the magnetic field to quasi-perpendicular geometries and some can even locally reflect/energize particles [e.g., Wilson et al, 2013; Wilson et al, 2016]

Spacecraft Observations

Kinetic Simulations

EXAMPLE OBSERVATIONS VERSUS SIMULATIONS

DISCUSSION

DATA AVAILABILITY STATEMENT