MMS Observations of Intense Whistler Waves Within Earth's Supercritical Bow Shock: Source Mechanism and Impact on Shock Structure and Plasma Transport

Abstract

The properties of whistler waves near lower‐hybrid frequencies within Earth's quasi‐perpendicular bow shock are examined using data from the Magnetospheric Multiscale (MMS) mission. These waves appear as right‐hand polarized wave packets propagating upstream obliquely to the magnetic field and shock normal with phase speeds from a few hundred up to 1,600 km/s. The wavelengths are near the ion inertial length scale (λ∼ 0.3–1.3 λi). Detailed analysis finds characteristics consistent with the modified two‐stream instability mechanism driven by the reflected ion and electron drift. Correlations between wave and electron anisotropy variations reveal that the whistlers are affecting electron dynamics and thus their perpendicular and parallel temperatures. The electron signatures are explainable via the interaction of magnetized electrons in the whistler induced nonmonotonic magnetic fields. These waves have intense magnetic fields (δB/B∘∼ 0.1–1) and carry sizable currents that are a significant fraction of the thermal current (|J/Jvte|∼ 0.1–0.5). The whistler‐induced currents and the electron anisotropies are sufficiently large to respectively excite high‐frequency (HF) electrostatic (>100 Hz) and HF whistler waves (f∼ 0.1–0.5 fce). Energy dissipation J·E from whistlers at 30 Hz and below range from a few thousandths to few hundredths of μW/m3. Comparisons reveal that plasma energy is converted to wave energy in the foot, whereas wave energy gets dissipated into the plasma in the ramp, where irreversible heating occurs. These observed features are indicative of an intricate coupling between small‐scale interaction processes and larger‐scale structure transpiring within the layer. Such a characterization is only made possible now with the MMS high‐time‐resolution measurements.

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

“…Note that similar current densities have been found using multi-spacecraft techniques(Hull et al, 2020) supporting the results inWilson III et al (2014a) andWilson III et al (2014b).13 Note that Q o in this context is not the quasi-static terms in quasi-linear or linear theory but that from the DC-coupled measurements. Further, δQ is the fluctuating terms from these theories but the AC-coupled measurements, thus there is no a priori requirement that 〈δQ〉 0.14 https://www.cfa.harvard.edu/shocks/wi_data/.Frontiers in Astronomy and Space Sciences | www.frontiersin.org January 2021 | Volume 7 | Article 592634…”

“…Prior to this study, the view by many in the community was that these fluctuating fields were completely negligible or just a minor, secondary effect. More recent, independent studies have performed similar analyses using different spacecraft and came to similar conclusions (Chen et al, 2018;Goodrich et al, 2018;Hull et al, 2020).…”

Magnetic reconnection and kinetic waves generated in the Earth's quasi-parallel bow shock

“…The electron beams accelerated within the LW waves can be the source of the secondary instability to generate whistler waves. 41,42 The electron distributions observed in the SW modes also resemble electron distributions in modified two stream instabilities (MTSIs), 43,44 which can generate oblique whistler waves with frequencies near the lower hybrid frequency, 0.07 X e in the simulation with the mass ratio of 200, although the waves by MTSIs have been discussed in quasi-perpendicular shock waves. 45,46 As shown in Fig.…”

Whistler Waves in the foot of Quasi-Perpendicular Super-Critical Shocks

“…We note that Hull et al (2020) studied whistler generation at a shock with θ Bn = 82 • and M a = 10. They found that the reflected ion beam is in resonance with the waves, generating a whistler at frequency ∼ f lh through the kinetic cross-field streaming instability.…”

“…The magnetospheric multiscale (MMS) spacecraft (Burch et al, 2016) is a constellation of 4 spacecraft in a tetrahedral formation equipped with high-resolution field and particle instruments that allows the exploration of the microphysics of collisionless shocks in an unprecedented way. Hull et al (2020) used MMS data to conduct a case study about the generation mechanism and energetics of whistler waves upstream of quasi-perpendicular and super-critical shocks, with Alfvenic Mach number M a larger than the nonlinear whistler critical Mach number M cwn , so that no precursor whistlers are to be found and non-stationary behaviour of the shock is expected (Krasnoselskikh et al, 2002). They found that the kinetic cross field streaming instability of the reflected ions with the solar wind plasma is the likely source of the observed whistlers.…”