Generation of Electron Whistler Waves at the Mirror Mode Magnetic Holes: MMS Observations and PIC Simulation

Abstract

The Magnetospheric Multiscale mission has observed electron whistler waves at the center and at the edges of magnetic holes in the dayside magnetosheath. The magnetic holes are nonlinear mirror structures since their magnitude is anticorrelated with particle density. In this article, we examine the growth mechanisms of these whistler waves and their interaction with the host magnetic hole. In the observations, as magnetic holes develop and get deeper, an electron population gets trapped and develops a temperature anisotropy favorable for whistler waves to be generated. In addition, the decrease in magnetic field magnitude and the increase in density reduce the electron resonance energy, which promotes the electron cyclotron resonance. To investigate this process, we used expanding box particle‐in‐cell simulations to produce the mirror instability, which then evolve into magnetic holes. The simulation shows that whistler waves can be generated at the center and edges of magnetic holes, which reproduces the primary features of the MMS observations. The simulation shows that the electron temperature anisotropy develops in the center of the magnetic hole once the mirror instability reaches its nonlinear stage of evolution. The plasma is then unstable to whistler waves at the minimum of the magnetic field structures. In the saturation regime of mirror instability, when magnetic holes are developed, the electron temperature anisotropy appears at the edges of the holes and electron distributions become more isotropic at the magnetic field minimum. At the edges, the expansion of magnetic holes decelerates the electrons, which leads to temperature anisotropies.

Observations of the Source Region of Whistler Mode Waves in Magnetosheath Mirror Structures

“…Whistler mode waves, which have a right‐hand polarization with frequencies below the electron cyclotron frequency in the plasma rest frame, are thought to play a crucial role in regulating the anisotropy of electrons (Gary et al, 2005; Gary & Wang, 1996; Kim et al, 2017; Tao et al, 2017), in scattering electrons, which causes precipitation into the atmosphere (e.g., Kasahara et al, 2018; Ni et al, 2008; Nishimura et al, 2010; Nishimura et al, 2011; Su et al, 2009), and accelerating electrons even up to relativistic energies in the inner magnetosphere (e.g., Horne et al, 2005; Kubota & Omura, 2018; Omura et al, 2019; Reeves et al, 2013; Thorne et al, 2013). In the terrestrial magnetosheath, intense whistler mode waves, called “Lion roars,” are often detected, and some of them are seen in the troughs of semiperiodic fluctuations of magnetic field intensity ( B ) (e.g., Ahmadi et al, 2018; Maksimovic et al, 2001; Smith et al, 1969; Smith & Tsurutani, 1976; Zhang et al, 1998). Although this type of whistler mode waves, which are seen only in the troughs of B in the magnetosheath, is focused on in the present study, similar modulation of whistler mode waves are also seen in the magnetosphere (Baumjohann et al, 2000; Dubinin et al, 2007; Li et al, 2011; Tenerani et al, 2013; Xia et al, 2016; Zhang et al, 2019).…”

“…The magnetic trap is an analogy with the dipole magnetic field, because both of them have a minimum of B (minimum‐ B ) along B and can trap charged particles in a certain pitch angle range. In magnetic bottles of mirror mode structures, whistler mode waves are thought to be generated by cyclotron instability of electrons with perpendicular anisotropy ( T ⊥e > T ||e ) (Ahmadi et al, 2018; Lee et al, 1987; Thorne & Tsurutani, 1981), where T e is the electron temperature. However, in general, it is not easy to discriminate whether a spacecraft that observes whistler mode waves had been in a wave source region or outside of it at the time of observation.…”

Mirror mode physics: Amplitude limit

“…At the one hand, if the bulk electron anisotropy T ⊥ (s)/T (s) > 1 becomes sufficiently large somewhere along the magnetic field, electrons may themselves evolve into an electron-scale electron-mirror branch (Noreen 135 et al, 2017) which appears as a small-scale low-amplitude structure on the ion-mirror mode. Such structures have recently been identified in both old (Treumann & Baumjohann, 2018b) and recent high resolution (Breuillard et al, 2018; spacecraft data. Trapped electrons may also, as will be demonstrated below, go into resonance with ion-sound waves which at finite temperature are always present in the plasma at least as thermal background.…”

Electron mirror branch: observational evidence from “historical” AMPTE-IRM and Equator-S measurements

“…(There is a wealth of literature on observations of mirror modes, large-scale electron holes, and lion roars, cf., e.g., Smith and Tsurutani, 1976;Tsurutani et al, 1982;Luehr & Kloecker N, 1987;Treumann et al, 1990;Czaykowska et al, 1998;Zhang et al, 1998;Baumjohann et al, 1999;Maksimovic et al, 2001;Constantinescu et al, 2003;Remya et al, 2014;Breuillard et al, 2018, to cite only the basic original ones, plus a few more recent papers). These observations confirmed their theoretical prediction based on fluid (cf., e.g., Chandrasekhar, 1961;Hasegawa, 1969;Thorne & Tsurutani, 1981;Southwood & Kivelson, 1993;Baumjohann & Treumann, 1996;Treumann & Baumjohann, 1997) and the substantially more elaborated kinetic theory (cf., Pokhotelov et al, 2000Pokhotelov et al, , 2002Pokhotelov et al, , 2004, and further references in Sulem, 2011), which essentially reproduces the linear fluid results, while including some additional higher order precision terms (like, for instance, finite Larmor radius effects). An attempt of modelling the final state of mirror modes by invoking pressure balance can be found in Constantinescu (2002).…”

“…For a recent unambiguous proof of their presence and the correctness of these arguments the reader is again directed toAhmadi et al (2018).…”