The Properties of Lion Roars and Electron Dynamics in Mirror Mode Waves Observed by the Magnetospheric MultiScale Mission

Abstract

Mirror mode waves are ubiquitous in the Earth's magnetosheath, in particular behind the quasi‐perpendicular shock. Embedded in these nonlinear structures, intense lion roars are often observed. Lion roars are characterized by whistler wave packets at a frequency ∼100 Hz, which are thought to be generated in the magnetic field minima. In this study, we make use of the high time resolution instruments on board the Magnetospheric MultiScale mission to investigate these waves and the associated electron dynamics in the quasi‐perpendicular magnetosheath on 22 January 2016. We show that despite a core electron parallel anisotropy, lion roars can be generated locally in the range 0.05–0.2fce by the perpendicular anisotropy of electrons in a particular energy range. We also show that intense lion roars can be observed up to higher frequencies due to the sharp nonlinear peaks of the signal, which appear as sharp spikes in the dynamic spectra. As a result, a high sampling rate is needed to estimate correctly their amplitude, and the latter might have been underestimated in previous studies using lower time resolution instruments. We also present for the first‐time 3‐D high time resolution electron velocity distribution functions in mirror modes. We demonstrate that the dynamics of electrons trapped in the mirror mode structures are consistent with the Kivelson and Southwood (1996) model. However, these electrons can also interact with the embedded lion roars: first signatures of electron quasi‐linear pitch angle diffusion and possible signatures of nonlinear interaction with high‐amplitude wave packets are presented. These processes can lead to electron untrapping from mirror modes.

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

“…Electrons trapped inside those structures on the electron mirror branch will, by the same reasoning (cf., e.g., Thorne & Tsurutani, 1981;Tsurutani et al, 1982Tsurutani et al, , 2011Baumjohann et al, 1999, and others), be capable of exciting the whistler instability and thus produce high frequency lion roars, still below the local electron cyclotron frequency, which in this case would be around f ∼ 0.5 − 0.7 kHz, in reasonable agreement with the majority of high intensity emissions below the local electron cyclotron frequency in Fig. 1. (It also corresponds to the MMS observations reported by Breuillard et al, 2018). These are found to coincide with the walls and maxima of the main ion mirror structures, and in some cases evolve even on top of the maxima (see the cases indicated in Fig.…”

“…They were, however, restricted to the ion mirror mode and the detection of electron-cyclotron waves (lion roars) which propagate in the whistler band deep inside the magnetic mirror configuration and are caused by trapped resonant anisotropic electrons. (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).…”

Statistical Study of the Properties of Magnetosheath Lion Roars

“…As LRs propagate from the bow shock toward the magnetopause, they play an important role in the regulation of the halo electron anisotropies in the magnetosheath. They also seem to be closely related to mirror mode structures and the regulation of the temperature distribution of the trapped electrons in these structures (Breuillard et al, ). Quasi‐linear and nonlinear particle‐wave interactions could lean to untrapping electrons from the mirror mode.…”

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

“…Overplotted on the pitch angle distributions is the trapping angle for B max = 45nT. The trapping angle is given by (Breuillard et al, ) …”

“…One of the unique features of MMS mission is the high time resolution of particle and field measurements when the spacecraft is in burst mode (Baker et al, 2016;Ergun et al, 2016;Fuselier et al, 2016;Le Contel et al, 2014;Lindqvist et al, 2014;Pollock et al, 2016;Russell et al, 2014;Torbert et al, 2016). Using burst mode MMS data, there have been recent studies of whistler waves in the magnetosheath associated with mirror mode structures (Breuillard et al, 2017). MMS satellites crossed the magnetosheath at approximately 16:20 UT on 1 December 2016.…”