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

Abstract

In the magnetosheath, intense whistler mode waves, called "Lion roars," are often detected in troughs of magnetic field intensity in mirror mode structures. Using data obtained by the four Magnetospheric Multiscale (MMS) spacecraft, we show that reversals of gradient of magnetic field intensity along the magnetic field correspond to reversals of the field-aligned component of Poynting flux of whistler mode waves in the troughs. Such a characteristic is consistent with the idea that the whistler mode waves are effectively generated near the local minima of magnetic field intensity because of the smallest cyclotron resonance velocity and propagate toward regions of larger magnetic field intensity along the magnetic field lines on both sides. We use the reversal of the Poynting flux as an indicator of wave source regions. In these regions, we find that pancake or an outer edge of butterfly electron distributions above~100 eV are good candidates for wave generation. Unclear correlations of phase difference and amplitude variations of whistler mode waves in cases of~40 km spacecraft separation indicate that a simple plane wave approximation with a constant amplitude is not valid at this spatial scale that is much smaller than the ion gyroradius. The whistler mode waves consist of small coherent wave packets from multiple sources with spatial scales smaller than tens of electron gyroradii transverse to the background magnetic field in a mirror mode structure.

Statistical properties of kinetic-scale magnetic holes in terrestrial space

“…These structures were first identified in the solar wind (Turner et al, 1977), and were subsequently observed frequently in the planetary magnetosheath (e.g., Cattaneo et al, 1998; Balikhin et al, 2009; Tsurutani et al, 2011; Yao ST et al, 2017), in the magnetotail (e.g., Ge YS et al, 2011; Yao ST et al, 2016; Zhang XJ et al, 2017), in the magnetospheric cusp (Shi QQ et al, 2009; Jasinski et al, 2017), and even in the magnetosphere of comets (Russell et al, 1987; Plaschke et al, 2018). They have been identified under various names according to their generation mechanisms and property features, for example, “magnetic dips”, “magnetic decreases”, “mirror modes”, and “solitary waves” (e.g., Song P et al, 1992, 1994; Baumgärtel, 1999; Lucek et al, 1999; Horbury et al, 2004; Stasiewicz, 2004; Tsurutani et al, 2011; Yao ST et al, 2018b; Tian AM et al, 2018, 2020; Zhang L et al, 2018; Treumann and Baumjohann, 2019; Wang GQ et al, 2020a; Kitamura et al, 2020). Of these terms, the first two describe the observable phenomenon, while the last two emphasize the generation mechanism and nature.…”

Energy Transfer Between Hot Protons and Electromagnetic Ion Cyclotron Waves in Compressional Pc5 Ultra‐low Frequency Waves

“…The situation seems to be similar to that of whistler-mode waves that are frequently modulated by compressional structures in the magnetosheath (e.g., Ahmadi et al, 2018;Breuillard et al, 2018;Kitamura et al, 2020;Maksimovic et al, 2001;Smith et al, 1969;Smith & Tsurutani, 1976;Y. Zhang et al, 1998) and sometimes even in the magnetosphere (Baumjohann et al, 2000;Dubinin et al, 2007;W.…”

“…A clear example is the whistler mode waves modulated by compressional mirror mode waves, called lion roars, that are frequently observed in the magnetosheath (e.g., Ahmadi et al., 2018; Breuillard et al., 2018; Kitamura et al., 2020; Maksimovic et al., 2001; Smith et al., 1969; Smith & Tsurutani, 1976; Y. Zhang et al., 1998). Whistler mode waves are generated in the troughs of magnetic field intensity in the compressional mirror mode waves and cannot escape from the troughs, in cases where the amplitude of the compressional mirror mode waves is large (Kitamura et al., 2020). Thus, whistler mode waves are observed by a spacecraft only in the troughs of magnetic field intensity, and this condition causes very clear modulation (on‐off) of whistler mode waves.…”

“…Modulation of one plasma wave mode by another wave mode is an interesting topic in studies of plasma waves. A clear example is the whistler mode waves modulated by compressional mirror mode waves, called lion roars, that are frequently observed in the magnetosheath (e.g., Ahmadi et al., 2018; Breuillard et al., 2018; Kitamura et al., 2020; Maksimovic et al., 2001; Smith et al., 1969; Smith & Tsurutani, 1976; Y. Zhang et al., 1998). Whistler mode waves are generated in the troughs of magnetic field intensity in the compressional mirror mode waves and cannot escape from the troughs, in cases where the amplitude of the compressional mirror mode waves is large (Kitamura et al., 2020).…”

“…Zhang et al, 1998). Whistler mode waves are generated in the troughs of magnetic field intensity in the compressional mirror mode waves and cannot escape from the troughs, in cases where the amplitude of the compressional mirror mode waves is large (Kitamura et al, 2020). Thus, whistler mode waves are observed by a spacecraft only in the troughs of magnetic field intensity, and this condition causes very clear modulation (on-off) of whistler mode waves.…”