Abstract
Abstract. A new mechanism to generate whistler waves in the course of collisionless magnetic reconnection is proposed. It is found that intense whistler emissions occur in association with plasmoid collisions. The key processes are strong perpendicular heating of the electrons through a secondary magnetic reconnection during plasmoid collision and the subsequent compression of the ambient magnetic field, leading to whistler instability due to the electron temperature anisotropy. The emissions have a bursty nature, completing in a short time within the ion timescales, as has often been observed in the Earth's magnetosphere. The whistler waves can accelerate the electrons in the parallel direction, contributing to the generation of high-energy electrons. The present study suggests that the bursty emission of whistler waves could be an indicator of plasmoid collisions and the associated particle energization during collisionless magnetic reconnection.
Highlights
The present study proposes a new mechanism for the generation of whistler waves in the course of collisionless magnetic reconnection
The present model can explain the bursty nature of whistler emissions as observed in the Earth’s magnetosphere (e.g., Deng and Matsumoto, 2001; Wei et al, 2007; Tang et al, 2013; Graham et al, 2016; Huang et al, 2016; Zhao et al, 2016)
The perpendicular electron heating occurs during a secondary reconnection forced by the plasmoid collision with the current sheet formed downstream of the main reconnection x-line
Summary
Whistler waves are fundamental plasma waves frequently observed in space in association with transient phenomena, such as collisionless shocks (e.g., Olson et al, 1969; Rodriguez and Gurnett, 1975; Lengyel-Frey et al, 1996; Zhang et al, 1999; Hull et al, 2012) and magnetic reconnection (e.g., Deng and Matsumoto, 2001; Wei et al, 2007; Tang et al, 2013; Graham et al, 2016; Huang et al, 2016; Zhao et al, 2016; Uchino et al, 2017). The waves have a righthand polarization with respect to the ambient magnetic field, so they can couple with the electrons through the cyclotron resonance and give rise to pitch-angle scattering and parallel acceleration (e.g., Kennel and Petschek, 1966; Gary and Wang, 1996; Schreiner et al, 2017) Such microscopic wave– particle interactions can cause anomalous transport in momentum and energy, resulting in anomalous magnetic dissipation in collisionless plasma where the classical Coulomb collision is negligibly weak. The intense electron beam can be coupled with the stationary background electrons, which directly (Fujimoto, 2014) or indirectly (Goldman et al, 2014) triggers obliquely propagating whistlers In both regions, the whistler emissions occur within a very narrow extent across the field lines, which explains the bursty nature in the observations. The resultant whistler emission is bursty and has characteristics different from other emissions, so it can work as an indicator of plasmoid and flux rope collisions
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