Excitation of Whistler Waves Through the Bidirectional Field‐Aligned Electron Beams With Electron Temperature Anisotropy: MMS Observations

Abstract

Using four‐point measurements from the Magnetospheric Multiscale (MMS) mission, we identify whistler waves at the boundary of an ion scale magnetic hole, which should be locally excited rather than propagated from other regions. Based on the measured local plasma parameters, which include those for bidirectional electron beams with electron temperature anisotropy, the frequency range with the growth rate derived from kinetic theory is consistent with the observations, while the growth rate in the absence of such beams is negative; hence, the observed whistler waves are locally excited by the bidirectional electron beams with electron temperature anisotropy. The dispersion relation, derived from one‐dimensional particle‐in‐cell simulations that are conducted using the inputs of the bidirectional electron beams with electron temperature anisotropy, agrees well with the one from kinetic theory, which further supports this excitation mechanism. Our results shed light on a new excitation mechanism for whistler waves.

Statistical Characteristics of Electron Pitch Angle Distributions Inside the Magnetopasue Based on MMS Observations

“…Substorm injected electrons are almost isotropic (Åsnes et al, 2005). The isotropy distribution could also result from pitch angle scattering by whistler waves at the separatrix (Fu et al, 2012; Z. Wang et al, 2019); (5) cigar distribution (also named as bidirectional PAD) with a maximum flux around α = 0° or/and 180° but a minimum at large α (Fu et al, 2011; Fu, Yi, et al, 2020; Huang et al, 2020; Liu, Fu, Xu, Cao, & Liu, 2017; Ni et al, 2020; Zhou et al, 2019); it is probably a result of Fermi acceleration contributed by the compression of magnetic field line (Angelopoulos & Fu, 2016); (6) rolling‐pin distribution with electron fluxes mainly at α = 0°, 90°, and 180° (Fu, Zhao, et al, 2020; Liu, Fu, Xu, Wang, et al, 2017; M. J. Zhao, Fu, et al, 2019) that is due to both global‐scale Fermi acceleration and local‐scale betatron acceleration (Liu, Fu, Xu, Wang, et al, 2017).…”

Helical Magnetic Cavities: Kinetic Model and Comparison With MMS Observations

“…In some other events, donut-shaped particle pitch angle distributions (PADs) have been identified (Yao et al, 2018;Ahmadi et al, 2018), which according to Li, Zhou, et al (2020) could originate from the deepening and/or shrinking processes of the magnetic cavities (Liu et al, 2019b(Liu et al, , 2020Yao, Hamrin, et al, 2020). It is the anisotropic particle distributions that provide the free energy for excitation of various plasma waves often observed in magnetic cavities (Ahmadi et al, 2018;Huang et al, 2018Huang et al, , 2020Yao, Shi, Yao, Li, et al, 2019). In recent years, the availability of high-resolution observations from the Magnetospheric Multi-Scale (MMS) constellation (Burch et al, 2016) enables identification of electron-scale (with the radius of several electron thermal gyroradii ρ e ) magnetic cavities (Gershman et al, 2016;Huang et al, 2017;Yao et al, 2017).…”

Statistical Characteristics in the Spectrum of Whistler Waves Near the Diffusion Region of Dayside Magnetopause Reconnection

“…The electron field‐aligned beams/crescent are a natural product of magnetic reconnection (Burch et al., 2016b; Ren et al., 2019). MMS studies show that field‐aligned electron beams are a common energy source for whistler waves (S. Huang et al., 2020; Ren et al., 2019; Zhao et al., 2020). In addition to the electron field‐aligned beams, a fraction of whistler emissions may be also from 1 to 10 keV electrons that are accelerated in the pile up region on the magnetosheath side.…”