Electron vortex magnetic holes: A nonlinear coherent plasma structure

Abstract

We report the properties of a novel type of sub-proton scale magnetic hole found in two dimensional particle-in-cell simulations of decaying turbulence with a guide field. The simulations were performed with a realistic value for ion to electron mass ratio. These structures, electron vortex magnetic holes (EVMHs), have circular cross-section. The magnetic field depression is associated with a diamagnetic azimuthal current provided by a population of trapped electrons in petal-like orbits. The trapped electron population provides a mean azimuthal velocity and since trapping preferentially selects high pitch angles, a perpendicular temperature anisotropy. The structures arise out of initial perturbations in the course of the turbulent evolution of the plasma, and are stable over at least 100 electron gyroperiods. We have verified the model for the EVMH by carrying out test particle and PIC simulations of isolated structures in a uniform plasma. It is found that (quasi-)stable structures can be formed provided that there is some initial perpendicular temperature anisotropy at the structure location. The properties of these structures (scale size, trapped population, etc.) are able to explain the observed properties of magnetic holes in the terrestrial plasma sheet. EVMHs may also contribute to turbulence properties, such as intermittency, at short scale lengths in other astrophysical plasmas. V C 2015 AIP Publishing LLC.

Observations of a Kinetic‐Scale Magnetic Hole in a Reconnection Diffusion Region

“…The energy of the parallel population was greater than that of the antiparallel population. This is because the Larmor radius of the higher energy electrons was larger than the size of the MH, similar to that reported by Haynes et al (2015). This is because the Larmor radius of the higher energy electrons was larger than the size of the MH, similar to that reported by Haynes et al (2015).…”

“…The electron bulk velocity V eN reverses from −400 km/s to 400 km/s (Figure 2c). This is a typical signature of electron vortex in the L-N plane, which have been reported in kinetic-scale MHs both in observations and simulation (Haynes et al, 2015;Huang, Du, et al, 2017;Yao et al, 2017). This electron vortex formed a current loop, which is manifested as the bipolar J N (Figure 2d).…”

“…These include the electron mirror mode or field-swelling instability (Gary & Karimabadi, 2006;Pokhotelov et al, 2013), solitary waves (Ji et al, 2014;Li et al, 2016;Stasiewicz et al, 2003;Yao et al, 2017), decaying turbulence (Haynes et al, 2015;Huang, Du, et al, 2017;Roytershteyn et al, 2015), and electron Kelvin-Helmholtz instability (Pritchett & Mozer, 2009). In contrast, electron vortices can be formed by the trapped electrons self-consistently within kinetic-scale MHs (Haynes et al, 2015). The magnetic field fluctuations in association with this X line were much smaller than those of the turbulence in the magnetosheath; hence, the MH was unlikely produced in decaying turbulence.…”

“…These include the electron mirror mode or field-swelling instability (Gary & Karimabadi, 2006;Pokhotelov et al, 2013), solitary waves (Ji et al, 2014;Li et al, 2016;Stasiewicz et al, 2003;Yao et al, 2017), decaying turbulence (Haynes et al, 2015;Huang, Du, et al, 2017;Roytershteyn et al, 2015), and electron Kelvin-Helmholtz instability (Pritchett & Mozer, 2009). These include the electron mirror mode or field-swelling instability (Gary & Karimabadi, 2006;Pokhotelov et al, 2013), solitary waves (Ji et al, 2014;Li et al, 2016;Stasiewicz et al, 2003;Yao et al, 2017), decaying turbulence (Haynes et al, 2015;Huang, Du, et al, 2017;Roytershteyn et al, 2015), and electron Kelvin-Helmholtz instability (Pritchett & Mozer, 2009).…”

Generation of magnetic holes in fully kinetic simulations of collisionless turbulence

“…Indeed, our small-scale three-dimensional simulations with realistic values of m i /m e demonstrated the formation of electron-scale holes with nearly identical structure. While this paper was under review, we also became aware of recent two-dimensional simulations with realistic mass ratios demonstrating the formation of small-scale magnetic holes [21] in a manner quite similar to that discussed here.…”