The Geometry of an Electron Scale Magnetic Cavity in the Plasma Sheet

Abstract

Electron scale magnetic cavities are electron vortex structures formed in turbulent plasma, while the evolution and electron dynamics of these structures have not been fully understood. Recently, high‐energy, angular, and temporal electron measurements from Magnetospheric Multiscale have enabled the application of an energetic particle sounding technique to these structures. This study analyzes an electron scale magnetic cavity observed by Magnetospheric Multiscale on 7 May 2015 in the plasma sheet. A comprehensive sounding technique is applied to obtain the geometry and propagation velocities of the boundaries. The result shows that the scale size of the structure is ∼90 km, and the leading and trailing boundaries are moving in the same direction but with different speeds (∼11.5 ± 2.2 and ∼18.1 ± 3.4 km/s, respectively). The speed difference suggests a shrinking of the structure that may play a significant role in magnetic energy dissipation and electron energization of electron scale magnetic cavities.

Statistical Properties of Sub‐Ion Magnetic Holes in the Solar Wind at 1 AU

“…The M component of the electron velocity V eM increases (decreases) on the left (right) side of the hole relative to the ambient flow. Such a bipolar electron velocity indicates an electron vortex inside the structure by assuming that its cross‐section is circular, which has been confirmed for the sub‐ion magnetic hole in the plasma sheet and magnetosheath (Liu, Zong, Zhang, Sun, et al, 2019; Liu, Zong, Zhang, Xiao, et al, 2019). The bottom three panels show that there is a distinct enhancement of the electron fluxes at 90° pitch angle in the energy range of 0.2–2 keV.…”

“…Simulations found that they can survive over 100 electron gyroperiods even in the mirror stable environment (Haynes et al, 2015). A bipolar electron velocity can be observed in the cross‐section, indicating the existence of an electron vortex in this plane by assuming that the cross‐section is circular (Haynes et al, 2015; Zhang et al, 2017), which has been confirmed based on multisatellite joint observations (Liu, Zong, Zhang, Sun, et al, 2019). Such a vortex contributed by the collective motion of the electron creates a ring‐like current inside the magnetic hole, which plays an important role in the maintenance of the hole‐like structure (Shustov et al, 2019; Zhang et al, 2017).…”

“…Our results show that there are also a lot of sub‐ion magnetic holes in the solar wind near the Earth and they may play a role in the interaction between the solar wind and Earth (Yao, Shi, Yao, Guo, et al, 2019). The bipolar feature in the electron velocity indicates the existence of an electron vortex inside the sub‐ion magnetic hole by assuming that its cross‐section is circular (Haynes et al, 2015; Huang, Sahraoui, et al, 2017), which has been confirmed in the plasma sheet and magnetosheath (Liu, Zong, Zhang, Sun, et al, 2019; Liu, Zong, Zhang, Xiao, et al, 2019). The electron vortex is essential, since an azimuthal current contributed by electrons is needed to be self‐consistent with the magnetic field depression inside the structure (Constantinescu, 2002; Haynes et al, 2015).…”

Self-consistent kinetic model of nested electron- and ion-scale magnetic cavities in space plasmas

“…The observed E y , albeit weak in magnitude, is in the same direction as the azimuthal electric current, and therefore indicates the conversion of electromagnetic energy to plasma thermal and/or kinetic energy. A possible mechanism to generate the azimuthal electric field is electromagnetic induction associated with cavity shrinkage, a process reported recently via MMS observations [57][58][59] . This could indeed happen in this event, since the observed magnetic field (in Fig.…”

Study of the Electron Velocity Inside Sub‐Ion‐Scale Magnetic Holes in the Solar Wind by MMS Observations

“…The cross‐section of the sub‐ion magnetic hole is found to be circular (Liu, Zong, Zhang, Sun, et al, 2019; Liu, Zong, Zhang, Xiao, et al, 2019). The radial direction can be regarded as the normal direction of the structure if we assume that the cross‐section is a circle; thus, V e,N is supposed to be 0.…”