MMS Observations of Kinetic-size Magnetic Holes in the Terrestrial Magnetotail Plasma Sheet

Abstract

Kinetic-size magnetic holes (KSMHs) in the terrestrial magnetotail plasma sheet are statistically investigated using the observations from the Magnetospheric Multiscale mission. The scales of KSMHs are found to be smaller than one ion gyroradius or tens of electron gyroradii. The occurrence distributions of KSMHs have dawn–dusk asymmetry (duskside preference) in the magnetotail, which may be caused by the Hall effect. Most events of KSMHs (71.7%) are accompanied by a substorm, implying that substorms may provide favorable conditions for the excitation of KSMHs. However, there is a weak correlation between KSMHs and magnetic reconnection. The statistical results reveal that for most of the events, the electron total temperature and perpendicular temperature increase while the electron parallel temperature decreases inside the KSMHs. The electron temperature anisotropy (T e⊥/ ) is observed in 72% of KSMHs. Whistler-mode waves are frequently observed inside the KSMHs, and most (92%) KSMHs associated with whistler waves have enhancements of electron perpendicular distributions and satisfy the unstable condition of whistler instability. This suggests that the observed electron-scale whistler waves, locally generated by the electron temperature anisotropy, could couple with the electron-scale KSMHs. The observed features of KSMHs and their coupling to electron-scale whistlers are similar to the ones in the turbulent magnetosheath, implying that they are ubiquitous in the space plasmas. The generation of KSMHs in the plasma sheet could be explained by an electron vortex magnetic hole, magnetosonic solitons, and/or ballooning/interchange instabilities.

Roles of electrons and ions in formation of the current in mirror-mode structures in the terrestrial plasma sheet: Magnetospheric Multiscale observations

“…Magnetic peaks can only exist in the mirror-unstable environments, while magnetic dips are able to survive in the mirror-stable region (Kuznetsov et al, 2007;Soucek et al, 2008). The typical scales of the mirror-mode structures are 10 s ρ i in the magnetosheath (Tsurutani et al, 1982;Horbury and Lucek, 2009), where ρ i is the ion gyro radius. Based on observations of the G. Wang et al: Roles of electrons and ions in formation of the current in mirror-mode structures four Cluster satellites, the longest scales of the mirror-mode structures in the magnetosheath are found to be 2-6 times the lengths of their shortest scales, and their shapes are approximately cigar-like (Horbury and Lucek, 2009).…”

“…The typical scales of the mirror-mode structures are 10 s ρ i in the magnetosheath (Tsurutani et al, 1982;Horbury and Lucek, 2009), where ρ i is the ion gyro radius. Based on observations of the G. Wang et al: Roles of electrons and ions in formation of the current in mirror-mode structures four Cluster satellites, the longest scales of the mirror-mode structures in the magnetosheath are found to be 2-6 times the lengths of their shortest scales, and their shapes are approximately cigar-like (Horbury and Lucek, 2009). By contrast, magnetic dips with a scale less than 1 ρ i also exist in the magnetosheath as well as in the plasma sheet, and electron vortices are found inside the structure (Ge et al, 2011;Huang et al, 2017Huang et al, , 2018Huang et al, , 2019Yao et al, 2017).…”

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

“…Recently, a statistical work has also shown that ESMCs could be sources of a variety of waves in magnetized plasmas (Yao et al, ). Actually, ESMCs could be favorable to the generations of whistler waves via electron temperature anisotropy instabilities (Huang et al, ; Huang et al, ), indicating that the whistler waves can couple with ESMCs in the turbulent plasmas.…”

“…They are thought to be sheet‐like or cylindrical structures with scales less than proton thermal gyroradii (Ji et al, ; Li et al, ; Liu et al, ; Sundberg et al, ). Recent observations and simulations have shown that they are characterized by electron vortex and may facilitate generations of various kinds of waves (Huang et al, ; Yao et al, ) and are also possibly linked to energy cascade in turbulent plasma (Haynes et al, ; Roytershteyn et al, ; Sahraoui et al, , ). However, the electron dynamics and generating mechanisms are still not fully settled.…”

Ion and Electron Dynamics in the Presence of Mirror, Electromagnetic Ion Cyclotron, and Whistler Waves

“…Electrons with pitch angles ∼90°are heavily trapped in magnetic troughs associated with mirror mode waves, and these electrons are the source of whistler waves (Tsurutani et al 1982;Ahmadi et al 2018;Breuillard et al 2018). Whistler waves can be also closely connected with the kinetic-scale magnetic holes (e.g., Huang et al 2019), where the electron perpendicular anisotropy appears in magnetic dips relating to (electron-vortex) magnetic holes (e.g., Huang et al 2017aHuang et al , 2017b. Besides the temperature anisotropy, charged particles usually have different streaming velocities in the magnetosheath.…”