Waves in Kinetic‐Scale Magnetic Dips: MMS Observations in the Magnetosheath

Abstract

Kinetic‐scale magnetic dips (KSMDs), with a significant depression in magnetic field strength, and scale length close to and less than one proton gyroradius, were reported in the turbulent plasmas both in recent observation and numerical simulation studies. These KSMDs likely play important roles in energy conversion and dissipation. In this study, we present observations of the KSMDs that are labeled whistler mode waves, electrostatic solitary waves, and electron cyclotron waves in the magnetosheath. The observations suggest that electron temperature anisotropy or beams within KSMD structures provide free energy to generate these waves. In addition, the occurrence rates of the waves are higher in the center of the magnetic dips than at their edges, implying that the KSMDs might be the origin of various kinds of waves. We suggest that the KSMDs could provide favorable conditions for the generation of waves and transfer energy to the waves in turbulent magnetosheath plasmas.

Magnetohydrodynamic and kinetic scale turbulence in the near-Earth space plasmas: a (short) biased review

“…Indeed, thanks to its unprecedented time resolution [in particular that of the plasma instruments ], MMS data allow us to probe into nearly electron-scale plasma dynamics, where new types of coherent structures were evidenced along with their role energizing locally the plasma particles. This is the case of sub-ion scale magnetic holes/dips, found to be associated to intense wave activity and electron dynamics (Gershman et al 2016;Yao et al 2017Yao et al , 2019 and ion-scale magnetic islands (Huang et al 2016), which had been reported earlier in global hybrid (Karimabadi et al 2014) and PIC (Roytershteyn et al 2015) simulations. In the latter, MMS data showed intense particle dynamics and wave activity (e.g., electron beams, electrostatic solitary structures, strong electromagnetic lower hybrid drift waves) (Huang et al 2016).…”

Observations of the Source Region of Whistler Mode Waves in Magnetosheath Mirror Structures

“…Because generation of whistler mode waves in electron scale magnetic dips has been reported recently (Huang et al, 2018; Yao et al, 2019), if any further electron scale magnetic dips exist, they may be expected to make small scale whistler mode waves. In addition, the existence of electron mirror mode structures in (ion) mirror mode structures was reported by Treumann and Baumjohann (2018), although it has not been expected because of the much lower growth rate compared with that of whistler mode waves which requires the same type of electron anisotropy ( T ⊥e > T ||e ) for wave growth (Ahmadi et al, 2016; Gary & Karimabadi, 2006).…”

Electron Mirror-mode Structure: Magnetospheric Multiscale Observations

“…In recent studies, MHs with scale sizes close to ρ i (kinetic-scale magnetic hole, KSMH) were observed in the Earthʼs plasma sheet and magnetosheath (e.g., Sun et al 2012; Sundberg et al 2015; Gershman et al 2016;Goodrich et al 2016;Yao et al 2016;Zhang et al 2017). The KSMH attracts attentions because it is closely related to electron-scale physical processes (e.g., Balikhin et al 2012;Li et al 2016;Liu et al 2019), various kinds of plasma waves and particle acceleration (Yao et al 2017(Yao et al , 2019, and energy conversion and dissipation (Huang et al 2017;Yao et al 2019). The KSMH displays an isolated structure whose generation mechanism is still unclear.…”