Energy Partition and Balance at Dipolarization Fronts

Abstract

Dipolarization fronts (DFs) have been documented as important structures contributing to energy conversion and flux transport in Earth's magnetotail. However, energy partition and balance at DFs still remain elusive. Using high‐cadence data from NASA's Magnetospheric Multiscale mission, we preform a comprehensive investigation of energy budgets at the DFs. We find that material derivatives of particle energy densities in the DF frame are basically close to zero, indicating that particles experience negligible heating and/or acceleration and that the energy released at the DFs is predominantly manifested in the form of energy flux. The energy flux is found to be dominated by ion and electron enthalpy flux, ion heat flux, and Poynting flux, with electron enthalpy flux being locally elevated. In addition, the energy flux tends to increase during the DFs' earthward propagation, without exhibiting clear asymmetry in the dawn‐dusk direction. These results help further understand energy budgets at the DFs.

Energy Transfer and Electron Heating in Turbulent Flux Pileup Region

“…DFs have been documented as the primary regions hosting the interaction between the BBFs and the ambient plasma, such as particle heating and acceleration (e.g., Birn et al, 2013;Duan et al, 2014;Fu, Khotyaintsev, Vaivads, André, Sergeev, et al, 2012;Fu et al, 2020;Gabrielse et al, 2016;Lu et al, 2016Lu et al, , 2020Runov et al, 2015;Xu et al, 2018;Zhou et al, 2013Zhou et al, , 2018 and wave-particle interactions (e.g., Divin et al, 2015;Fu et al, 2014;Hwang et al, 2014;Khotyaintsev et al, 2011;Liu et al, 2019Liu et al, , 2021Zhou et al, 2009). Presence of these dynamical activities indicates that the DFs typically host strong energy transfer, which has been confirmed by multiple-point spacecraft measurements (e.g., Huang et al, 2015;Khotyaintsev et al, 2017;Liu, Cao, & Pollock, 2022;Liu, Fu, Vaivads, et al, 2018;Liu, Fu, Xu, et al, 2018;Yao et al, 2017;Zhong et al, 2019). DF-driven energy conversion primarily occurs right at the ion-scale front region, where intense currents and electric fields usually develop due to sharp gradients of fields and particles (e.g., Angelopoulos et al, 2013;Huang et al, 2015;Khotyaintsev et al, 2017;Liu, Fu, Vaivads, et al, 2018;Yao et al, 2017).…”

Electron‐Scale Front of Magnetic Pile‐Up Region in Reconnection Exhaust

“…The front edge of the magnetic pile‐up region is sometimes referred to as a dipolarization front if it propagates earthward or anti‐dipolarization front (ADF) in cases of tailward propagation, and also a reconnection front or jet front if it is near to or inside the reconnection site (Li et al., 2014; Liu, Vaivads, et al., 2019; Xu et al., 2021). Such fronts have been shown to be important sites for energy conversion (Liu et al., 2021, 2022; Yao et al., 2017; Zhong et al., 2019), in which electromagnetic energy is converted into particle energy. Occasionally, such energy conversion processes are associated with various plasma waves, for example, lower hybrid drift waves, whistler waves, and electrostatic waves (Divin et al., 2015; Huang et al., 2015; Hwang et al., 2014; Khotyaintsev et al., 2017; Sitnov et al., 2018).…”

Turbulent Energy Transfer at Dipolarization Fronts

“…We find that compared to the well‐recognized LET, TET hosts very different features. Due to the random nature of turbulence itself, energy transfer driven by turbulent processes is case‐sensitive: energy loads and generators equally govern local energy budgets, different from LET which is typically dominated by energy loads (e.g., Liu, Cao, & Pollock, 2022). In this sense, TET can be easily smoothed out in a superposed epoch analysis, as typically used for investigating the LET (e.g., Huang et al., 2015; Wang et al., 2020).…”

“…MMS data during 2017-2021 tail seasons have been surveyed to collect DF events. With previously-employed criterion for the DF selection (e.g., Liu, Cao, & Pollock, 2022;Schmid et al, 2011), such as magnetic field inclination angle 𝐴𝐴 𝐴𝐴 𝐴 45 • and magnetic field B z increase 𝐴𝐴 ∆𝐵𝐵𝑧𝑧 > 4 𝑛𝑛𝑛𝑛 , we have identified from the MMS burst-mode data 320 DF events, which are used to perform a statistical analysis of TET at DFs. Since separation of the MMS tetrahedron in the tail seasons is usually close to 50 km (approximately several local electron inertial lengths d e ), local energy transfer at electron scale can thus be resolved by MMS.…”

“…MMS data during 2017–2021 tail seasons have been surveyed to collect DF events. With previously‐employed criterion for the DF selection (e.g., Fu, Khotyaintsev, Vaivads, André, & Huang, 2012; Liu, Cao, & Pollock, 2022; Schmid et al., 2011), such as magnetic field inclination angle $$\theta > 45{}^{\circ}$$ and magnetic field B z increase $${\increment}{B}_{z} > 4\,nT$$, we have identified from the MMS burst‐mode data 320 DF events, which are used to perform a statistical analysis of TET at DFs.…”