Magnetic Energy Conversion and Transport in the Terrestrial Magnetotail Due to Dipolarization Fronts

Abstract

Dipolarization front (DF) is an important region involved with energy conversion and flux transport in the planet's magnetotail. Using observation data from the recent Magnetospheric Multiscale (MMS) mission, we have identified and analyzed 215 DF events. Significant magnetic energy conversion and transport are observed at the DFs, which show strong dawn-dusk asymmetry. We find that the transport process dominates the change of local magnetic energy. Nearly 70% of the events show net Poynting flux flowing into the DF region. Such effect is more significant than local magnetic energy dissipation, averagely. When DF approaches the near-Earth region, the net flow-in Poynting flux decreases and magnetic energy dissipation increases. Our results suggest that the downstream magnetic energies of transient magnetic reconnections in the midtail may be transported to the near-Earth region by one DF event after another. These intensive strikes to the magnetosphere in the near-planet region may drive stronger storms, compared with the previous knowledge about DF. Plain Language Summary Dipolarization front (DF), a sharp boundary with a scale of ion inertial length, is frequently observed at the nightside of the terrestrial magnetosphere. It is widely believed that DF plays a key role in magnetic flux transport and energy conversion in the magnetotail. Reliable data from Magnetospheric Multiscale (MMS) give us an opportunity to investigate the magnetic energy conversion and transport associated with DF. We have statistically studied 215 DF events and found that the magnetic dissipation increases when DF moves toward the near-Earth region. It is interesting that the change of local magnetic energy mainly results from the net inflow Poynting flux, which is much larger than the magnetic energy dissipation in the midtail. These results suggest that DF has a long lifetime during the transport in the magnetotail and might produce a more intense impact on the inner magnetosphere and ionosphere.

Electron‐Scale Measurements of Antidipolarization Front

“…However, the energy conversion at the ADF in this study is much larger than previously reported (Angelopoulos et al, 2013) and is comparable to that observed at the DFs Khotyaintsev et al, 2017;C. M. Liu, Fu, Vaivads, et al, 2018;Wang et al, 2020). This is possibly because the distance from Earth makes a difference on the energy conversion rates: the location of the ADF observed by MMS in this study (−19R E ) is much closer to the Earth than that reported in Angelopoulos et al (2013)…”

Energy Flux Densities at Dipolarization Fronts

“…Recent statistical studies investigating j E e ⋅ and j E i ⋅ terms which quantify energy exchange between particles and magnetic fields have suggested that energy conversion at DFs is mainly controlled by ion dynamics (j E j E i e ! ) (e.g., Wang et al, 2020;Zhong et al, 2019), that is, ion reflection ahead of the front (e.g., Zhou et al, 2018). This may lead to the expectation that energy transport is dominated by ion physics as well, as indeed suggested by recent numerical simulations which shows that ion kinetic and enthalpy flux carry the greatest energy near the DFs, although electron enthalpy flux contributes to an important portion to the energy budget (Lapenta et al, 2020).…”

“…Energy conversion at the DFs is typically dominated by energy loads ( E·J > 0), that is, electromagnetic energy (primarily magnetic field energy) being transferred into particle energy, and magnetic field energy can be converted into several energy channels (e.g., Huang et al., 2015; Khotyaintsev et al., 2017; Liu, Fu, Vaivads, et al., 2018; Sitnov et al., 2018; Wang et al., 2020; Yao et al., 2017; Zhong et al., 2019). At a general level, energy exchange between the fields and particles can be expressed by the following equation: …”

“…Energy conversion at the DFs is typically dominated by energy loads (E•J > 0), that is, electromagnetic energy (primarily magnetic field energy) being transferred into particle energy, and magnetic field energy can be converted into several energy channels (e.g., Huang et al, 2015;Khotyaintsev et al, 2017;Liu, Fu, Vaivads, et al, 2018;Sitnov et al, 2018;Wang et al, 2020;Yao et al, 2017;Zhong et al, 2019). At a general level, energy exchange between the fields and particles can be expressed by the following equation: where U B , U i e , , S, H i e , , K i e , , Q i e , , respectively, denote magnetic field energy, particle total energy, Poynting flux, particle enthalpy flux, kinetic energy flux, and heat flux (here the subscripts i and e represent ion and electron species, respectively).…”

Hall Nature Ahead of Dipolarization Fronts in the Earth's Magnetotail: A Statistical Study for MMS Data

“…All the data are presented in the Geocentric Solar Magnetospheric (GSM) coordinate system. We utilize the event list of fairly sharp DFs from Wang et al (2020). 215 events are identified by the four MMS spacecraft when they were located in the magnetotail plasma sheet, within the region−25 R E ≤ X GSM ≤ −10 R E , 𝐴𝐴 |𝑌𝑌GSM| ≤ 12 𝑅𝑅𝐸𝐸 and 𝐴𝐴 |𝑍𝑍GSM| ≤ 5 𝑅𝑅𝐸𝐸 (R E is the Earth' radius) from 1 May 2017 to 30 September 2018.…”