Hall effect control of magnetotail dawn‐dusk asymmetry: A three‐dimensional global hybrid simulation

Abstract

Magnetotail reconnection and related phenomena (e.g., flux ropes, dipolarizing flux bundles, flow bursts, and particle injections) occur more frequently on the duskside than on the dawnside. Because this asymmetry can directly result in dawn‐dusk asymmetric space weather effects, uncovering its physical origin is important for better understanding, modeling, and prediction of the space weather phenomena. However, the cause of this pervasive asymmetry is unclear. Using three‐dimensional global hybrid simulations, we demonstrate that the Hall physics in the magnetotail current sheet is responsible for the asymmetry. The current sheet thins progressively under enhanced global convection; when its thickness reaches ion kinetic scales, some ions are decoupled from the magnetized electrons (the Hall effect). The resultant Hall electric field Ez is directed toward the neutral plane. The Hall effect is stronger (grows faster) on the duskside; i.e., more ions become unmagnetized there and do not comove with the magnetized dawnward Ez × Bx drifting electrons, thus creating a larger additional cross‐tail current intensity jy (in addition to the diamagnetic current) on the duskside, compared to the dawnside. The stronger Hall effect strength on the duskside is controlled by the higher ion temperature, thinner current sheet, and smaller normal magnetic field Bz there. These asymmetric current sheet properties are in turn controlled by two competing processes that correspond to the Hall effect: (1) the dawnward E × B drift of the magnetic flux and magnetized ions and electrons and (2) the transient motion of the unmagnetized ions which do not execute E × B drift.

Formation of Dawn‐Dusk Asymmetry in Earth's Magnetotail Thin Current Sheet: A Three‐Dimensional Particle‐In‐Cell Simulation

“…Figure shows the Hall electric field E z on the dawnside and duskside ( y = ± 10 d i ). This electric field is directed toward the neutral plane ( z = 0) in the TCS, which is consistent with previous numerical and theoretical results (e.g., Hesse et al, ; Lu et al, ; Pritchett & Coroniti, , ; Yoon & Lui, ). This electric field E z also has a dawn‐dusk asymmetry, stronger on the duskside, suggesting a stronger Hall effect on the duskside as expected from the thinner current sheet and smaller B z there.…”

Studying Dawn‐Dusk Asymmetries of Mercury's Magnetotail Using MHD‐EPIC Simulations

“…However, the Hall effect is the only mechanism that could produce dawn‐dusk asymmetry in the Hall‐A simulation, so that the Hall effect must be the reason to create higher dawnside plasma density in Hall‐A as well as the MHD‐EPIC simulations. Figure f shows that the average E z component of MHD‐EPIC‐A is stronger on the duskside, which is a key for the E × B drift explanation and is consistent with Lin et al (), Lu et al (), and Lu et al (). The MESSENGER data indicate slight proton pressure enhancement on the dawnside (Figure b), but our simulations do not show any significant preference of the proton pressure.…”

Ion motion in a polarized current sheet

“…This may explain why spacecraft observations in Earth's magnetotail have shown electron currents to be much stronger than ion currents 6,39,54 . Similar results are found in numerical simulations 28,29 . An electric field E z can play an essential role in ion energization within thin current sheets located near the reconnection region.…”