[Paper Review] MMS Observations of a Compressed Current Sheet: Importance of the Ambipolar Electric Field

Ami DuBois, Chris Crabtree+3 more|arXiv (Cornell University)|Jan 19, 2022

Ionosphere and magnetosphere dynamics62 references6 citations

TL;DR

MMS observations reveal a transverse ambipolar electric field in a compressed magnetotail current sheet, driving intense E×B velocity shear that excites shear-driven lower hybrid waves. This mechanism explains non-ideal current sheet structures, non-gyrotropic particle distributions, and anomalous dissipation critical for magnetic reconnection initiation.

ABSTRACT

Spacecraft data reveals a nonuniform ambipolar electric field transverse to the magnetic field in a thin magnetotail current sheet that leads to intense ExB velocity shear and non-gyrotropic particle distributions. The ExB drift far exceeds the diamagnetic drift and drives lower hybrid waves localized to the magnetic field reversal region, which is ideally suited for the anomalous dissipation necessary for reconnection. It also reveals substructures embedded in the current density, indicating the formation of a non-ideal current sheet.

Motivation & Objective

• To investigate the role of transverse ambipolar electric fields in shaping thin, non-ideal current sheets in Earth's magnetotail.
• To determine how plasma compression leads to velocity shear and wave activity in the current sheet.
• To identify the origin of intense lower hybrid fluctuations and their link to anomalous dissipation processes.
• To establish the significance of E×B drift over diamagnetic drift in driving turbulence and wave activity.
• To demonstrate that shear-driven lower hybrid waves, not just pressure gradients, are key to current sheet dynamics.

Proposed method

• Utilized in-situ measurements from the four MMS spacecraft during a magnetotail current sheet crossing on July 3, 2017.
• Applied minimum variance analysis (MVA) to determine the current sheet normal direction (N̂), with high consistency across spacecraft (angular difference ~1°).
• Rotated magnetic, electric, and plasma data into LMN coordinates (normal, in-plane, guide field) to analyze structure relative to the current sheet.
• Employed boxcar averaging to reduce noise and infer quasi-static profiles of plasma parameters.
• Calculated ion gyro-radius (ρi = 841 km) and used it to normalize spatial scales, enabling comparison to ion-scale dynamics.
• Analyzed electric field components, particularly the transverse ambipolar field (EN), and correlated them with E×B drift and lower hybrid wave activity.

Experimental results

Research questions

• RQ1 What is the role of the ambipolar electric field in generating E×B velocity shear in a compressed current sheet?
• RQ2 How does the E×B drift compare to the diamagnetic drift in driving lower hybrid wave activity?
• RQ3 What causes the formation of non-ideal current sheet structures with substructures in current density?
• RQ4 Why are shear-driven lower hybrid waves dominant over lower hybrid drift instability in this event?
• RQ5 How does plasma compression lead to intense, localized wave activity and anomalous dissipation?

Key findings

• A transverse ambipolar electric field (EN ≈ -45 mV/m) was observed with scale size < ρi, driving strong E×B velocity shear.
• The E×B drift velocity far exceeded the diamagnetic drift, indicating dominance of ambipolar electric field effects.
• Electrostatic lower hybrid fluctuations were localized to the magnetic field reversal region, peaking at the lower hybrid frequency.
• Shear-driven lower hybrid waves were identified as the dominant instability mechanism, not pressure-gradient-driven LHDI.
• Non-gyrotropic electron distributions and vortex structures were observed, consistent with strong velocity shear and wave activity.
• The current sheet width was comparable to ρi (841 km), confirming ion-scale thinning and conditions favorable for anomalous dissipation.