[Paper Review] Double layer electric fields aiding the production of energetic flat-top distributions and superthermal electrons within the exhausts from magnetic reconnection

J. Egedal, W. Daughton+2 more|arXiv (Cornell University)|Apr 29, 2015

Plasma Diagnostics and Applications44 references4 citations

TL;DR

This paper demonstrates that double layer electric fields aligned with magnetic fields (E∥) in magnetic reconnection exhausts enable large-scale electron energization, producing superthermal electrons and flat-top phase-space distributions. The mechanism, driven by non-adiabatic electron dynamics when βₑ∞ < 0.02, allows sustained energization via perpendicular electric fields within E∥-confined regions, matching in situ spacecraft observations in Earth's magnetotail and explaining electron acceleration in solar flares.

ABSTRACT

Using a kinetic simulation of magnetic reconnection it was recently shown that magnetic-field-aligned electric fields (E||) can be present over large spatial scales in reconnection exhausts. The largest values of E|| are observed within double layers. The existence of double layers in the Earth's magnetosphere is well documented. In our simulation their formation is triggered by large parallel streaming of electrons into the reconnection region. These parallel electron fluxes are required for maintaining quasi-neutrality of the reconnection region and increase with decreasing values of the normalized electron pressure upstream of the reconnection region. A threshold normalized pressure is derived for strong double layers to develop. We also document how the electron confinement, provided in part by the structure in E||, allows sustained energization by perpendicular electric fields. The energization is a consequence of the confined electrons' chaotic orbital motion that includes drifts aligned with the reconnection electric field. The level of energization is proportional to the initial particle energy and therefore is enhanced by the initial energy boost of the acceleration potential, acquired by electrons entering the region. The mechanism is effective in an extended region of the reconnection exhaust allowing for the generation of superthermal electrons in reconnection scenarios, including those with only a single x-line. An expression for the phase-space distribution of the superthermal electrons is derived, providing an accurate match to the kinetic simulation results. The numerical and analytical results agree with detailed spacecraft observations recorded during reconnection events in the Earth's magnetotail.

Motivation & Objective

• To resolve the discrepancy between limited kinetic-scale dissipation regions and the large-scale electron energization observed in space plasmas.
• To investigate how electron energization can occur over extended spatial scales in single x-line magnetic reconnection.
• To identify the role of non-adiabatic parallel electron dynamics and E∥ structures in enabling sustained electron heating.
• To derive and validate an analytical model for superthermal electron phase-space distributions matching kinetic simulations and in situ observations.

Proposed method

• Kinetic particle-in-cell (PIC) simulations of anti-parallel magnetic reconnection to model electron dynamics in the exhaust.
• Identification of large-scale E∥ structures via charge separation, confirming their double layer nature with localized electric fields.
• Use of guiding center approximation to estimate electron energization rates despite chaotic electron orbits in low-B regions.
• Derivation of an analytical expression for electron energy gain per bounce orbit, linking E∥ confinement and E⊥ heating.
• Comparison of simulated electron distribution functions with in situ measurements from Earth’s magnetotail.
• Threshold derivation for double layer formation based on upstream electron beta (βₑ∞ < 0.02).

Experimental results

Research questions

• RQ1 Can superthermal electron populations be generated over large spatial scales in single x-line magnetic reconnection, beyond kinetic-scale dissipation regions?
• RQ2 What physical mechanism enables sustained electron energization in reconnection exhausts despite chaotic electron motion?
• RQ3 How do double layer electric fields (E∥) contribute to electron confinement and energization in the exhaust?
• RQ4 What is the critical threshold in upstream electron beta (βₑ∞) for strong double layer formation and effective electron energization?
• RQ5 To what extent do the simulated electron phase-space distributions match in situ spacecraft observations in Earth’s magnetotail?

Key findings

• Double layer electric fields (E∥) form over large spatial scales in reconnection exhausts when βₑ∞ < 0.02, with the strongest E∥ values localized within the double layers.
• E∥ structures confine electrons within the exhaust, enabling multiple bounce orbits and sustained energization by perpendicular electric fields (E⊥).
• Electron energization is proportional to initial particle energy and enhanced by the parallel acceleration potential eΦ∥ = e∫E∥dl, leading to superthermal electron populations.
• The derived analytical expression for the phase-space distribution of superthermal electrons matches kinetic simulation results with high accuracy.
• The simulation and analytical results are in strong agreement with in situ observations from spacecraft in Earth’s magnetotail, particularly during reconnection events.
• The mechanism is applicable to large-scale systems such as solar flares, where βₑ∞ is also expected to be low, explaining long-lived hard X-ray sources.