On the origin of “patchy” energy conversion in electron diffusion regions

Abstract

During magnetic reconnection, field lines interconnect in electron diffusion regions (EDRs). In some EDRs, the reconnection and energy conversion rates are controlled by a steady out-of-plane electric field. In other EDRs, the energy conversion rate [Formula: see text] is “patchy,” with electron-scale large-amplitude positive and negative peaks. We investigate 22 EDRs observed by NASA's Magnetospheric Multiscale mission in a wide range of conditions to determine the cause of patchy [Formula: see text]. The patchiness of the energy conversion is quantified and correlated with seven parameters describing various aspects of the asymptotic inflow regions that affect the structure, stability, and efficiency of reconnection. We find that (1) neither the guide field strength nor the asymmetries in the inflow ion pressure, electron pressure, nor number density are well correlated with the patchiness of the EDR energy conversion; (2) the out-of-plane axes of the 22 EDRs are typically fairly well aligned with the “preferred” axes, which bisect the time-averaged inflow magnetic fields and maximize the reconnection rate; and (3) the time-variability in the upstream magnetic field direction is best correlated with the patchiness of the EDR [Formula: see text]. A 3D fully kinetic simulation of reconnection with a non-uniform inflow magnetic field is analyzed; the variation in the magnetic field generates secondary X-lines, which develop to maximize the reconnection rate for the time-varying inflow magnetic field. The results suggest that magnetopause reconnection, for which the inflow magnetic field direction is often highly variable, may commonly be patchy in space, at least at the electron scale.

Evidence of a Nonorthogonal X-line in Guide-field Magnetic Reconnection

“…The idea of a preferred X-line direction has been discussed by Genestreti et al (2022) and small-angle deviations are seen in numerical simulations (Hesse et al 2013;Daughton et al 2011). To our knowledge, however, many kinetic simulations may not accept a nonorthogonal X-line due to a 2D system or boundary conditions that force an orthogonal X-line.…”

“…M is set to N × L. L obtained in our analysis is within 5°of that reported by Li et al (2021), and the M-directions are nearly coaligned. A variance analysis using differing time periods and methods (Denton et al 2018;Genestreti et al 2022) indicates an uncertainty up to 10°.…”

“…Understanding the 3D behavior of magnetic reconnection on the large scale (e.g., Priest & Schrijver 1999) and in the electron diffusion region (EDR) is a current challenge. Even though patchy reconnection (Paschmann et al 2013) and deflections of the X-line (Hesse et al 2013;Genestreti et al 2018Genestreti et al , 2022 are reported, the basic theoretical premise of quasi-2D magnetic reconnection has been that the X-line is orthogonal to the L-N plane, where L is the direction of the reversing and reconnecting B and N is normal to the current sheet. In this Letter, we explore the possibility that the X-line is not orthogonal to the L-N plane in a guide-field magnetic reconnection event.…”

Three-Dimensional Ion-Scale Magnetic Flux Rope Generated from Electron-Scale Magnetopause Current Sheet: Magnetospheric Multiscale Observations

“…The suggested patchy or transient X-line(s) on the sub-ion scale is consistent with 3D fully kinetic simulations of magnetic reconnection with guide field, as reported by Daughton et al (2011) and Nakamura et al (2016), in which turbulent reconnection generates 3D filamentary ion-scale flux ropes. On the other hand, turbulent fluctuations of the upstream magnetosheath magnetic field may also have led to the localized and/or intermittent nature of secondary X-line(s) (Genestreti et al, 2022). 4).…”

“…What is peculiar is that j • E ′ often exhibits oscillatory or bipolar features, with both positive and negative values (Genestreti et al, 2017;. Genestreti et al (2022) discussed possible causes of these intense and oscillatory energy conversion signatures and effects of upstream (magnetosheath) and boundary conditions. However, since the reconnection region geometry and spacecraft path with respect to reconnection layer structures are often hardly known from in-situ measurements, their origins are not fully understood.…”