Spacecraft Observations of Oblique Electron Beams Breaking the Frozen-In Law During Asymmetric Reconnection

Abstract

Fully kinetic simulations of asymmetric magnetic reconnection reveal the presence of magnetic-field-aligned beams of electrons flowing toward the topological magnetic x line. Within the ∼6d_{e} electron-diffusion region, the beams become oblique to the local magnetic field, providing a unique signature of the electron-diffusion region where the electron frozen-in law is broken. The numerical predictions are confirmed by in situ Magnetospheric Multiscale spacecraft observations during asymmetric reconnection at Earth's dayside magnetopause.

Crescent‐Shaped Electron Distributions at the Nonreconnecting Magnetopause: Magnetospheric Multiscale Observations

“…In and near EDRs observed by MMS at the dayside magnetopause, crescent-shaped agyrotropic electron distributions perpendicular to the magnetic field have been observed at the magnetospheric side of the EDR (e.g., Chen et al, 2016;Webster et al, 2018) as a result of electron meandering motion in the thin magnetic field reversal (Lapenta et al, 2017), which are consistent with particle simulations (e.g., Bessho et al, 2017;Hesse et al, 2014;Shay et al, 2016). These electron crescents can become oblique due to the presence of the parallel electric field or the electron inertia if the magnetic field lines rotate rapidly (Egedal et al, 2018). Electron crescents were also found at the magnetosheath side of the Hall region during an MMS near-EDR crossing owing to electron finite gyroradius effect.…”

“…(5) The agyrotropic electrons here are mainly distributed around the perpendicular plane. These agyrotropic distributions are explained by electron gyromotion at thin electron-scale boundaries and could be different from the EDR cases, where the transition from perpendicular crescents into parallel crescents has been observed owing to the parallel electric field and/or the electron inertia Egedal et al, 2018). Based on the observations above, we conclude that reconnection here is not active, and a similar electron-scale current sheet without bursty reconnection signatures has been reported in the magnetotail (Wang et al, 2018).…”

“…Electron crescents were also found at the magnetosheath side of the Hall region during an MMS near-EDR crossing owing to electron finite gyroradius effect. These electron crescents can become oblique due to the presence of the parallel electric field or the electron inertia if the magnetic field lines rotate rapidly (Egedal et al, 2018). Therefore, agyrotropic electron crescents, along with other signatures (such as intense current density and enhanced energy dissipation), are taken as direct observational indicators of an EDR or near-EDR event (e.g., Fuselier et al, 2017;Webster et al, 2018).…”

Pressure Tensor Elements Breaking the Frozen-In Law During Reconnection in Earth’s Magnetotail

“…NASA's Magnetospheric Multiscale (MMS) mission [14] was specially designed to characterize the fine scale structure of magnetic reconnection in Earth's magnetosphere and has successfully recorded reconnection events for a range of environments in the magnetosphere [15]. These include asymmetric reconnection events in the dayside magnetopause [16], where electron beams oblique to the magnetic lines of force break the frozen-in law [17]. However, so far, no direct observations have been reported on how the electron frozen-in law is broken during symmetric reconnection in the magnetotail.…”

Three-dimensional stability of current sheets supported by electron pressure anisotropy

“…The interpretation of measurements of reconnection regions relies heavily on comparing data to the signatures derived from kinetic numerical models (e.g., [7][8][9][10][11][12][13][14][15][16]). In recent work, we found that the details of the reconnection region, including both the electron diffusion region and the reconnection exhaust out to ion scales, depend sensitively on the parameters of the numerical simulations [17].…”