A New Look at the Electron Diffusion Region in Asymmetric Magnetic Reconnection

Abstract

Magnetic reconnection is arguably the most important energy conversion and transport process in collisionless plasmas. Magnetic reconnection is believed to be a key driver in astrophysical plasmas (e.g., Uzdensky, 2011), the mechanism behind solar eruptions (e.g., Antiochos et al., 1999), and it facilitates both energy entry into the magnetosphere as well as energy dissipation inside the magnetosphere (e.g., M. Hesse and Cassak, 2020). For these reasons, magnetic reconnection is also the ultimate engine behind many of the deleterious effects associated with space weather. For both reasons of basic physical understanding as well as space weather-related applications reconnection has been a prime space physics research target. This began early on in solar research and continues Abstract A new look at the structure of the electron diffusion region in collision less magnetic reconnection is presented. The research is based on a particle-in-cell simulation of asymmetric magnetic reconnection, which includes a temperature gradient across the current layer in addition to density and magnetic field gradient. We find that none of X-point, flow stagnation point, and local current density peak coincide. Current and energy balance analyses around the flow stagnation point and current density peak show consistently that current dissipation is associated with the divergence of nongyrotropic electron pressure. Furthermore, the same pressure terms, when combined with shear-type gradients of the electron flow velocity, also serve to maintain local thermal energy against convective losses. These effects are similar to those found also in symmetric magnetic reconnection. In addition, we find here significant effects related to the convection of current, which we can relate to a generalized diamagnetic drift by the nongyrotropic pressure divergence. Therefore, only part of the pressure force serves to dissipate the current density. However, the prior conclusion that the role of the reconnection electric field is to maintain the current density, which was obtained for a symmetric system, applies here as well. Finally, we discuss related features of electron distribution function in the electron diffusion region (EDR). Specifically, we analyze both new crescent substructures as well as outer, higher energy crescents generated by accelerated magnetospheric particles. Plain Language Summary Magnetic reconnection is arguably the most important mechanism to release energy stored in magnetic fields explosively. Magnetic reconnection is believed to be the driver between as diverse a set of phenomena as solar eruptions, astrophysical radiation bursts, magnetic storms in near-Earth space, and the aurora. Quite amazingly, magnetic reconnection facilitates energy conversion over huge regions of space with size of many Earth radii by means of a tiny core region, the so-called diffusion region, with dimensions of a few to a few hundreds of kilometers. The delicate interaction between charged particles and electromagnetic fields in this central re...

Distribution of Negative J·E′ in the Inflow Edge of the Inner Electron Diffusion Region During Tail Magnetic Reconnection: Simulations Vs. Observations

“…Both the inner EDR and the outer EDR can be balanced between $${E}_{\text{pres},y}$$ and $${E}_{y}^{\prime }$$ (e.g., Egedal et al., 2019). The electric field balance from the term $$-\partial {p}_{yz}/\partial z$$ is essential throughout the EDR and impels the heating and acceleration of electrons (e.g., Hesse et al., 2011, 2018, 2020). There is only one negative dip of $$\boldsymbol{J}\cdot {\boldsymbol{E}}^{\prime }$$ and $$\boldsymbol{J}\cdot {\boldsymbol{E}}_{\text{pres}}$$ corresponding with each other in the observation due to the large distance of the satellites.…”

“…Both the inner EDR and the outer EDR can be balanced between 𝐴𝐴 𝐴𝐴pres,𝑦𝑦 and 𝐴𝐴 𝐴𝐴 ′ 𝑦𝑦 (e.g., Egedal et al, 2019). The electric field balance from the term 𝐴𝐴 − 𝜕𝜕𝜕𝜕𝑦𝑦𝑦𝑦∕𝜕𝜕𝑦𝑦 is essential throughout the EDR and impels the heating and acceleration of electrons (e.g., Hesse et al, 2011Hesse et al, , 2020. There is only one negative dip of 𝐴𝐴 𝑱𝑱 ⋅ 𝑬𝑬 ′ and 𝐴𝐴 𝑱𝑱 ⋅ 𝑬𝑬pres corresponding with each other in the observation due to the large distance of the satellites.…”

Identification of Electron Diffusion Regions with a Machine Learning Approach on MMS Data at the Earth's Magnetopause

“…Null or relatively low magnetic intensities are thus theoretically expected in these regions. On the dayside of the magnetosphere, the X-point and the flow stagnation point are separate and the EDR exhibits slightly different physics at these two points (Hesse et al, 2021). Another indicator of diffusion regions is the specific terms that support the electric field among those of the generalized Ohm's law that can be written:…”

Asymmetrically Varying Guide Field During Magnetic Reconnection: Particle‐In‐Cell Simulations

“…Great progress has been made toward understanding symmetric, asymmetric, and guide field magnetic reconnection, both through modeling and observations (e.g., Burch et al., 2016; Cassak & Fuselier, 2016; Chen et al., 2017; Fuselier et al., 2017; Hesse et al., 2016, 2021; Torbert et al., 2018; Wilder et al., 2018). Several simulation studies compare how similar reconnection schemes are modified by changing one or more of the initial conditions (Dargent et al., 2020; Kolstø et al., 2020a, 2020b; Spinnangr et al., 2021; Tenfjord et al., 2019, 2020).…”