The Self-Generation of Core Fields and Electron Scattering in Flux Ropes during Magnetic Reconnection

Two-dimensional particle-in-cell simulations with a realistic mass ratio reveal the mechanisms underlying the generation of out-of-plane magnetic fields in magnetic islands/flux ropes during magnetic reconnection. In the absence of an initial guide field, reconnection produces a large electron temperature anisotropy (∼4.5) that drives the Weibel instability inside magnetic islands and produces strong out-of-plane fields (B_z/B₀ ∼ 0.4, greatly exceeding the Hall field) with a regular bipolar structure. A space–time analysis reveals a one-to-one correspondence between the temperature anisotropy and the development of the Weibel magnetic field. The instability relaxes the anisotropy, but island merging leads to anisotropy reemergence and re-excitation. In the presence of a strong ambient guide field (B_g/B₀ = 0.5), the electron outflow from the X-point deflects along the separatrices and forms a circular current loop that wraps the flux ropes. This current generates an out-of-plane magnetic field that reinforces the ambient guide field, reaching B_z/B₀ ∼ 1.4. The current can, in some cases, drive the electron Kelvin–Helmholtz instability, which produces electron vortices and strengthens the magnetic field. These self-generated out-of-plane magnetic fields scatter electrons and reduce their temperature anisotropy, which can potentially affect electron heating via Fermi reflection. The simulation results are supported by spacecraft observations suggesting that magnetic fields can be enhanced within flux ropes in near-Earth space and in the solar wind. The presence of flux-rope core fields can also affect particle energy gain during reconnection in various astrophysical environments.