Wave Generation and Electron Scattering During Magnetic Reconnection

Abstract

Magnetic reconnection is a fundamental process in space and astrophysical plasmas that releases magnetic energy and produces energetic particles. The resulting nonthermal electrons and anisotropic distributions provide free energy for kinetic instabilities, which generate magnetic structures and plasma waves that can significantly influence particle dynamics and transport. This dissertation employs particle-in-cell (PIC) simulations to investigate how reconnection-driven kinetic processes generate magnetic structures and waves, and how these self-generated fluctuations regulate energetic electron scattering and transport.

First, two-dimensional PIC simulations with a realistic mass ratio are used to investigate the origin of out-of-plane magnetic fields in magnetic islands (flux ropes) formed during reconnection. Without an initial guide field, reconnection generates a large electron temperature anisotropy inside magnetic islands that drives the Weibel instability, producing strong bipolar out-of-plane magnetic fields with amplitudes reaching $B_z/B_0 \sim 0.4$ with $B_0$ the amplitude of the reconnecting magnetic field. The instability relaxes the anisotropy after which island merging regenerates the free energy and re-excites the instability. With a strong guide field ($B_g/B_0 = 0.5$), electron outflows are redirected along separatrices and form a circular current loop around flux ropes. This current enhances the guide field to $B_z/B_0 \sim 1.4$ and can drive the electron Kelvin--Helmholtz instability, further amplifying the magnetic field. These self-generated core fields scatter electrons, reduce temperature anisotropy, and may influence Fermi acceleration during reconnection. The results are consistent with spacecraft observations of enhanced magnetic fields within flux ropes in the magnetotail and solar wind.

Second, PIC simulations are performed to investigate the scattering of electron heat flux by self-generated oblique whistler waves in the context of solar flares. Multiple observations suggest the inhibition of energetic-electron transport and even the confinement of electrons in the flaring region. In the simulations, an initial distribution representing a reconnection-driven electron heat flux excites strong oblique whistler waves together with filamentary Weibel-like structures. These instabilities efficiently scatter electrons, suppress heat flux, and drive the electron distribution toward isotropy. Electron trajectories show that particles with initially large parallel velocities can be scattered to negative parallel velocities. The key parameter is the ratio of the heat-flux drift speed to the electron Alfvén speed, with larger drift speeds producing stronger waves and more effective heat flux suppression. A scaling relation between the scattering rate and drift speed is also derived.

Finally, the role of the $90^\circ$ pitch-angle scattering barrier is investigated. In quasilinear theory, the pitch-angle diffusion coefficient vanishes at $90^\circ$, preventing electrons from being scattered across this boundary. As a result, an electron heat flux evolves into a nearly uniform distribution over pitch angles $0<\theta<90^\circ$, while remaining unable to diffuse into the negative parallel-velocity region. Simulations show that this highly anisotropic distribution contains substantial free energy that drives the growth of large-amplitude oblique whistler waves. These waves strongly scatter electrons near $90^\circ$ pitch angle, enabling a significant fraction of particles to cross the barrier and diffuse to negative parallel velocities. Consequently, the electron distribution evolves toward an approximately isotropic state. These findings provide new insight into the regulation of energetic-electron transport and heat flux in the solar corona, solar wind, and intracluster medium of galaxy clusters.