Vlasov simulation of electrons in the context of hybrid global models: An eVlasiator approach

Abstract

<p>Modern investigations of dynamical space plasma systems such as magnetically complicated topologies within the Earth's magnetosphere make great use of supercomputer models as well as spacecraft observations. Space plasma simulations can be used to investigate energy transfer, acceleration, and plasma flows on both global and local scales. Simulation of global magnetospheric dynamics requires spatial and temporal scales achievable currently through magnetohydrodynamics or hybrid-kinetic simulations, which approximate electron dynamics as a charge-neutralizing fluid. We introduce a novel method for Vlasov-simulating electrons in the context of a hybrid-kinetic framework in order to examine the energization processes of magnetospheric electrons. Our extension of the Vlasiator hybrid-Vlasov code utilizes the global simulation dynamics of the hybrid method whilst modelling snapshots of electron dynamics on global spatial scales and temporal scales suitable for electron physics. Our eVlasiator model is shown to be stable both for single-cell and small-scale domains, and the solver successfully models Langmuir waves and Bernstein modes. We simulate a small test-case section of the near-Earth magnetotail plasma sheet region, reproducing a number of electron distribution function features found in spacecraft measurements.</p>

Electron Signatures of Reconnection in a Global eVlasiator Simulation

“…In this work, we employed the novel eVlasiator method (Battarbee et al., 2021) to model electron distributions in a global magnetosphere simulation, using an ion‐scale, geomagnetically active background from a previous Vlasiator 2D‐3V simulation. We present VDFs at the dayside magnetopause, the tail current sheet, and the PSBL and show that they have a remarkable agreement with MMS observations.…”

“…Only the electron distributions are propagated according to the Vlasov equation, which is solved in a leapfrog fashion, alternating spatial translations and velocity space accelerations. In addition to magnetic force and convective and electron pressure gradient electric fields, the electrons experience electric fields $${\vec{E}}_{{J}_{\mathrm{e}}}$$ as a result of their oscillations, solved in tandem with the electron bulk velocity $${\vec{V}}_{\mathrm{e}}$$ (Battarbee et al., 2021): $$$\delta {\vec{E}}_{{J}_{\mathrm{e}}}=\delta t\,{c}^{2}\left(\nabla \times \vec{B}+{\mu }_{0}e\left({n}_{e}{\vec{V}}_{e}-{n}_{p}{\vec{V}}_{p}\right)\right)$$$ $$$\delta {\vec{V}}_{e}=\delta t\,\frac{e}{{m}_{e}}{\vec{E}}_{{J}_{\mathrm{e}}}$$$ in a coupled Runge‐Kutta 4 scheme, with $${\vec{E}}_{{J}_{\mathrm{e}},{t}_{0}}=0$$ for the initial eVlasiator step, δt the Runge‐Kutta timestep, and c the speed of light. See Battarbee et al.…”

“…Vlasiator solves the ion Vlasov‐Maxwell system with the Darwin and quasineutral approximations and describes electrons as a charge‐neutralizing, massless fluid. eVlasiator (Battarbee et al., 2021) is a specialized solver building on the Vlasiator platform to model kinetic electrons with Vlasov methods at ion scales.…”

“…Working toward globally described electron VDFs, Battarbee et al. (2021) presented the technical details and local‐scale validation of a new electron propagation scheme, eVlasiator, which uses existing Vlasiator ion‐kinetic global simulations as a starting point for global electron simulations. The eVlasiator method propagates electron distribution functions in constant background fields, describing the VDF evolution with the Vlasov equation and including electron plasma oscillations.…”

“…In this work, we apply eVlasiator (Battarbee et al., 2021) to model the global magnetosphere for the first time. This approach probes which parts of the electron VDFs can be understood in terms of global ion‐scale physics and which parts—not modeled by eVlasiator—stem from electron‐scale kinetic effects.…”