Comparing Simulated and Observed Particle Energy Distributions through Magnetic Reconnection in Earth's Magnetotail

TL;DR

This study addresses the question of how well data-driven fully kinetic simulations reproduce MMS observations of particle energization during magnetic reconnection in Earth's magnetotail. It employs fully kinetic 2D PIC simulations (ECsim/RelSIM) initialized with MMS-derived upstream parameters and systematically varies $m_i/m_e$, domain size, and upstream temperatures to assess their impact on ion and electron energy distributions, including a fit to observed nonthermal tails where applicable. The results show that the simulations capture the overall nonthermal shapes and energy transfer, with the electron distribution following a nonthermal form $f(E) \propto E^{-p}$ in practice, but they generally underestimate the very high-energy tail of the electron spectrum; numerical parameters have little effect while upstream temperatures strongly influence energization. The work highlights the value and limits of 2D data-driven simulations for magnetotail reconnection, demonstrates the need for 3D modeling and better upstream constraints to reproduce extreme energization, and suggests incorporating virtual spacecraft probes for localized MMS-like comparisons in future studies.

Abstract

Magnetic reconnection is an explosive process that accelerates particles to high energies in Earth's magnetosphere, offering a unique natural laboratory to study this phenomenon. This study investigates how well data-driven fully kinetic simulations can reproduce the ion and electron energy distributions observed during a reconnection event by the Magnetospheric Multiscale (MMS) mission.We performed fully kinetic 2D simulations initialized with plasma parameters derived from the MMS event and compared the resulting ion and electron energy distributions with observations. Key numerical and physical parameters were systematically varied to assess their influence on the resulting particle spectra. The simulations capture the overall shape and evolution of nonthermal energy distributions for both species, but generally underestimate the very high-energy tail of the electron spectrum. Variations in numerical parameters have negligible effects on the resulting spectra, while the initial upstream temperatures instead play a more pronounced role in reproducing the observed distributions.We present a novel analysis of data-driven fully kinetic simulations of MR, showing that key aspects of particle acceleration can be captured, while also highlighting the limitations of 2D simulations and the need for more realistic (e.g., 3D) setups to reproduce the observed particle energization accurately.

Figures (3)

• Figure 1: Electron energy distributions for different simulations, considering the variation in specific parameters: the mass ratio, $m_i/m_e$, in panel a); the simulation domain size, $L_x \times L_y$, in panel b); and the initial temperature ratio, $T_{0,i}/T_{0,e}$, in panel c). The dashed lines show the initial energy distributions and the solid lines represent the distributions at $t = 90\Omega_{0,i}^{-1}$.
• Figure 2: Energy distributions from MMS3 observations (gray dots) and simulation R7. Solid lines show the simulated electrons (a) and ions (b) at different times until $135\Omega_{0,i}^{-1}$, with later times in darker shades. Dotted lines represent Maxwellian fits of the core of the observed distributions, and dashed lines show the simulated energy distributions at initialization.
• Figure 3: Evolution of different energy quantities throughout the simulation $R0$ in a), calculated as the energy change $\Delta E = (E (t) - E(0))/E_\mathrm{tot}(0)$, with $E$ the specific energy quantity (magnetic or kinetic) and $E_\mathrm{tot}$ the total energy within the system. Energy distributions $f(E)\equiv\mathrm{d}N/\mathrm{d}E$ are shown for electrons (b)) and ions (c)) at different times throughout the same simulation. Both panels show the evolution until $t = 105 \Omega_{0,i}^{-1}$, with darker shades indicating later times. The dashed lines show the initial distribution of each particle species.