Plasma Energization in Colliding Magnetic Flux Ropes

TL;DR

This study uses 2D PIC simulations to investigate how magnetic flux ropes merge and energize plasma, revealing multiple mechanisms contributing to energy transfer with weak dependence on system size or mass ratio.

Contribution

It provides new insights into plasma energization mechanisms during flux rope coalescence, highlighting the roles of various electric fields and pressure tensors.

Findings

Particles are accelerated during flux rope merging.

Multiple energization mechanisms contribute comparably.

Energization depends weakly on system size and mass ratio.

Abstract

Magnetic flux ropes are commonly observed throughout the heliosphere, and recent studies suggest that interacting flux ropes are associated with some energetic particle events. In this work, we carry out 2D particle-in-cell (PIC) simulations to study the coalescence of two magnetic flux ropes (or magnetic islands), and the subsequent plasma energization processes. The simulations are initialized with two magnetic islands embedded in a reconnecting current sheet. The two islands collide and eventually merge into a single island. Particles are accelerated during this process as the magnetic energy is released and converted to the plasma energy, including bulk kinetic energy increase by the ideal electric field, and thermal energy increase by the fluid compression and the non-ideal electric field. We find that contributions from these different energization mechanisms are all important and comparable with each other. Fluid shear and a non-gyrotropic pressure tensor also contribute to the energy conversion process. For simulations with different box sizes ranging from $L_x \sim 25$--$100 d_i$ and ion-to-electron mass ratios $m_i / m_e = 25$, 100 and 400, we find that the general evolution is qualitatively the same for all runs, and the energization depends only weakly on either the system size or the mass ratio. The results may help us understand plasma energization in solar and heliospheric environments.