Multiple ion temperature effects on collisionless magnetic reconnection

Aohua Mao,Zhibin Wang

doi:10.1063/5.0039137

Abstract

Plasmas with a component of low-temperature (colder) ions are frequently observed in solar-terrestrial space with plasmaspheric and ionospheric origins. In addition, in fusion plasmas, massive fueling in the burning plasma provides a colder ion source in the fueling region. Therefore, such mixtures of multiple-temperature ions are common and inevitable in plasmas. In this paper, we focus on multiple ion temperature effects on magnetic reconnection by two-dimensional particle-in-cell simulations. Two typical scenarios are discussed. From group 1 runs, it can be found that if the colder ion density is increased but the warmer ion density is kept the same, the reconnection rate is reduced and the onset of the fast reconnection is delayed, mainly due to the mass-loading effect. Meanwhile, an additional spatial scale is introduced by the multi-temperature ions to form a nested structure of diffusion regions of the electrons, as well as colder and warmer ions, which are demagnetized (inflow) and remagnetized (outflow) in different spatial positions and accelerated to different levels. For the three species, the closer accessible position from the X-point is that the higher speed can be reached by acceleration in the diffusion regions. On the other hand, from group 2 runs, it can be found that if one keeps the total background ion density fixed while only changing the ratio of the warmer to the colder ions, the mass-loading effect can then be ignored. As the colder ion proportion increases, the peak inflows and outflows of both warmer and colder ions are getting higher as more ions can get closer to the X-point, leading to the rise of the reconnection rate with reconnection characteristics undergoing a transition from the warmer ion dominant to the colder ion dominant.

Highlights

Magnetic reconnection is a process that rapidly converts large amounts of magnetic energy into kinetic and thermal energies of plasmas to accelerate heat particles with the reconfiguration of magnetic topology.1 This process has been extensively observed and studied in space, astrophysical, fusion, and other laboratory plasmas,2–5 which is related to fast growing phenomena, such as magnetospheric substorms, solar flares, and sawteeth as well as disruptions in fusion plasmas
Magnetic field lines are reconnected in a so-called “diffusion region,” where ions and electrons become demagnetized in turn and a multi-scale structure, namely, a smaller electron diffusion region surrounded by a larger ion diffusion region, is formed
Low-energy ion components of plasmaspheric and ionospheric origins have been extensively observed in different regions of the magnetosphere

Summary

INTRODUCTION

Magnetic reconnection is a process that rapidly converts large amounts of magnetic energy into kinetic and thermal energies of plasmas to accelerate heat particles with the reconfiguration of magnetic topology. This process has been extensively observed and studied in space, astrophysical, fusion, and other laboratory plasmas, which is related to fast growing phenomena, such as magnetospheric substorms, solar flares, and sawteeth as well as disruptions in fusion plasmas. Magnetic reconnection is a process that rapidly converts large amounts of magnetic energy into kinetic and thermal energies of plasmas to accelerate heat particles with the reconfiguration of magnetic topology.1 This process has been extensively observed and studied in space, astrophysical, fusion, and other laboratory plasmas, which is related to fast growing phenomena, such as magnetospheric substorms, solar flares, and sawteeth as well as disruptions in fusion plasmas. Observations and simulations suggest that the introduction of dense low-energy ions leads to an increase in the total density, a reduction in the characteristic Alfven velocity, and the Hall electric field, resulting in an impact on the reconnection rate due to the mass-loading effect..

SIMULATION SETUPS

Overview

G1: Effects of additional colder ions

G2: Impacts of the density ratio of the colder to warmer ions

CONCLUSIONS