Multiple ion temperature effects on collisionless magnetic reconnection

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.