Various astrophysical environments involve extremely rapid and very luminous gamma-ray flares, such as the blazars and the Crab pulsar wind nebula (PWN). Particle acceleration mechanisms are needed to explain how magnetic energy is released and converted to kinetic energy. Currently, magnetic reconnection is one of the very promising candidates (for a review, see references [1, 2]).
The essential condition for triggering magnetic reconnection includes oppositely directed magnetic fields and a very thin current sheet that can be produced by certain plasma instabilities. The traditional magnetic field configuration is the Harris-layer-type kinetic equilibrium [3], in which the kinetic-scale current layer limits the outflow (reconnection rate) to the order of
Owing to the improvements in computational power, we can study the properties of magnetic reconnection by practicing numerical simulation. Based on the ABC fields, we particularly focus on the difference in the behaviors of electrons and ions, especially their kinetic energy gains.
We performed particle-in-cell (PIC) numerical simulations using the Zeltron code(1) [9] of 2-D periodic magnetic equilibria known as the ABC fields [10–12]:
where
The particle density is given by the expression
where 〈β〉 is the average dimensionless speed of ions and electrons, and
where
We investigate a wide range of relativistic ion temperatures as 10−2 ≤ Θi ≤ 102 and electron temperatures as 10−2 ≤ Θe ≤ 103. We set
For kinetic simulations, it is necessary to resolve the gyroradii of both ions and electrons at the same time. Limited by the grid sizes and resolutions of both species of particles, we choose mass ratios of μ = 10,100 instead of the real ion:electron mass ratio 1836. Previous studies configured with mass ratios either as 1836 [13] or a sequence such as 10, 25, 50, and 1836 [14]. Differences caused by the choices of the mass ratio can be ignored by simultaneously increasing the electron temperatures, because the plasma magnetization 〈σini〉∝Θe/μ in the limit Θe >> 1.
In conclusion, our configurations yield initial plasma magnetiza tion 〈σini〉 ≲ <14.38.
In Fig. 1, we present the particle momentum distributions
However, Fig. 2 shows that the other species dominates both the nonthermal particle number and the energy fractions, and both species reach a similar level of the maximum particle energy
In Fig. 3, we show the kinetic energy partition between ions and electrons. We find that a significant part of our results does not agree with those of previous studies [13, 14]. Our results show that the energy partition is strongly connected with the initial enthalpy ratio
In this contribution, we show that the initial ion–electron enthalpy ratio determines the partition of kinetic energy between the particle species, such that the species dominating the initial enthalpy also dominates the peak of the spectra and the kinetic energy gains. However, because both species reach a similar level of the maximum particle energy by the end of the simulations, the species not dominating the initial enthalpy dominates the nonthermal particle number fractions and the nonthermal energy fractions. These results differ from the findings of previous studies, because not all our results follow their predicted trends. One most likely reason is that the majority of the previous papers’ choices of the ion–electron temperature combinations are limited in or near the relation Θi = Θe/μ, but the cases other than this relation are not taken into consideration. On the other hand, our results cover relatively larger temperature combinations. Meanwhile, we are currently analyzing whether these differences may also result from using different initial magnetic field configurations.