On the Presence and Thermalization of Cold Ions in the Exhaust of Antiparallel Symmetric Reconnection

Abstract

Using fully kinetic 2.5 dimensional particle-in-cell simulations of anti-parallel symmetric magnetic reconnection, we investigate how initially cold ions are captured by the reconnection process, and how they evolve and behave in the exhaust. We find that initially cold ions can remain cold deep inside the exhaust. Cold ions that enter the exhaust downstream of active separatrices, closer to the dipolarization front, appear as cold counter-streaming beams behind the front. In the off-equatorial region, these cold ions generate ion-acoustic waves that aid in the thermalization both of the incoming and outgoing populations. Closest to the front, due to the stronger magnetization, the ions can remain relatively cold during the neutral plane crossing. In the intermediate exhaust, the weaker magnetization leads to enhanced pitch angle scattering and reflection. Cold ions that enter the exhaust closer to the X line, at active separatrices, evolve into a thermalized exhaust. Here, the cold populations are heated through a combination of thermalization at the separatrices and pitch angle scattering in the curved magnetic field around the neutral plane. Depending on where the ions enter the exhaust, and how long time they have spent there, they are accelerated to different energies. The superposition of separately thermalized ion populations that have been accelerated to different energies form the hot exhaust population.

Kinetics of Cold Ions in Asymmetric Reconnection: Particle-in-Cell Simulation

“…In addition, significant positive gradient is formed in 𝑓(𝑣) as clearly shown in Figure 10. This type of VDF is unstable to trigger various plasma waves (e.g., Deng et al, 2010;Khotyaintsev et al, 2019;Toledo-Redondo et al, 2021;, which may feed back to the reconnection process and affect the cold ion dynamics. Further studies are worth performing to investigate the wave-particle interactions caused by cold ions.…”

“…During the reconnection process, cold ions can be heated or accelerated (e.g., Toledo-Redondo et al, 2016;2017;Norgren et al, 2021;Zhang et al, 2018). Toledo-Redondo et al (2016;2017;2021) conducted comprehensive investigations using Cluster and Magnetospheric Multiscale (MMS) observations to examine the effects of asymmetric reconnection on cold ions in the dayside magnetopause.…”

Asymmetrically Varying Guide Field During Magnetic Reconnection: Particle‐In‐Cell Simulations

“…A higher reconnection rate means more flux transport in the outflow, while a lower rate leads to slower transport of B z flux. These variations in formation rate and propagation speed of B z lead to the formation of flux pile‐up regions where the magnitude of B z is enhanced, when fast moving field lines catch up with slower moving field lines (Norgren et al., 2021). In Figure 7, we can see that the flux pile‐up regions form during times of increased reconnection rate.…”

Quantification of Cold-Ion Beams in a Magnetic Reconnection Jet

“…The lower-hybrid waves can contribute to the cold-ion heating. Norgren et al (2021) showed that the cold ions can be heated through a combination of thermalization at the separatrices and pitch angle scattering in the curved magnetic field around the neutral plane. Here, we calculate the temperature ( ∼320 eV) of the cold-ion beams inside the reconnection jet, which corresponds to about 6% of the total energy in the spacecraft frame.…”