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.
Highlights
Magnetic reconnection is a fundamental plasma process that converts energy stored in the magnetic fields to plasma energy by enabling the reconfiguration of the magnetic field topology
Schriver and Ashour-Abdalla (1990) suggested that cold inflow plasma in the plasma sheet boundary layer could be heated through an ion-ion streaming instability due to the cold inbound plasma and the hotter parallel ion beams commonly observed there (e.g., Eastman et al, 1986; Nagai et al, 1998). These findings suggest that the separatrices, and the proximity to the X line, may play a role in how cold ions are heated
We focus on the distinction between the cold ions being swept up by the exhaust as it expands (e.g., Eastwood et al, 2015) and the cold ions entering the exhaust closer to the X line (e.g., Zenitani et al, 2013)
Summary
Magnetic reconnection is a fundamental plasma process that converts energy stored in the magnetic fields to plasma energy by enabling the reconfiguration of the magnetic field topology. In the vicinity of dipolarization fronts, both Nagai et al (2002) and Xu et al (2019) found, in addition to a hot component, two cold beams counter-streaming parallel to the magnetic field They proposed that the cold ions moved along the reconnected field lines directly from the lobes due to the topological change enabled by reconnection. The counter-streaming beams are attributed to Fermi acceleration of ions initially dwelling on flux tubes downstream of the front that are being swept up as the front expands past them This process is described by Eastwood et al (2015), the authors attributed the source population to pre-existing plasma sheet, not the lobes. Closer to the jet front, in the magnetic pile-up region, the radius of magnetic field curvature typically exceeded the ion gyroradii, and the inflow population could remain magnetized and formed the cold counter-streaming beams mentioned above.
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