Matthew R. Argall

Spatiotemporal evolution of electron characteristics in the electron diffusion region of magnetic reconnection: Implications for acceleration and heating

Based on particle-in-cell simulations of collisionless magnetic reconnection, the spatiotemporal evolution of electron velocity distributions in the electron diffusion region (EDR) is reported to illustrate how electrons are accelerated and heated. Approximately when the reconnection rate maximizes, electron distributions in the vicinity of the X line exhibit triangular structures with discrete striations and a temperature (Te) twice that of the inflow region. Te increases as the meandering EDR populations mix with inflowing electrons. As the distance from the X line increases within the electron outflow jet, the discrete populations swirl into arcs and gyrotropize by the end of the jet with Te about 3 times that of the X line. Two dominant processes increase Te and produce the spatially and temporally evolving EDR distributions: (1) electric field acceleration preferential to electrons which meander in the EDR for longer times and (2) cyclotron turning by the magnetic field normal to the reconnection layer.

Highly structured electron anisotropy in collisionless reconnection exhausts

Results from two-dimensional particle-in-cell simulations of collisionless magnetic reconnection with zero guide field discussed in this paper reveal that around the time when the reconnection rate peaks, electron velocity distributions become highly structured in magnetic islands and open exhausts. Rings, arcs, and counterstreaming beams are generic and lasting components of the exhaust electron distributions. The temporal dependence of electron distributions provides a perspective to explain an outstanding discrepancy concerning the degree of electron anisotropy in reconnection exhausts and enables inference of the reconnection phase based on observed anisotropic electron distributions. Some of the structures predicted by our simulations are confirmed by measurements from the Cluster spacecraft during its encounter with reconnection exhausts in the magnetotail.

ACE Observations of Magnetic Waves Arising from Newborn Interstellar Pickup Helium Ions

We report low-frequency magnetic waves that were observed by the Advanced Composition Explorer (ACE) spacecraft on day-of-year (DOY) 180 of 1999 with characteristics consistent with the predictions of waves excited by newborn interstellar pickup He+ ions. This event was found by examining daily spectrograms of MAG data, a new data product that is now available to the community via the ACE Science Center. The event shown here is one of approximately 20 similar events that will be analyzed in future studies. This event is fairly typical of those we have found so far. The waves exist at spacecraft-frame frequencies between the He+ cyclotron frequency and approximately twice the H+ cyclotron frequency. Fluctuations are transverse to the mean magnetic field, are non-compressive, circularly polarized, have field-aligned minimum variance directions, and are left-hand polarized in the spacecraft frame as predicted by theory. The event lasts for just under one hour.