The Formation and Evolution of Ion Beams near Dipolarization Fronts in Magnetotail Reconnection Exhausts

Abstract:

Reconnection exhausts in Earth’s magnetotail are characterized by complex ion velocity distributions, which often exhibit two or more maxima indicative of multiple beam-like populations. 2D implicit PIC simulations of tail reconnection with a weak guide field (B_g=0.1B₀) are used to study the evolution of these ion beams between the x-point and the magnetic pileup region at the interface between the exhaust and the pre-reconnection current sheet. This pileup region, which is characterized by a local increase in B_z-GSM, is also known as a dipolarization front (DF). We follow, both forward and backward in time, the trajectories of collections of ions from the beam-like populations at different spatial locations. By doing so, we are able to determine from where these ions originate as well as to where they ultimately propagate. For example counter-streaming ion beams near the DF are found to originate on the flanks of the pre-reconnection current sheet, consistent with the model of Nagai et al. [Phys. Plasmas, 9, 3705 (2002)]. These ions get swept up via complex interactions with both the “reconnection” (E_y-GSM) and “Hall” (E_z-GSM) electric fields before crossing just upstream of the DF. After crossing, the ions in the beams alternately gain and lose energy, primarily through interactions with E_y-GSM, which has a maximum amplitude in the vicinity of the DF. The weak guide field introduces a significant North-South asymmetry in the beam trajectories. The ions in these beams, which have a small energy spread when the beams cross, ultimately develop a large spread in both energy and position, indicative of heating through interactions with the DF electric field. These simulation studies are used to help interpret recent Themis observations of multiple ion populations near the DF. These results are also relevant to the upcoming MMS mission, which will provide multipoint particle distribution measurements with unprecedented temporal resolution.