Ion dynamics in magnetotail reconnection in the presence of density asymmetry

We present the first dual-spacecraft observations of antiparallel, asymmetric magnetic reconnection in the Earth's magnetotail. The measurements were made with the comprehensive instrumentation of the ARTEMIS mission at the lunar distance near the dusk flank. The north lobe had high-density mantle and boundary layer plasma, while the south lobe had a much lower density, consistent with the prevailing solar wind conditions. The two spacecraft observed a finite width reconnection flow channel emanating from a tailward moving X line. Within the jet, the Hall magnetic field polarity corresponding to the high-density north lobe was enhanced, and the peak flow velocity was shifted toward the low-density south side, consistent with expectations from simulations. Two counterstreaming populations are seen in the distribution functions in most parts of the exhaust, in contrast to single-population convective flows reported in a previous asymmetric magnetotail reconnection event. Furthermore, the density difference of the two distinct mixing populations of different density is visible in the exhaust distributions up to similar to 150 d(i) downstream of the X line. We observe no signatures of Speiser-like ion motion near the neutral plane, in agreement with theoretical expectations for asymmetric boundary conditions.