Ion temperature anisotropy across a magnetotail reconnection jet

A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by antiparallel reconnection in conjunction with particle-in-cell simulations, showing spatial variations in the anisotropy across the outflow far (>100d ) downstream of the X line. A consistent pattern is found in both the spacecraft data and the simulations: While the total temperature across the exhaust is rather constant, near the boundaries T dominates. The plasma is well above the firehose threshold within patchy spatial regions at |B |∈[0.1,0.5]B₀, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the midplane ( |BX|≲0.1B0), T>T and ions undergo Speiser-like motion despite the large distance from the X line.