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 (>100 d i ) 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 i ,|| dominates. The plasma is well above the firehose threshold within patchy spatial regions at | B X |∈[0.1,0.5] B 0 , suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the midplane ( | B X | ≲ 0 . 1 B 0 ), T i ,⊥ > T i ,|| and ions undergo Speiser‐like motion despite the large distance from the X line.