Dr. Heli Hietala
Imperial College London
Ion temperature anisotropies in magnetotail reconnection jets
Magnetic reconnection redistributes energy by releasing magnetic energy into plasma kinetic energy - high speed bulk flows, heating, and particle acceleration. A significant portion of the energy released by magnetotail reconnection appears to go into ion heating, and the heating is anisotropic with the plasma temperature parallel to the magnetic field generally increasing more than the perpendicular temperature. Simulations and theory indicate that this temperature anisotropy can balance part of the magnetic tension force that accelerates the jet, and may even exceed it leading to firehose instability. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by anti-parallel reconnection in conjunction with Particle-In-Cell simulations, showing spatial variations in the anisotropy across the outflow far downstream (>100 ion inertial lengths) of the X-line. A consistent pattern is found in both the spacecraft data and the simulations: whilst the total temperature across the exhaust is rather constant, near the boundaries the parallel temperature dominates. The plasma is well-above the firehose threshold in portions of the exhaust, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. In contrast, the perpendicular temperature dominates at the mid-plane, indicating that (1) the increase in perpendicular heating is not simply the result of scattering, and (2) despite the large distance to the X-line, particles undergo Speiser-like motion. We also analyse the characteristics of the particle distributions leading to these anisotropies at different distances from the mid-plane.