Magnetotail Reconnection Jets at Lunar Distances

Abstract:

Magnetic reconnection redistributes energy by releasing magnetic energy into particle energies—high speed bulk flows, heating, and particle acceleration. With near-Earth in situ observations, we have access to different parameter regimes: The magnetotail has typically a very large magnetic shear and symmetric boundary conditions. Reconnection at the magnetopause, in contrast, usually takes place under asymmetric boundary conditions and a variety of shear angles. Finally, reconnecting current sheets in the solar wind are typically large scale and not affected by nearby obstacles, and observations are typically made extremely far downstream from the X-line. As such, magnetotail reconnection, especially at lunar distances where the effect of the Earth's dipole is small, should be closest to simple models.

Ion heating has recently been studied systematically in solar wind and magnetopause reconnection, but not in the magnetotail. The energetics of magnetotail reconnection jets are particularly interesting as the available magnetic energy per particle (B_in²/μ₀n_in = m_iV_A,in²) is typically orders of magnitude higher and the inflow plasma beta much lower than in the solar wind and at the magnetopause.

We survey ARTEMIS data from 2011-2014 for fast reconnection flows and analyse their statistical properties. In particular, we address (i) the ion temperature increase (ii) ion temperature anisotropy and firehose instability, and (iii) the underlying ion dynamics. We examine the spatial structure of the ion temperature across the exhaust, and compare with particle-in-cell simulations. We find that the temperature parallel to the magnetic field dominates near the edges of the jet, while the very center of the exhaust has T_perp > T_para, indicating Speiser-like ion motion.