Speaker
Heli Hietala
(Imperial College London, UK)
Description
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.
Primary author
Heli Hietala
(Imperial College London, UK)
Co-authors
Jim Drake
(University of Maryland, US)
Jim McFadden
(University of California, Berkeley, US)
Jonathan Eastwood
(Imperial College London, UK)
Tai Phan
(University of California, Berkeley, US)
