Physics – Plasma Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm13c2092l&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM13C-2092
Physics
Plasma Physics
[2723] Magnetospheric Physics / Magnetic Reconnection, [2748] Magnetospheric Physics / Magnetotail Boundary Layers, [7526] Solar Physics, Astrophysics, And Astronomy / Magnetic Reconnection, [7851] Space Plasma Physics / Shock Waves
Scientific paper
A 2-D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Particle-in-Cell (PIC) simulations are carried out for various ratios of normal magnetic field to the transverse upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). When the angle is sufficiently oblique, the simulations reveal a large firehose-sense (P||>P⊥) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a non-coplanar rotational mode. In the downstream region the firehose stability parameter ɛ =1-μ0(P||-P⊥)/B2 tends to plateau at 0.25. This balance arises from the competition between counter-streaming ions, which drives ɛ down, and the scattering due to ion inertial scale waves, which are driven unstable by the downstream rotational wave. At very oblique propagating angles, 2-D turbulence also develops in the downstream region. An explanation for the critical value 0.25 is proposed by examining anisotropic fluid theories, in particular the Anisotropic Derivative Nonlinear-Schrodinger-Burgers equations, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes, and corresponds to the lower bound of the transition point in a compound slow shock(SS)/rotational discontinuity(RD) wave. This work implies that it is a pair of compound SS/RD waves that bounds the reconnection outflow, instead of a pair of switch-off slow shocks as in Petschek's model. In large 2-D anti-parallel reconnection PIC simulations, the signature of ɛ =0.25 in the downstream exhausts which extend 200 ion-inertial scales away from the reconnection site is identified. Its implication on in-situ observations of magnetotail reconnection is discussed.
Daughton W. S.
Drake James F.
Li Handong
Liu Ya-Ying
Swisdak Michael
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