Structure of the Diffusion Region During Reconnection in the Magnetosphere

Abstract:

Observables depicting the reconnection diffusion region
at the magnetopause and the magnetotail
will be addressed based on
data from the four Cluster spacecraft
and particle-in-cell simulations.
Initial results from the newly launched Magnetospheric Multi-scale (MMS)
mission, when relevant,
will be discussed in light of these observables.
The DC field structure
as well as plasma flow patterns and distribution functions
in the diffusion region of anti-parallel reconnection with symmetric upstream
parameters
are dramatically modified by the presence of guide fields
and asymmetries in upstream conditions.
For example, a guide field that is 0.1 times of the reconnecting field
and a density asymmetry of 10 introduce two outstanding features
along the direction normal to the reconnection layer:
(1) significant electron outflows above and below the
the X line, and
(2) electric field pointing towards the high density side (En) present at
the X line and its close vicinity within a few electron skin depths on the low density side. For asymmetric reconnection withzero guide field, En at the X line is also finite and is mainly due to the electron pressure gradient. The primarily uni-directional En affects electron acceleration and structures of electron distribution functions. The above knowledge sets the context for the MMS mission to create new frontiers in reconnection physics with its unprecedented high resolution measurements.