PROBLEMS OF GEOCOSMOS

Proceedings of the 7th International Conference "Problems of Geocosmos" (St. Petersburg, Russia, 26-30 May 2008)
Figure 2: Large-scale structure of the diffusion region. Left top: out-of-plane current density; left bottom:
out-of-plane magnetic field. Right top: electron (thin) and ion (thick) velocity, dark: vx, gray: vz; thick dotted
line: convection flow velocity vE = c E×B
B 2 . Right bottom: components of the Ohm‘s law at x = x X(·), dark
solid: − 1
c [vi × B]z, dark dotted: − 1
c [ve × B]z, gray solid: − 1
ne ∇Pe, gray thin: Ez
Reconnection electric field, first generated near the X-line, then spreads throughout all domain and ignites
global convection. Magnetic field is frozen into plasma in inflow region and is pulled to the reconnection site
near y=0. Configuration of smaller Run 1 represents quasistationary X-point (Fig.1 , upper, out-of-plane current
is shown on background).
Existence of two distinct species in the simulation (namely, ions and electrons) suggests that their motion
is, strictly speaking, different at the regions of sharp magnetic field and flow gradients, reconnection being the
case. That fact manifests itself as appearance of the Hall term in the Ohm’s law and formation of the typical
quadrupolar pattern of out-of-plane magnetic field Bz. Value of Bz reaches its maximum of 0.2 ÷ 0.3 within
several di from the X-line. Spatial extent of the quadrupolar pattern marks ion diffusion region as the area,
where electrons and ions follow different trajectories.
Spatial extent of the X-point is considerably larger than di estimations derived earlier. This significant result
is further elaborated in recent works by [6],[8],[14] and others. Thus we increased domain size in the next Run
2 to minimize the influence of boundaries on the reconnection dynamics, keeping results of Run 1 to deepen
further into the vicinity of X-point by extracting particles distribution function there. Larger Run 2 better
describes dynamics on ion scales and shows the opening of the exhaust in outflow with formation of shock-like
structures between inflow and outflow regions. Electrons get accelerated first in EDR up to the electron Alfven
velocity cAe =
B0 √ , whereas ions remain under-Alfvenic well beyond the computational region. At Fig.
4πneme
2 (top right) magnetic field lines convection velocity vE = c E×B
B 2 is marked by thick dotted line. Left part of
the simulation box is displayed. vE velocity represents E × B drift and thus, for magnetized particles, should
control their mean velocity. Fig. 2 suggests that magnetization happens somewhere far away from the X-line
and require much larger box to observe it in simulation. In [6], [7], [8] , [9] computational boxes of the order
of 100di were implemented to show that demagnetization region stretches up to the boundary and supports
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