26pC01
Collisionless Plasma Dynamics in Driven Magnetic Rconnection
C. Z. Cheng(1), S. Inoue(2), Y. Ono(1), R. Horiuchi(3)(1) School of Frontier Sciences, University of Tokyo, Japan(2) Japan Atomic Energy Agency, Japan(3) National Institute for Fusion Science, Japan
Physics of magnetic reconnection has been studied
by using various models and experiments. Based on
the MHD model, there are two well-known
steady-state magnetic reconnection models: theSweet-Parker model and the Petschek model. In theSweet-Parker model, both electrons and ions flow
together from the upstream region through themagnetic reconnection diffusion layer into thedownstream region and the outflow velocity speeds up
to the Alfven speed by the reconnection electric field.
The reconnection rate (and the inflow speed) is
proportional to the square root of the plasma
resistivity and is too low to account for observations.
To overcome the slow reconnection rate and inflow
bottleneck problems, the Petschek model proposesthat both electrons and ions flow together mainly
across the field line separatrix into the downstream
and the plasma outflow velocity is accelerated by theJ  B force at the slow mode shock which is locatedin the downstream region. Then, the magnetic
reconnection rate can be enhanced to realistic value.
On the other hand, based on the Hall-MHD model, theelectron flow decouples from the ion flow. Theelectron flow velocity perpendicular to B is 
governed by the cE×B /B 2 drift velocity, and the ion 
flow velocity is controlled by the J × B and pressure
gradient forces. However, the charge quasi-neutrality
is assumed so that there is no electrostatic electricfield, the plasma pressure is assumed to be isotropic
and obey the adiabatic pressure law, and there is no
separate information on the electron and ion parallel
flow velocities along the magnetic field. Moreover,there is no information on plasma heating andacceleration. The main additional feature of theHall-MHD model is the generation of the quadrupole
out-of-plane magnetic field because the electron
outflow velocity is much larger than the ion outflow
velocity in the downstream region.
However, the physical processes of magnetic
reconnection based on the full kinetic model are
drastically different from those based on the MHD andHall-MHD models. In this paper we present thephysical mechanisms of key plasma dynamical
processes during driven magnetic reconnection ofanti-parallel magnetic fields in collisionless plasmas
by examining the 2-1/2 dimensional particle-in-cell(PIC) simulation results [1]. In particular, we provide

explanation on the generation of the electric andmagnetic fields, the electron and ion flow dynamics,
and acceleration and heating of electrons and ions.

Fig. 1 Poloidal distributions of (a) driving inductive electricfield E z , (b) electrostatic electric field Ees , (c) quadrupolemagnetic field B z , and (d) parallel electric field E || atcet=481.91.

One of the key kinetic reconnection physics is thedecoupling of electron and ion dynamics around themagnetic reconnection and separatrix regions. Whenoppositely directed magnetic fields are driven by the
inductive electric field E z (shown in Fig. 1(a))
toward the neutral sheet to reconnect, both theelectrons and ions convect together with the magnetic
field. Around the neutral sheet region, the particle
motion becomes meandering because the magnetic
field is weakened and reversed across the neutral sheet.
Because the ion meandering width is (Ti mi /Te me )1/4
times the electron meandering width, the electron andion orbits are decoupled. Inside the orbit meandering
region, the particle density accumulates and is roughly
uniform, and thus the ion density is larger (smaller)
than the electron density outside (inside) the electron
orbit meandering region. The charge separation1
Ees is mainly perpendicular to the ambient magnetic
field because fast moving electrons can smear out thecharge separation along the field line and thus Ees is
also perpendicular to Ez . Then, the perpendicularelectric field is E  Ees  (1  Bz2 /B2 ) Ez  ( Ez Bz /B2 ) Bp
due to both Ees and Ez .
With the physical understanding of how E es
and Bz and thus E || and E  are produced, weproduces a pair of in-plane
bipolar converging
electrostatic electric field E es pointing toward theneutral sheet (shown in Fig. 1(b)).

can understand the electron and ion dynamics andtheir velocity distributions and flow structures during
the driven reconnection as they move from the
upstream to the downstream. In particular, we address
the following key physics issues:(1) around the reconnection neutral sheet region, howelectrons and ions are accelerated/decelerated by theconverging bipolar Ees and accelerated by the
inductive Ez , and how the electron and ion inflow
velocities are slowed down;(2) why the electron outflow velocity from thereconnection region reaches super-Alfvenic speed andthe ion outflow velocity reaches Alfvenic speed;
(3) how electrons and ions are accelerated by E ||Fig. 2 Poloidal distributions of (a) electron flow velocity V e ,and (b) ion flow velocity V i . Arrows indicate poloidal
components and color indicate the z-components.

Electrons flow from the upstream region mainly
through the magnetic field reconnection region into
the downstream region as shown in Fig. 2(a).
Although some ions flow through the reconnection
current layer into the downstream, most ions flow
across the field line separatrix into the downstream asshown in Fig. 2(b). The electron outflow velocity fromthe reconnection region dominates over the ionoutflow velocity in the downstream exhaust region,and thus a pair of currents flow inward toward thereconnection region in the reconnection plane togenerate the out-of-plane quadrupole magnetic fieldB z as shown in Fig. 1(c). The quadrupole magnetic
field concentrates mainly around the separatrix region 
and causes parallel electric field E||  (Ez  Bz /B2 )B
(mainly from the inductive Ez ) to accelerate particlesalong the magnetic field line. Because E || points

around the separatrix region, and why electrons have aflat-top parallel velocity distribution in the upstream
just outside the reconnection region;
(4) how ions gain energy from both E || and E  asthey move across the separatrix region into thedownstream, and how the ion velocity distribution isthermalized in the downstream; and(5) how electrons move across the separatrix region
and from reconnection layer into the downstream andhow the electron velocity distribution is thermalized inthe downstream.
   Finally, we present the distribution of Ez •Je, Ees •Je,  Ez • Ji and Ees •Ji energy dissipation in the poloidal
plane and show that electrons gain both kinetic andthermal energy from the inductive Ez in thereconnection current layer and the separatrix region,
but spend energy to maintain Ees . Ions gain energyfrom Ez in the reconnection current layer and thedownstream regions and from E es in the separatrix
region. Therefore, the main energy source for driving
magnetic reconnection and particle accelerationand
heating is the inductive electric field Ez , which
accelerates both electrons and ions around thereconnection current layer and separatrix regions.

mainly outward from the reconnection region toward
the downstream direction as shown in Fig. 1(d),electrons are accelerated by E || around the separatrix
region to flow along the field lines toward thereconnection region.
Around the separatrix region, electrons flow very
fast mainly along the field line and ions flow mainly
across the field line slowly. The decoupling of theelectron and ion flow dynamics produces net positive(negative) charge on upstream (downstream) side ofthe separatrix and thus an electrostatic electric field
E es pointing toward the downstream midplane
direction (shown in Fig. 1(b)) is produced. Note that

[1] C. Z. Cheng, S. Inoue, Y. Ono, R. Horiuchi, Phys. Plasmas,22, 101205, doi: 10.1063/1.4932337 (2015).

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