04aB01
多階層物理に支配されたエネルギー開放系での磁気リコネクション
Magnetic Reconnection Controlled by Multi-Hierarchy Physics in an OpenSystem堀内 利得1),2)､大谷寛明1),2)、宇佐見俊介1)
HORIUCHI Ritoku, OHTANI Hiroaki, USAMI Shunsuke
核融合研、2)総研大

1)
1)NIFS, 2) Sokendai

Magnetic reconnection is a typical nonlinear
complexphenomenoncontrolledby
multi-hierarchy physics from microscopic physics
relating to electron and ion dynamics throughmacroscopic one such as plasma transport in a
global scale. We have developed two different
types of simulation models in order to investigate
multi-hierarchyphysicsof
collisionlessreconnection in an open system where an external
driving plasma flow and a strong guide field exist.
One is an electromagnetic PIC simulation modelfor a microscopic open system (“PASMO”) whichenables us to simulate long time-scale behavior ofcollisionless reconnection [1-4]. The other is an
MHD-PIC interlocked model (“MARIS”) in which
a simulation region is split into PIC, MHD, andinterface domains, and the micro- andmacrophysics in each domain is solved with PIC
and MHD models while exchanging theirinformation through the interface domain [4,5].
Long time-scale behaviors of collisionlessreconnection have been disclosed by a series ofsimulation studies using the PASMO codes. The
frozen-in condition is broken due to microscopic
kinetic effects and collisionless reconnection istriggered when current sheet is compressed as thin
as ion kinetic scales under the influence of external
driving flow. A reconnection system evolves into aquasi-steady state after an initial transient phase if
the driving flow satisfies some condition [2,3]. In a
quasi-steady state there exist two microscopic
mechanisms which break the frozen-in conditionand sustain magnetic reconnection in acollisionless plasma, i.e. microscopic kinetic
eﬀects of particles with finite mass and finite orbit
amplitude [2-4], and anomalous resistivity
associated with plasma instabilities [1,4,6].
For a no guide field case a microscopic kinetic
eﬀect originating from stochastic motion of
charged particles near a reconnection point, called
meandering motion, becomes dominant inside theorbit amplitude in a quasi-steady state. The
reconnection electric field is sustained by pressuretensor term at this stage. Two kinds of plasma
instabilities are observed to grow gradually inside
an ion-scale current sheet at the quasi-steady state[1,4,6]. The lower hybrid drift instability (LHDI)
grows in the periphery of the current sheet, but,
this mode does not penetrate into the central high-β
region of the current sheet because it undergoes
strong damping there. Thus, it cannot be a directcause of anomalous resistivity at the neutral sheet.
Later, a drift kink instability (DKI) is triggered at
the neutral sheet as a result of nonlinear
deformation of the current sheet by LHDI. The
reconnection electric field grows at the neutral
sheet in accordance with the excitation of DKI.
The detailed analysis [4,6] reveals that DKI
consists of two modes with different wavenumbers.
The short kink mode depends on the electrondynamics and its growth rate decreases as theion-to-electron mass ratio approaches a realistic
one. On the other hand, the longer mode isdominantly controlled by the ion dynamics, and its
growth rate is kept constant independent of themass ratio. Thus, the anomalous resistivity
generated by the longer king mode or its
reconnection rate is large enough to explain theobserved value even for a realistic mass ratio case.
For a strong guide field case off-plane magnetic
field evolves dynamically into a complex form,
which is controlled by initial uniform guide field
Bz0, quadrupole field created by the Hall current,
and the compression by the external driving flow,
as shown in Fig.1. The electron current layer grows
in a long and narrow region with strong magnetic
shear and its shape become asymmetric against thevertical axis passing through a reconnection pointin contrast to that for a no guide field case, as
shown in Fig. 2. Most of charged particles become
magnetized due to the strong guide field. In orderto clarify the relationship between stochastic

particle motion in a vicinity of a reconnection pointand current layer structure we carried out test
particle simulations under fixed electromagnetic
field, which was chosen at some simulation periods.
Figure 3 demonstrates the spatial profiles ofcurrent density and number density along thevertical axis passing through a reconnection point
for the same case as Fig. 1. The width of central
peak profile of the current density is given by theelectron orbit amplitude lme, and its half-width isnearly equal to the electron skin depth deskn. On theother hand, the typical spatial scale of number
density profile is given by the ion orbit amplitude
lmi. Strong guide field affects energy conversion
processes from magnetic energy to particle energy
in the reconnection process. Figure 4 shows theenergy spectra of electrons for four different guidefields. The growth of a non-thermal component issuppressed at the lower level for the stronger guidefield. This result corresponds to the fact thatelectron kinetic region shrinks as the guide field
increases.
Because a microscopic kinetic system such as
discussed above is always imbedded in a global
microscopic system and its location varies in bothtime and space, it is very important to locate akinetic regime in a dynamically evolving
macroscopic system and clarify the role of kinetic
effect in the global reconnection phenomena
through the interaction between macro- andmicrophysics. In order to investigate full picture ofmagnetic reconnection phenomena in nature, wehave developed a multi-hierarchy model,“MARIS”, in which MHD and PIC models areinterconnected through the interface region. The
model is applied to a few numerical test programs
and collisionless driven reconnection in a simple
geometry and is confirmed to work well [4,5]. We
are now applying the model to a fusion plasma
with a strong guide field.
[1] R. Horiuchi and T. Sato., Phys. Plasmas 6,4565 (1999).
[2] W. Pei, R. Horiuchi, and T. Sato, Phys. Plasmas,8 (2001), pp. 3251-3257.
[3] A. Ishizawa, and R. Horiuchi, Phys. Rev. Lett.,
95, 045003 (2005).
[4] R. Horiuchi, S. Usami, H. Ohtani, and T.
Moritaka, Plasma Fusion Res. 5, (2010) S2006.
[5] S. Usami, R. Horiuchi, H. Ohtani, and M. Den,Phys. Plasmas 20, 061208 (2013).
[6] T. Moritaka, and R. Horiuchi, Physics ofPlasmas, 15 (2008), 092114.

Fig. 1 Spatial profile of off-plane magnetic field atpet=5528 for a strong guide field of Bz0=2B0,
where B0 is an initial distant value of in-planemagnetic field.

Fig. 2 Color contours of the off-plane component ofelectron current density and vector plots of electron
flow velocity in (x,y) plane for the same case as Fig.
1.
Bz0=2.0, T=5528-Jz [10+6]
6-Jiz2lme-Jz
45000

2deskn
2lmi

20-2.50y/(c/ ce2.50Fig. 3 Spatial profile of current density and number
density along the vertical axis for the same case asFig. 1.
Electron Energy Spectrum at Kstep=70000

101086104102

Bz0=0.0
Bz0=0.5
Bz0=1.0
Bz0=2.0

10-2010Particle Energy

Fig. 4 Energy spectra of electrons for four different
guide fields.