Transcript 20040812100011001-148634

Kinetic Structure of the Reconnection Layer
and of Slow Mode Shocks
Manfred Scholer
Centre for Interdisciplinary Plasma Science
Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
Kaspar Arzner
Michael Cremer
Irina Sidorenko
Isaac Newton Institute, August 2004
First a few vgiewgraphs about
Onset of collisionless reconnection in thin current sheets:
Anti-parallel and sheared magnetic fields
(3-D PIC simulations)
Scholer, Sidorenko, Jaroschek, Treumann, 2003
Initial Equilibrium and Numerical Set-up
Double Harris-sheet
Current sheet width = 1 ion inertial length
Periodicity in all three directions
L x  L y  L z  6i
i  c / pi
mi / me  150
c / VA  15
Ti / Te  2.7
200  106 particles of each species
(note the coordinate system:
z is the current direction)
I. Thin Current Sheet with Antiparallel Magnetic Field
Right: Reconnected flux versus
time. The whole flux between the
two current sheets is reconnected
when   0
Left: Magnetic field pattern at
four different times (Isointensity
contours of (x, y) )
 2  Jz
z
Lower Hybrid Drift Instability at the
Current Sheet Edge
Color coded electron density (left)
and electric field (right) in the current
directionin the plane perpendicular
to the magnetic field.
Cuts of various parameters before reconnection starts
t=0
Cuts of electron contribution to current
density (top) and electron density across
the current sheet. Profiles at t=0 are
shown dashed for reference.
t=4
Reduced electron distribution function
f(v_z) versus v_z in the current sheet gradient
region (top) and in the center (bottom) of
the current sheet exhibiting electron
acceleration in the electric field of the LHD
waves.
Development of the Reconnection Line
Reconnecrtion channel
Shown is the normal magnetic field
component in the center of the current
sheet (blue: negative field, red: positive
field). Transitions from red to blue
indicate positions of a neutral line.
Initially, several single neutral lines
emerge. Eventually one single
reconnection channel results.
Two reconnection channels
Development of Neutral Line (Guide Field Case Bz=1)
With a sufficiently strong
guide field reconnection
is two-dimensional, i.e., a
single X line develops and
extends through the whole
system
Time for electrons to move
across the system smaller
than growth time of instability
Guide Field = 20% of Main Field
Deviation of main field above
(right) and below (left) current
sheet at wo different times
LH waves propagate
perpendicular to the
Magnetic field
In this guide field case the LHDI develops as well.
After reconnection onset the reconnection rate is
about the same as in the exactly antiparallel case.
Development of the Reconnection Channels
Guide field = 0.2 of main field
‘Antiparallel reconnection‘
‘Component reconnection‘
Reconnection Layer
MHD
Collisionless
Petschek, 1964
Hill, 1975
2 switch-off shocks
bound the reconnection layer
Current sheet where particles
Perform Speiser-type orbits
1. Investigation of the reconnection layer by large-scale hybrid
simulations
2. Analysis of slow mode shocks by solving an equivalent Riemann
problem (decay of a current sheet)
2-D hybrid simulation of tail reconnection
Hybrid code with massless electrons
Initial equilibrium is Birn et al. (1975) equilibrium with flaring tail
(but can also be a Harris sheet)
Simulation box of 500 x 126 ion inertial lengths (O=c/pi , where
plasma frequency refers to current sheet center density)
(corresponds to about 100 x 25 RE)
Lobe density to center density at x=0 is 0.4
Temperature is chosen such that overall prssure balance is
maintained (bi=be=0.05 in the lobe)
Reconnection is initialized by localized resistivity at 100 O
Computed up to 500 Wci-1
We are back to the
magnetospheric
coordinare system
By – field (out-of-plane)
B  B  (m / e)  u
y-component of ion vorticity
is frozen into ion fluid u. In the inflow lobe region both terms
are zero. Occurrence of vorticity has to be cancelled by By
1.Near diffusion region:
interpenetrating cold beams
2.Center of current sheet:
Partial ring distribution due to
Speiser-type orbitsin CS with
thickness larger than ion gyroradius
3.Boundary layer:
Speiser accelerated ions ejected
onto reconnected field lines
4.Post-plasmoid plasma sheet:
Hot thermalized distribution
1.,2.,3. closely resembles Hill (1975) scenario
Instability and breakup of current sheet
Waves are standing in the
outflow frame
Instability and breakup of the current sheet
Bulk velocity shear insufficient to drive KH instability.
Beams in the BL provide additional energy to drive
instabiliy. Due to sharp gradients in BL a homogeneous
scenario does not apply.
3-layer anisotropic MHD model.
Free energy mainly produced by velocity anisotropy;
bulk velocity shear plays auxiliary role
Comparison of solution of dispersion relation with simulation
Arzner and Scholer, 2001
Simulation: Dominant wavelength   20 O,
corresponding to k = 0.3; wavelength
increases with x. Phase velocity and group
velocity about equal to outflow velocity.
3-layer model (red) compares favorably
with simulation.
Development of the instability with distance
monochromatic
higher harmonics
cascading to lower wavelengths
Simulated power spectra of B time series averaged over the turbulent
range in the post plasmoid plasma sheet. Agrees with observed temporal
power spectra in the tail (AMPTE: Bauer et al., 1995; GEOTAIL:
Hoshino et al., 1994).
Hot thermalized post-plasmoid
plasma sheet
Boundary layer
Lobe distribution
Dissipation Mechanism in Slow Mode Shocks
Bounding the Reconnection Layer
Cremer and Scholer, 2000
Solution of an Equivalent Riemann Problem
(Decay of a 1D current sheet wit normal magnetic field)
Perpendicular Heating of Lobe by the EMIIC Instability
Temperature in lobe vs time
Ion distribution function
In the LOBE!
Nonresonant and right-hand resonant
ion-ion beam instabilities are stable.
Oblique propagating AIC waves are
excited by the EMIIC instability.
They heat lobe and beam ions
perpendicular to the magnetic field.
Large perpendicular to parallel
temperature anisotropy excites parallel
propagating AIC waves.
These AIC waves lead downstream to
ion phase-mixing and thermalization.
Excitation of parallel propagating AIC waves by the perp to parallel
temperature anisotropy
T / T  1
2D Fourier analysis in the kx-kz plane
W-kx power spectrum and result from linear
theory; hodogram of kx=1 mode
Suggested cycle of processes leading to slow mode shocks bounding
the reconnection layer