Abstract
Roles of ion and electron kinetic effects in the trigger mechanism of magnetic reconnection due to current sheet instabilities are investigated by means of explicit particle simulation. The simulation is performed for the Harris equilibrium without guide fields in the plane perpendicular to the antiparallel magnetic fields. The instabilities excited in the vicinity of the neutral sheet are classified into two modes, i.e., one is a longer wavelength kink mode and the other is a shorter wavelength kink mode. The growth of the longer kink mode depends only on the ion mass, while the growth of the shorter one depends only on the electron mass. Before the growth of these kink modes, the lower hybrid drift instability leads to two types of plasma diffusion:diffusion at the periphery controlled by ions and diffusion in the vicinity of the neutral sheet controlled by electrons. The diffusion at the periphery affects the ion distribution function at the neutral sheet through the ion meandering motion, and the ionion kink mode is destabilized as the electronindependent longer kink mode. The generation of the reconnectionelectric field at the neutral sheet due to the longer wavelength kink mode is characterized only by the ion dynamics and can take place commonly in ionscale current sheets observed in the magnetosphere and laboratories.
One of the authors (T.M.) is grateful to Professor H. Sakagami for his constant encouragement.
This work was partially supported by a GrantinAid for Scientific Research from the Japan Society for the Promotion of Science (Grant No. 18340188), the Research Cooperation Program on Hierarchy and Holism in Natural Science at the National Institute of Natural Sciences, and General Coordinated Research at the National Institute for Fusion Science (Grant No. NIFS08KTAN005).
I. INTRODUCTION
II. SIMULATION APPROACH
A. Simulation model
B. Initial equilibrium
III. SIMULATION RESULTS
A. Linear and nonlinear properties of LHDI
B. Growth of kink modes and dcelectric field at the neutral sheet
C. Origin of the electronindependent kink mode
IV. SUMMARY AND DISCUSSION
Key Topics
 Macroinstabilities
 90.0
 Electric fields
 23.0
 Magnetic reconnection
 23.0
 Diffusion
 22.0
 Magnetic fields
 19.0
Figures
Schematic image of series A and B. The top figure represents the initial condition in the case of mass ratio 220 which is common in series A and B, while the bottom figures represent the initial conditions for lower mass ratio in series A (left) and in series B (right). The curves convex upward (blue) indicate the initial density profiles. Two curves which have a shape similar to the ∞ symbol in each figure indicate typical meandering orbits. Larger one (green) and smaller one (red) correspond to ion and electron orbits, respectively.
Schematic image of series A and B. The top figure represents the initial condition in the case of mass ratio 220 which is common in series A and B, while the bottom figures represent the initial conditions for lower mass ratio in series A (left) and in series B (right). The curves convex upward (blue) indicate the initial density profiles. Two curves which have a shape similar to the ∞ symbol in each figure indicate typical meandering orbits. Larger one (green) and smaller one (red) correspond to ion and electron orbits, respectively.
Colorcoded contour plot of electric field profile at in the run A2, where the profile along the axis is enlarged.
Colorcoded contour plot of electric field profile at in the run A2, where the profile along the axis is enlarged.
Mass ratio dependence of the linear growth rate of LHDI. The growth rates normalized by the generalized lower hybrid frequency are plotted with blue squares for series A and with red crosses for series B, respectively. The relationship between the mass and the growth rate is approximately described by the green lines.
Mass ratio dependence of the linear growth rate of LHDI. The growth rates normalized by the generalized lower hybrid frequency are plotted with blue squares for series A and with red crosses for series B, respectively. The relationship between the mass and the growth rate is approximately described by the green lines.
(a), (b) Evolution of the fastestgrowing LHDI modes (electric field) at for series A [panel (a)] and series B [panel (b)], respectively. (c), (d) Spatial profiles of the LHDI amplitudes for series A [panel (c)] and series B [panel (d)]. The profiles are computed by averaging over the linear growth phase of each mode. Red, green, blue, and purple lines denote the runs A1/B1, A5/B5, A7/B7, and A8/B8, respectively. Numbers attached to the margin denote corresponding mass ratios and wave numbers normalized by the electron gyroradius .
(a), (b) Evolution of the fastestgrowing LHDI modes (electric field) at for series A [panel (a)] and series B [panel (b)], respectively. (c), (d) Spatial profiles of the LHDI amplitudes for series A [panel (c)] and series B [panel (d)]. The profiles are computed by averaging over the linear growth phase of each mode. Red, green, blue, and purple lines denote the runs A1/B1, A5/B5, A7/B7, and A8/B8, respectively. Numbers attached to the margin denote corresponding mass ratios and wave numbers normalized by the electron gyroradius .
(a), (b) Spatial profiles of electron velocity at the saturation phase of LHDI. (c), (d) Spatial profiles of charge density in the vicinity of neutral sheet at the saturation phase of LHDI. The panels (a) and (c) represent the profiles at for series A, and the panels (b) and (d) correspond to those at for runs B1, B4, B5, and B7, and that at for run B8. These profiles are averaged over the direction and the numbers attached to the margin denote corresponding mass ratios.
(a), (b) Spatial profiles of electron velocity at the saturation phase of LHDI. (c), (d) Spatial profiles of charge density in the vicinity of neutral sheet at the saturation phase of LHDI. The panels (a) and (c) represent the profiles at for series A, and the panels (b) and (d) correspond to those at for runs B1, B4, B5, and B7, and that at for run B8. These profiles are averaged over the direction and the numbers attached to the margin denote corresponding mass ratios.
(a), (b) Colorcoded contour plots of magnetic field at (a) and (b) in the run A2. (c), (d) Mass ratio dependence of the linear growth rate of the kink modes excited at the neutral sheet in series A (c) and in series B (d), where the red points denote the dominant longerwavelength mode and the blue points denote the dominant shorterwavelength mode .
(a), (b) Colorcoded contour plots of magnetic field at (a) and (b) in the run A2. (c), (d) Mass ratio dependence of the linear growth rate of the kink modes excited at the neutral sheet in series A (c) and in series B (d), where the red points denote the dominant longerwavelength mode and the blue points denote the dominant shorterwavelength mode .
Time evolutions of wavy magnetic force terms at the neutral sheet due to the kink modes. (a) Shorterwavelength kink mode for series A. (b) Shorterwavelength kink mode for series B. (c) Longerwavelength kink mode for series A. (d) Longerwavelength kink mode for series B. The wavy terms due to the longer modes are estimated from the summation of three modes of , 0.472, and 0.703, while those for the shorterwavelength kink modes are estimated from the summation of three modes of , 1.88, and 2.12. The red, green, and purple lines in panels (a), (b), (c), and (d) denote the runs A1/B1, A3/B3, and A5/B5, respectively.
Time evolutions of wavy magnetic force terms at the neutral sheet due to the kink modes. (a) Shorterwavelength kink mode for series A. (b) Shorterwavelength kink mode for series B. (c) Longerwavelength kink mode for series A. (d) Longerwavelength kink mode for series B. The wavy terms due to the longer modes are estimated from the summation of three modes of , 0.472, and 0.703, while those for the shorterwavelength kink modes are estimated from the summation of three modes of , 1.88, and 2.12. The red, green, and purple lines in panels (a), (b), (c), and (d) denote the runs A1/B1, A3/B3, and A5/B5, respectively.
Time evolutions of dc electric field at the neutral sheet in series A where the electric field is plotted in the logarithmic scale. The red (solid), green (dashed), and blue (dotted) lines denote the runs A1, A3, and A5, respectively.
Time evolutions of dc electric field at the neutral sheet in series A where the electric field is plotted in the logarithmic scale. The red (solid), green (dashed), and blue (dotted) lines denote the runs A1, A3, and A5, respectively.
Time evolutions of ion temperature anisotropy at the neutral sheet for series A (left) and B (right), respectively. The red, green, blue, and purple lines denote the runs A1/B1, A3/B3, A4/B4, and A5/B5, respectively. The numbers attached to the margin denote corresponding mass ratios.
Time evolutions of ion temperature anisotropy at the neutral sheet for series A (left) and B (right), respectively. The red, green, blue, and purple lines denote the runs A1/B1, A3/B3, A4/B4, and A5/B5, respectively. The numbers attached to the margin denote corresponding mass ratios.
Colorcoded contour plots of ion distribution function at initial equilibrium (left) and its variation from the initial equilibrium at (right) for the run A3, where the purple vertical line represents . White dots (a), (b), (c), (d), and (e) on the plane correspond to the velocities , , , , and , respectively.
Colorcoded contour plots of ion distribution function at initial equilibrium (left) and its variation from the initial equilibrium at (right) for the run A3, where the purple vertical line represents . White dots (a), (b), (c), (d), and (e) on the plane correspond to the velocities , , , , and , respectively.
Turning points of meandering ions as a function of particle velocity at the neutral sheet in the initial equilibrium for series A. Colored curved lines indicate the location of the meandering particles with the same turning point in the velocity space at the neutral sheet. The black curved lines represent contours of the absolute value of the velocity. The blue points (a), (b), (c), (d), and (e) correspond to the examples of the velocity at the neutral sheet in the same manner as in Fig. 10.
Turning points of meandering ions as a function of particle velocity at the neutral sheet in the initial equilibrium for series A. Colored curved lines indicate the location of the meandering particles with the same turning point in the velocity space at the neutral sheet. The black curved lines represent contours of the absolute value of the velocity. The blue points (a), (b), (c), (d), and (e) correspond to the examples of the velocity at the neutral sheet in the same manner as in Fig. 10.
Relationship between the growth rate of the kink mode and the ion temperature anisotropy. The red crosses (+) denote the simulation results for series A and numbers in the figure denote mass ratio for each run. The blue diagonal crosses (×) denote those for series B. Solid line represents the theoretical relationship of the ionion kink mode estimated from Fig. 15 in Ref. 42.
Relationship between the growth rate of the kink mode and the ion temperature anisotropy. The red crosses (+) denote the simulation results for series A and numbers in the figure denote mass ratio for each run. The blue diagonal crosses (×) denote those for series B. Solid line represents the theoretical relationship of the ionion kink mode estimated from Fig. 15 in Ref. 42.
Tables
Simulation parameters for simulation series A.
Simulation parameters for simulation series A.
Simulation parameters for simulation series B.
Simulation parameters for simulation series B.
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