Origin of Anomalous Resistivity in Collisionless Reconnection

Transcription

1 APFA & APPTC, Oct , 2009, Aomori, Japan Origin of Anomalous Resistivit in Collisionless Reconnection Ritoku Horiuchi 1) Toseo Moritaka 2) 1) National Institute for Fusion Science and the Graduate Universit for Advanced Studies, Toki , Japan, 2) Kobe Universit, Nada, Kobe , Japan

2 Purpose : To investigate plasma instabilities excited in an ionscale current sheet b means of two-dimensional particle simulation To clarif an origin of anomalous resistivit in collisionless reconnection in relation to plasma instabilities. Two plasma instabilities have so far been investigated, i.e., 1. Lower hbrid drift instabilit (LHDI) is excited in the peripher of the current sheet in a relativel earl phase. 2. Drift kink instabilit (DKI) is excited at the central region after nonlinear saturation of LHDI. DKI is a strong candidate for an anomalous resistivit, but there is a big discrepanc between theor and experiments.

3 1. Simulation model EM Particle Simulation (PIC method) Initial condition : Harris-tpe equilibrium q n u B p Neutral sheet =0 J 2 q n vusech,0,0 L Jx B 0,0, B0 Bz tanh L Bz Jx x Localized current densit Anti-parallel B- field No E-field Non-uniformit scale ion groradius

4 2. Simulation result (1) LHDI in peripher t ci Dispersion at peripher(=l) / ce wavenumber x/l LHDI Theor k e ~ 1 (2) Kink mode at NS frequenc t ci ~ LH kl Dispersion at NS / ce wavenumber x/l k L ~1 frequenc ~ ci kl

5 2.1 Effect of electrostatic fluctuation at LHDI growing phase n E n ~ ~ E n E ~ ~ n v B n v B n v B Nonlinear coupling term Profile of averaged force in x direction ( t ci ) electron ion n o F x cb L -3.21L -3.21L -3.21L n ~ E LHDI ~ LHDI x nv out B z No DC electric field is generated at NS, but LHDI leads to outward plasma diffusion.

6 Plasma waves near reconnection region Electrostatic wave Excitation of lower hbrid drift instabilit (LHDI) at peripher Theor of LHDI is consistent with observation not direct cause of collisionless reconnection Theor Satellite Observation Unstable due to gradient J B Stabilize at NS F.Mozer et al PRL 2002

7 2.2 Generation of DC electric field at DKI growing phase n E n ~ ~ E n E ~ n v B n v B n v B Profile of averaged force in x direction electron ion ( t ci 24.3) n o F x cb 0 n E ~ n v B -3.21L E +3.21L -3.21L ~ KI KI DC x B x v~ +3.21L EM fluctuation excited b DKI sustains DC electric field at neutral sheet.

8 Estimation of generated DC electric field Vin Erec E KI ±L MHD Estimate b inflow speed BL Time evolution of inflow speed v Dissipation region(±l) +stead MHD model in,l Erec B 2 B L L 0.1 v v 0.01v a,l in,l a,l t ci Generated electric field is large enough to explain the observations

9 Low frequenc EM waves near reconnection region Theor Experiment (MRX) Unstable due to relative drift motion between ion and electron (drift-kink instabilit) δb eff J B H.Ji et al PRL 2004 DKI is a strong candidate for an anomalous resistivit, but there is a big discrepanc between theor and experiments.

10 3. Role of ion and electron dnamics Low frequenc EM mode, called drift kink mode, plas an important role in generating anomalous resistivit. However, linear theor predicts that growth rate of this mode becomes negligibl small in real mass-ratio case. We need check whether this prediction is true or not? Mass-ratio dependence of DKI 1) Series A: electron mass is changed with fixed ion mass. 2) Series B: ion mass is changed with fixed electron mass. W.Daughton PoP 1999

11 Growth rate of kink mode for Series A Short kink mode (k i ~1.7) Long kink mode (k i ~0.5) /L x/l Growth rate vs mass ratio x/l Stabilize with mass ratio Independent of electron mass

12 Growth rate of kink mode for Series B Short kink mode (k i ~1.7) Long kink mode (k i ~0.5) /L x/l Growth rate vs mass ratio x/l Independent of ion mass Decrease with ion mass

13 3.1 Influence of LHDI on IDF at neutral sheet Initial ion distribution function Ion orbit effect Pressure tensor effect suggests modification of ion distribution function due to ion finite orbit effect LHDI active region NS Deviation from initial distribution at LHDI saturation phase B Ion meandering motion B t ci 5.31

14 Anisotrop in ion distribution function Relative drift motion between initial profile and ions affected b LHDI Anisotrop in ion temperature Two-component plasma with foreground and background ions Deviation from initial distribution Background Ions without an drift v Vd vx Foreground ions with finite drift (particle number: Nf) Two-component plasma becomes unstable against ion-ion kink instabilit due to relative drift motion. Energ of relative drift motion Er can be related to anisotrop in ion temperature Tx/T. Number ratio N N b f T T x 1 N N b f (particle number: Nb) v v ave t 2 energ E r T T x N E r total v 2 t

15 Growth rate of long kink mode Comparison with linear theor of ion-ion kink instabilit W.Daughton PoP 1999 ci Red:Series A (electron mass changes) Blue:Series B (ion mass changes) T T xi i 1 LHDI Ion temperature anisotrop after nonlinear saturation of LHDI Behavior of long kink mode is in good agreement with that of ion-ion kink mode.

16 Summar Anomalous resistivit due to plasma instabilities and their influence on collisionless reconnection are investigated b means of two-dimensional particle simulation. 1. Lower hbrid drift instabilit (LHDI) and two kinds of drift kink instabilities (DKI) are excited in ion-scale current laer. 2. LHDI is excited onl at the peripher of current laer, and thus does not cause anomalous resistivit at the center directl. However, it causes plasma diffusion at the peripher and modifies ion distribution at the center through the interaction between meandering ions and electrostatic fluctuations. 3. DKI grows at the center, and leads to the generation of reconnection electric field, which is large enough to explain observations in earth magnetosphere and reconnection experiments. 4. Short wavelength kink mode depends strongl on electron mass, while long wavelength kink mode is controlled mainl b ion dnamics. Thus, long kink mode is considered to have a finite growth rate for real mass ratio, and be a potential candidate of triggering mechanism of collisionless reconnection in a thin current laer. Reference [1] T. Moritaka, R. Horiuchi and H. Ohtani, Phs. Plasmas, vol. 14 (2007), [2] T. Moritaka, and R. Horiuchi, Phs. Plasmas, vol. 15 (2008), [3] W. Daughton, Phs. Plasmas, vol. 6 (1999), [3] H. Ji, et al, Phs. Rev. Lett., 92 (2004)