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Three Species Collisionless Reconnection: Effect of O + on Magnetotail Reconnection

Three Species Collisionless Reconnection: Effect of O + on Magnetotail Reconnection. Michael Shay – Univ. of Maryland Preprints at: http://www.glue.umd.edu/~shay/papers. Overview. 3-species reconnection What length scales? Signatures? Reconnection rate? Examples and background

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Three Species Collisionless Reconnection: Effect of O + on Magnetotail Reconnection

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  1. Three Species Collisionless Reconnection: Effect of O+ on Magnetotail Reconnection Michael Shay – Univ. of Maryland Preprints at: http://www.glue.umd.edu/~shay/papers

  2. Overview • 3-species reconnection • What length scales? • Signatures? • Reconnection rate? • Examples and background • Linear theory of 3-species waves • 3-Fluid simulations

  3. Magnetospheric O+ March 18, 2002 • Earth’s magnetosphere • ionospheric outflows can lead to significant O+ population. • Active Times • Oct. 1, 2001: Geomagnetic storm • CLUSTER, spacecraft 4 • CIS/CODIF data • More O+ than protons. • Chicken or Egg?

  4. Astrophysical Plasmas • Star and planet forming regions • Molecular clouds and protoplanetary disks. • Lots of dust. • Wide range of conditions. • Dust • negatively charged • mass >> proton mass. • Collisions with neutrals important also. Hubble Orion Nebula Panorama

  5. Previous Computational Work • Birn et al. (2001, 2004) • Global MHD magnetotail simulations. • Test particle O+ to examine acceleration and beam generation. • Winglee et al. (2002, 2004) • Global MHD 2-fluid magnetospheric simulations. • Reduction of cross polar cap potential. • Did not resolve inner reconnection scales. • Hesse et al., 2004 • 3-species full particle simulations. • O+ had no effect on reconnection, although an increase in proton density did. • Simulation size not large enough to fully couple O+.

  6. Three-Fluid Equations • Three species: {e,i,h} = {electrons, protons, heavy species} • mh* = mh/mi • Normalize: t0 = 1/Wi and L0 = di c/wpi • E = Ve B  Pe/ne

  7. Vout X Vin Y -Z d 1D Linear waves • Examine linear waves • Assume k || Bo • Compressional modes decouple.

  8. Dispersion Relation Slow Alfven • w << Wh • 2nd and 4th terms • Fast Waves • w >> Wh, Wi >>Wh

  9. da = c/wpa Smaller Larger ni = 0.05 cm-3no+/ni = 0.64 3-Species Waves: Magnetotail Lengths • Previous Astrophysical Work. • Heavy dust whistler (nh << ni, mhnh >> mini) has been examined but not in the context of reconnection. • Shukla et al, 1997. • Rudakov et al., 2001. • Ganguli et al., 2004.

  10. Heavy Whistler 1 dh • Assume: • Vh << Vi,Ve • Ignore ion inertia => Vi Ve

  11. Y Z -X Y Z -X D The Nature of Heavy Whistlers • Heavy species is unmagnetized and almost unmoving. • Primary current consists of frozen-in ions and electrons E B drifting. Ions+Electron fluid has a small net charge:charge density = e zh nh. • This frozen-in current drags the magnetic field along with it. Frozen-in Ion/Electron current

  12. Vin Vout y x z Effect on Reconnection? • Dissipation region • 3-4 scale structure. • Reconnection rate • Vin ~ d/D Vout • Vout ~ CAt • CAt = [ B2/4p(nimi + nhmh) ]1/2 • nhmh << nimi • Slower outflow, slower reconnection. • Signatures of reconnection • Quadrupolar Bz out to much larger scales. • Parallel Hall Ion currents • Analogue of Hall electron currents.

  13. Vin CA z x y Simulations: Heavy Ions • Initial conditions: • No Guide Field. • Reconnection plane: (x,y) => Different from GSM • 2048 x 1024 grid points • 204.8 x 102.4 c/wpi. • Dx = Dy = 0.1 • Run on 64 processors of IBM SP. • me = 0.0, 44B term breaks frozen-in, 4 = 5 • 10-5 • Time normalized to Wi-1, Length to di c/wpi. • Isothermal approximation, g = 1

  14. Reconnection Simulations Current along Z Density • Double current sheet • Reconnects robustly • Initial x-line perturbation Y t = 0 X X Y t = 1200 X X

  15. Equilibrium Bx Jz • Double current sheet • Double tearing mode. • Harris equilibrium • Te = Ti • Ions and electrons carry current. • Background heavy ion species. • nh = 0.64. • Th = 0.5 • mh = {1,16,104} • dh = {1,5,125} • Seed system with x-lines. • Note that all differences in cAt is due to mass difference. Z Electrons density Ions Heavy Ions Z nVz Z

  16. By with proton flow vectors Z 2-Fluid case mh* = 1 • Quadrupolar By • about di scale size. • Vix = Vhx X Vix with B-field lines. Z X Vhx Z X

  17. By with proton flow vectors Z O+ Case: mh* = 16 Heavy Whistler Light Whistler • Quadrupolar By • Both light and heavy whistler. • Vi participates in Hall currents. • Vhx acts like Vix in two-fluid case. X Vix with B-field lines. Z Vhx

  18. Whistler dominated mh* = 104 By with proton flow vectors • Quadrupolar By • System size heavy whistler. • Vix • Global proton hall currents. • Vhx basically immovable. Vix with B-field lines. Vhx

  19. Reconnection Rate Reconnection Rate • Reconnection rate is significantly slower for larger heavy ion mass. • nh same for all 3 runs. This effect is purely due to mh.. • Slowdown in mh* = 104? • System size scales: • Alfven wave: V  cAh • Whistler: V  k dh cAh V  dh cAh/L => As island width increases, global speed decreases. mh* = 1mh* = 16mh* = 104 Time Island Width Time

  20. symmetry axis Cut through x=55 Key SignaturesO+ Case mh* = 1mh* = 16 By • Heavy Whistler • Large scale quadrupolar By • Ion flows • Ion flows slower. • Parallel ion streams near separatrix. • Maximum outflow not at center of current sheet. • Electric field? Z Cut through x=55 mh* = 16 proton Vx O+ Vx Velocity Z Heavy Whistler Z Light Whistler X

  21. light whistler light Alfven Physical Regions Z mh* = 1 VexVix • Cuts through x-line along outflow direction. • Inner regions substantially compressed for mh* = 104. • Vix minimum. X light Alfven light whistler heavy whistler heavy Alfven Z VexVixVhx mh* = 16 X heavy whistler Z mh* = 104 X

  22. Scaling of Outflow speed • Maximum outflow speed • mh* = 1: Vout1 1.0 • mh* = 16: Vout16  0.35 • Expected scaling: • Vout  cAt CAt = [ B2/4p(nimi + nhmh) ]1/2 • Vout1/Vout16  2.9 • cAt1/cAt16  2.6

  23. Consequences for magnetotail reconnection • When no+mo+ > ni mi • Slowdown of outflow normalized to upstream cAi • Slowdown of reconnection rate normalized to upstream cAi. • However: • Strongly dependent on lobe Bx. • Strongly active times: cAi may change dramatically.

  24. Specific Signatures: O+ Modified Reconnection • O+ outflow at same speed as proton outflow. • Reduction of proton flow. • Larger scale quadrupolar By (GSM). • Parallel ion currents near the separatrices. • Upstream ions flow towards x-line. • The CIS/CODIF CLUSTER instrument has the potential to examine these signatures.

  25. Questions for the Future • How is O+ spatially distributed in the lobes? • Not uniform like in the simulations. • How does O+ affect the scaling of reconnection? • Will angle of separatrices (tan q  d/D) change? • Effect on onset of reconnection? • Effect on instabilities associated with substorms? • Lower-hybrid, ballooning,kinking, …

  26. Conclusion • 3-Species reconnection: New hierarchy of scales. • 3-4 scale structure dissipation region. • Heavy whistler • Reconnection rate • Vin ~ d/D Vout • Vout ~ CAt • CAt = [ B2/4p(nimi + nhmh) ]1/2 • nhmh << nimi • Slower outflow, slower reconnection. • Signatures of reconnection • Quadrupolar Bz out to much larger scales. • Parallel Hall Ion currents • Analogue of Hall electron currents.

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