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Magnetic Reconnection in Multi-Fluid Plasmas

Magnetic Reconnection in Multi-Fluid Plasmas. Michael Shay – Univ. of Maryland. Magnetic Reconnection in Multi-Fluid Plasmas General Theory and Simulations of O + Modified Reconnection. Michael Shay – Univ. of Maryland. Background. 2-species 2D reconnection has been substantially studied.

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Magnetic Reconnection in Multi-Fluid Plasmas

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  1. Magnetic Reconnection in Multi-Fluid Plasmas Michael Shay – Univ. of Maryland

  2. Magnetic Reconnection in Multi-Fluid PlasmasGeneral Theory and Simulations of O+ Modified Reconnection. Michael Shay – Univ. of Maryland

  3. Background • 2-species 2D reconnection has been substantially studied. • Many plasma have 3 or more charged species. • Magnetotail: • O+ due to ionospheric outflows: CLUSTER CIS/CODIF (kistler) • no+ >> ni sometimes, especially during active times. • Astrophysical plasmas • Dust species present • Neutrals also. • What will reconnection look like? • What length scales? Signatures? • Reconnection rate? • Previous work • Global 3-fluid magnetospheric codes (Winglee). • Tracer particle stepping in global MHD models (Birn). • Full particle codes (Hesse).

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

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

  6. da = c/wpa Smaller Larger ni = 0.05 cm-3no+/ni = 0.64 3-Species Waves: Magnetotail Lengths • Heavy whistler: Heavy species are unmoving and unmagnetized. • Electrons and ions frozen-in => Flow together. • But, their flow is a current.Acts like a whistler. • Heavy Alfven wave • All 3 species frozen in.

  7. 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.

  8. Vin CA y x z The Simulations • 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

  9. 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

  10. 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. Y Electrons density Ions Heavy Ions Y nVz Y

  11. By with proton flow vectors Z Out-of-plane B • mh* = 1 • Usual two-fluid reconnection. • mh* = 16 • Both light and heavy whistler. • Parallel ion beams • Analogue of electron beams in light whistler. • mh* = 104 • Heavy Whistler at global scales. X Heavy Whistler Z Light Whistler X Z X

  12. 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.. • Eventually, the heavy whistler is the slowest. mh* = 1mh* = 16mh* = 104 Time Island Width Time

  13. 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

  14. Outflow shows all 4 wave regions • Outflow region • 4 different physics regions • Maximum outflow speed • mh* = 1: Vout1 1.0 • mh* = 16: Vout16  0.35 • Expected scaling: • Vout  cAtCAt = [ B2/4p(nimi + nhmh) ]1/2 • Vout1/Vout16  2.9 • cAt1/cAt16  2.6 Cut through x-line along outflow light Alfven light whistler heavy whistler heavy Alfven VexVixVhx X

  15. 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.

  16. 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.

  17. 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, …

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