1. MHD Simulations of magnetotail reconnection (J. Birn) Observations MHD simulation: overview Propagation of dipolarization signals Generation of pulsations: bouncing, “breathing” Field-aligned current generation Role of entropy reduction Ackn.: Thanks to M. Henderson, M. Hesse, R. Nakamura, E. Panov Focus on “bursty bulk flows” and “dipolarization”

2. Observations of flow bursts and “dipolarization fronts” Sergeev et al., 1996 North Sun Dusk Sergeev et al., 2010 time field-aligned currents auroral streamer

3. Observations of flow bursts and dipolarization fronts Runov et al., 2009 Nakamura et al., 2009

4. Observations of flow bursts: bouncing and vorticity Panov et al., 2010 Ohtani et al., 2009

5. flow bursts = depleted flux tubes (“bubbles”)? reduced S from reconnection (Fig. after Forsyth et al., 2008) Kim et al., 2010 Pontius & Wolf, 1990: reduced “entropy” or slippage? ( Wolf et al., 2009)

6. Schindler, Birn Voigt Lee&Wolf

7. MHD Simulations: Overview (Static equilibrium:  p = j  B) 1. INITIAL (GROWTH) PHASE: SPATIALLY AND TEMPORALLY LIMITED INFLOW, COMPRESSION, THIN CURRENT SHEET FORMATION 2. DYNAMIC (EXPANSIVE) PHASE: ANOM. RESISTIVITY CAUSES RECONNECTION, PLASMOID EJECTION 3. ENERGETIC PARTICLES: TEST PARTICLES IN MHD FIELDS Time-dependent simulations: Initial State addition of dipole, + relaxation into equilibrium Initial Phase

8. Pre-onset phase: Adiabatic deformation  thin current sheet formation, loss of equilibrium y 2-D quasi-static theory 3-D MHD simulation JyJy JyJy x x y z

9. 3D MHD Simulation of Tail Reconnection

10. Magnetic Field B z in the x,z plane z x 0-2-4-6-8-10-12 B z (x,z)

11. Electric Field E y in the x,z plane z x

12. Electric Field E y and velocity in the x,y plane y x E y (x,y)

13. Local characteristics as function of x

14. 3D MHD Simulations & Observations THEMIS observations (Panov et al., 2010) MHD simulations (Birn et al., 2011) Bouncing, pulsating, dipolarization sequences

15. Field-aligned current generation t = 128 t = 132 (equilibrium) -120 0 -6-6 6 y -6-6 6 y

16. Onset Timing E y at x-point J || at x = 0 integrated J || (region 1) max. J || t = 61: finite resistivity t = 90: neutral line formation t = 120: onset of fast reconnection

17. Onset of fast reconnection

18. Entropy function in the x,y plane -15 -10 -5 0 x y -10 -5 B z contours 5 1010

19. Comparison 2D vs. 3D, enthalpy flux H x = 2.5pv x 2D3D

20. Comparison 2D vs. 3D

21. y x Test particle simulations (electrons) α = 35° at x=1.5, y=0,z=0

22. Test particle simulations (electrons) pitch angle averaged passage of dipolarization front x = 3, y = 0, z = 0

23. Summary: MHD Simulations of Tail Reconnection Reconnection enables ballooning/interchange Ballooning enables - fast reconnection, significant energy release - cross-tail structure, leading to Fast flows and dipolarization fronts - propagating earthward - stopping & tailward expansion - bouncing, vorticity, & breathing - field-aligned currents Localized electric field accelerates particles