Particle Simulations of Magnetic Reconnection with Open Boundary Conditions

1. Particle Simulations of Magnetic Reconnection with Open Boundary Conditions A. V. Divin1,2, M. I. Sitnov1, M. Swisdak3, and J. F. Drake1 1Institute for Research in Electronics and Applied Physics, University of Maryland 2Saint-Petersburg State University, Russia 3Naval Research Laboratory Fall AGU Meeting December 13, 2006

2. Acknowledgements P. Pritchett W. Daughton D. Swift

3. Motivation • Until recently simulations of magnetic reconnection were largely performed using a combination of periodic and conducting boundary conditions. • Recent results, including [Daughton et al., 2006], reveal interesting new effects that appear in case of the so-called ‘open’ boundary conditions. • We adjust the code P3D [Zeiler et al., 2002] to explore the effects of different open boundary conditions.

4. Simulations of reconnection: New aspect • Another new aspect of our studies is shifting the focus of interest from the central X-line vicinity to outflow regions, which resemble the tail of the magnetosphere. Earth’s magnetotail ? Reconnection onset (NENL) Distant neutral line

5. P3D: Simulations of reconnection with conducting/periodic BC Box size: (lx/d,lz/d)=(19.2x19.2), d= c/pi, mi /me=64, Ti/Te=3/2, c/vA=15, L=0.5oi, Initial GEM-type perturbation: Conducting BC Out-of-plane current Jy density is color-coded Periodic BC Periodic BC Z X Y Conducting BC

6. Construction of open boundaries: particles New particles are injected with the shifted Maxwellian distribution, whose parameters are chosen to preserve first two moments of the distribution function Temperature [e.g., Pritchett, 2001]

7. Construction of open boundaries: fields Pritchett [1998, 2001]: Horiuchi et al. [2001]: Daughton et al., [2006]: 1st order radiation BC for light waves 2nd order radiation (non-PML) BCs [Lindman, 1975; Engquist and Majda, 1977; Higdon, 1986; Renaut, 1992]: similar to 1st order radiation conditions in [Daughton et al., 2006], but often result in numerical instabilities. Open field BC used in this work (x-boundary): + or (Pritchett BC) ( Radiation BC)

8. Simulations of reconnection with open boundary conditions: Radiation BC 0it=6 0it=7 0it=12 0it=8

9. Simulations of reconnection with open boundary conditions: Pritchett BC 0it=6 0it=7 0it=12 0it=8

10. Evidence of the ion tearing instability Schindler [1974]: In the presence of the finite Bz the electron tearing instability can be replaced by the ion tearing, which is even faster: Ion tearing (0=0.3, open BC) Electron tearing (0=0, periodic BC) Ion tearing develops 3 times faster

11. Electric field evolution Ion tearing (0=0.3, open BC) Electron tearing (0=0, periodic BC) ion tearing Ey electron tearing Ey global Ey 0 10 t 20 30 0 t 10 Ion tearing develops spontaneously

12. What causes the destabilization? According to the theory [Sitnov et al., 2002], the stabilizing effect of trapped electrons [Lembege and Pellat, 1982], which appears in the presence of a finite Bz, can be eliminated by passing electrons. Same field BC with particle reintroduction Fully open BC Particle reintroduction stabilizes ion tearing

13. More detail on the case with Pritchett BC Normal magnetic field Bz(z=0) t=0 (white), t=8 (green) Out-of-plane magnetic field By 0it=8 Field-aligned current j|| Out-of-plane Electric field Ey 0it=8 0it=8

14. The new effect has been detected for different mass ratios: mi/me=25, 64, 128.However, it disappears with doubling the current sheet thickness: L=0i.

15. Summary • Simulations with open BC show some CS stretching beyond electron scales, compared to periodic/conducting BC case. • Simulations with open BC reveal the excitation of the ion tearing instability, predicted by Schindler [1974] as a mechanism of magnetospheric substorms. • They also confirm the destabilizing effect of passing electrons [Sitnov et al., 2002]. • A key parameter, which controls the reconnection onset in the tail, is current sheet thickness.

16. Our main result THEMIS MMS