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

Acknowledgements P. Pritchett W. Daughton D. Swift

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.

Simulations of reconnection: New aspect ? Distant neutral line Reconnection onset (NENL) Earth’s magnetotail 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.

P3D: Simulations of reconnection with conducting/periodic BC Box size: (l x /d,l z /d)=(19.2x19.2), d= c/  pi, m i /m e =64, T i /T e =3/2, c/v A =15, L=0.5  oi, Initial GEM-type perturbation: Out-of-plane current J y density is color-coded Z X Y Conducting BC Periodic BC

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]

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

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

Simulations of reconnection with open boundary conditions: Pritchett BC  0i t=6  0i t=7  0i t=8  0i t=12

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

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

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 B z, can be eliminated by passing electrons. Same field BC with particle reintroduction Fully open BC Particle reintroduction stabilizes ion tearing

More detail on the case with Pritchett BC Normal magnetic field B z (z=0) t=0 (white), t=8 (green)  0i t=8 Out-of-plane magnetic field B y Out-of-plane Electric field E y Field-aligned current j ||  0i t=8

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

Summary Simulations with open BC show some CS stretching beyond electron scales, compared to periodic/conducting BC case. 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. 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]. 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. A key parameter, which controls the reconnection onset in the tail, is current sheet thickness.

Our main result THEMIS MMS