On reformation of quasi-perpendicular collisionless shocks

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We review recent studies on mass ratio dependence of self-reformation of high Mach number, low b, highly oblique quasi-perpendicularshocks by Scholer et al. (Scholer, M., Shinohara, I., Matsukiyo, S. Quasi-perpendicular shocks: length scale of the cross-shockpotential, shock reformation, and implication for shock surfing, J. Geophys. Res., 108, 1014, doi:10.1029/2002JA009515, 2003), Matsukiyoand Scholer (Matsukiyo, S., Scholer, M. Modified two-stream instability in the foot of high Mach number quasi-perpendicularshocks, J. Geophys. Res., 108, 1459, doi:10.1029/2003JA10080, 2003), and Scholer and Matsukiyo (Scholer, M., Matsukiyo, S. Nonstationarityof quasi-perpendicular shocks: a comparison of full particle simulations with different ion to electron mass ratio, Ann. Geophys.,in press, 2004). A one-dimensional full particle simulation code is utilized. In small ion to electron mass ratio runs,reformation is due to the accumulation of gyrating reflected ions at the upstream edge of the foot. Furthermore, at an extremely smallmass ratio, the Buneman instability is generated in the foot. In the realistic mass ratio run, however, the modified two-stream instabilityexcited in the foot plays an important role in a shock reformation process.