Interactions of two supercritical quasi-perpendicular collisionless shocks:
Résumé
"Collisions" of shocks are often invoked in the interaction of interplanetary shocks, or interplanetary/planetary shocks, or during the impulsive phase of solar flares to explain the heating and particle acceleration arising from a large number of localized regions. Such collisions have been mainly analyzed with stationary shocks (Cargill, 1991). However, 1D/2D numerous PIC (Particle in Cell) - and more recently hybrid- numerical simulations have clearly evidenced that the front of individual supercritical quasi-perpendicular shocks may be strongly nonstationary. One well recognized nonstationary process is the so called "self-reformation" mainly driven by the accumulation of reflected ions over a foot distance from the ramp. In the present work, interactions between two supercritical quasi-perpendicular shocks are analyzed by using 1D full PIC simulations where space charge/induced effects, and this cyclic self-reformation are fully and self-consistently included. At the location of the shock fronts collision (in short SFC), strong local B fields build up associated to large magnetic pressure effects (accumulation of inter-penetrating ions). This accumulation is due to the local co-rotation of individual ion vortices (self-reforming shocks) which takes place before ion phase mixing establishes. Then, local new shock fronts build up and separate each other. In order to cover a large number of collisional situations, different conditions of SFC have been performed covering high and low Mach number shock regimes. Key results show that (a) shocks cross each other (instead of being reflected) so that each incident shock keeps in propagating within the downstream region of the other one, (b) each well organized self-reforming shock (before the SFC) may be replaced by a very turbulent shock (after the SFC) where the self-reformation totally disappears, according to the concerned shock regime, and (iii) strong ion energisation takes place after local shock interactions