Role of core losses in drift-vortex interactions

Dipole drift vortices in the Hasegawa-Mima-Charney equation are studied by means of particle-in-cell (PIC) calculations. Apart from providing an efficient and accurate solution of the equations, PIC provides additional information about the fluid flow such as exchange of fluid between regions interior and exterior to the dipoles. Several cases of perturbed dipoles are studied with particular emphasis on the evolution of the fluid that is initially trapped inside the separatrix of the co-moving stream function of each unperturbed dipole. In particular, the effect of a finite tilt of the dipole axis is analyzed. Here, asymmetric losses from the two dipole halves are found to play a crucial role in the qualitative evolution of the dipole trajectory: dipoles initially moving in the unstable direction are found to reverse their average velocity perpendicular to the density gradient. Very large perturbations are obtained in dipole collisions. Here symmetry of the initial conditions plays an important role: collisions of aligned dipoles appear almost solitonlike, while for nonaligned dipoles the collision at least generates a tilt of the axes of the dipoles, but may also lead to a complete destruction of one of the poles. In all cases a significant loss of initially trapped fluid is demonstrated.