Generation and Evolution of Two Opposite Types of Mesoscale Plasma Sheet Bubbles

Previous observations showed that there are two major types of mesoscale plasma sheet bubbles (plasma with relatively lower total entropy than ambient plasma): the type-1 has density lower and temperature higher than ambient plasma, and the type-2 has the opposite differences. We first show, from statistical analysis of observations, that both the type-1 and type-2 bubbles can be possibly generated locally by magnetic reconnection in the magnetotail. We then conduct simulations based on the Rice Convection Model to show that the density and temperature characteristics of a bubble can affect the bubble's evolution and associated magnetic field and flow perturbations through magnetic drift and precipitation. For both types, the bubble extends azimuthally as it moves inward, and B-z is enhanced near the bubble's earthward front. The flow perturbations within the bubble consist of two interchange flow vortices with a fast earthward-flow channel near the center and return flows on the two sides. The low entropy of a bubble is contributed by the reduction in the higher energy ions in the type-2 bubble but in the lower energy ions in the type-1 bubble. Duskward magnetic drift of the bubble's depleted high-energy ions allows the type-2 bubble and the B-z enhancement to expand azimuthally wider toward dusk than does the type-1 bubble. The electrons within the type-2 bubble produce weaker ionospheric conductance, in particular on the dawnside of the bubble because these electrons drift toward dawn, which results in stronger flows within the dawnside flow vortex, as compared to the type-1 bubble.