Ion velocity distributions in dipolarization events: Distributions in the central plasma sheet

Using combined MHD/test particle simulations, we further explore characteristic ion velocity distributions in the central plasma sheet (CPS) in relation to dipolarization events. Distributions in the CPS within the dipolarized flux bundle (DFB) that follows the passage of a dipolarization front typically show two opposing low subthermal‐energy beams with a ring‐like component perpendicular to the magnetic field at about twice the thermal energy. The dominance of the perpendicular anisotropy and a field‐aligned peak at lower energy agree qualitatively with ion distribution functions derived from “Time History of Events and Macroscale Interactions during Substorms” observations. At locations somewhat off the equatorial plane the field‐aligned peaks are shifted by a field‐aligned component of the bulk flow, such that one peak becomes centered near zero net velocity, which makes it less likely to be observed. The origins of the field‐aligned peaks are low‐energy lobe (or near plasma sheet boundary layer) regions, while the ring distribution originates mostly from thermal plasma sheet particles on extended field lines. The acceleration mechanisms are also quite different: the beam ions are accelerated first by the E × B drift motion of the DFB and then by a slingshot effect of the earthward convecting DFB (akin to first‐order Fermi, type B, acceleration), which causes an increase in field‐aligned speed. In contrast, the ring particles are accelerated by successive, betatron‐like acceleration after entering the high electric field region of an earthward propagating DFB.