Characterizing Ion Flows Across a Magnetotail Dipolarization Jet

The structure of dipolarization jets with finite width in the dawn‐dusk direction relevant to magnetic reconnection in the Earth's magnetotail is explored with particle‐in‐cell simulations. We carry out Riemann simulations of the evolution of the jet in the dawn‐dusk, north‐south plane to investigate the dependence of the jet structure on the jet width in the dawn‐dusk direction. We find that the magnetic field and Earth‐directed ion flow structure depend on the dawn‐dusk width. A reversal in the usual Hall magnetic field near the center of the current sheet on the duskside of larger jets is observed. For small widths, the maximum velocity of the earthward flow is significantly reduced below the theoretical limit of the upstream Alfvén speed. However, the ion flow speed approaches this limit once the width exceeds the ion Larmor radius based on the normal magnetic field, Bz. Plain Language Summary Magnetic reconnection is a phenomenon occurring in the Earth's magnetotail in which energy stored in the magnetic field is converted into kinetic energy. This process creates a high‐speed dipolarization jet of plasma. However, the physical dimension of these jets in the dawn‐dusk direction is not well constrained. We study how the structure of dipolarization jets depends on their width in the dawn‐dusk direction and find that both the shape of the magnetic field and the profile of the ion outflow speed across the jet vary as a function of the jet width. In particular, the maximum ion outflow can fall well short of the theoretically expected outflow speed for small widths. Our results can be used by satellites to determine their positions relative to observed dipolarization jets and place limits on the jet widths. Key Points The dependence of the structure of magnetotail dipolarization jets on cross‐tail width is studied through 2‐D Riemann simulations For small widths the jet velocity falls well below expectations based on predictions of conventional reconnection models In wide jets the self‐generated Hall magnetic field reverses sign compared with traditional predictions on the dawn edge of the jet