Buildup of the ring current during periodic loading‐unloading cycles in the magnetotail driven by steady southward interplanetary magnetic field

Abstract

During prolonged intervals of negative interplanetary magnetic field (IMF) Bz the magnetosphere often enters a state in which quasi‐periodic, large‐amplitude oscillations of energetic particle fluxes are observed at the geosynchronous orbit. We use the global magnetosphere MHD code BATS‐R‐US output during a long period of steady southward IMF Bz to drive the Fok Ring Current Model. Previous simulations of such events demonstrated flat behavior of the energetic particle fluxes after the initial injection. Periodical north/south IMF turning was required to reproduce oscillations in particle fluxes at geosynchronous orbit. In the present study we use a global magnetosphere MHD code that reproduces fast magnetotail reconnection rates observed in kinetic simulations. This results in periodical loading‐unloading cycles in the magnetotail even for steady southward Bz and can explain quasi‐periodic oscillations of geosynchronous energetic particle fluxes. The total proton energy within geosynchronous orbit exhibits overall growth in time due to quasi‐steady convection and oscillates due to injection through inductive electric field caused by multiple dipolarization. The flux oscillation amplitude is stronger in the outer regions of the ring current although the regions close to the geosynchronous orbit experience substantial perturbations as well.