Cluster observations and numerical modeling of energy-dispersed ionospheric H+ ions bouncing at the plasma sheet boundary layer

摘要

The Cluster mission offers a unique opportunity to investigate the origin of the energy-dispersed ion structures frequently observed at 4.5-5 RE altitude in the auroral region. We present a detailed study of the 14 February 2001 northern pass, characterized by the successive observation by three spacecraft of a series of energy-dispersed structures at ~72-75° ILAT in a region of poleward convection. Equatorward, the satellites also observed a localized, steady, and intense source of outflowing energetic (3-10 keV) H~+ and O+ ions. These substructures were modeled by launching millions of H+ ions from this ionospheric source and following them through time-dependent electric and magnetic fields obtained from a global MHD simulation of this event. Despite the complexity of ion orbits, the simulations showed that a large number of ions returned to the Cluster location, poleward of their source, in a number of adjacent or overlapping energy-latitude substructures with the correct dispersion. The first dispersed echo was unexpectedly generated by "half-bouncing" ions that interacted with the current sheet to return to the same hemisphere. The time-shifted observations made by two Cluster (SC 1 and SC3) spacecrafts were correctly reproduced. Almost all the ions returning to the spacecraft underwent a ,2-5 keV nonadiabatic acceleration at each interaction with the current sheet in a very confined resonant region. This acceleration explains the overall energy increase from one structure to the next. This event confirms the importance of the ionospheric source in populating bouncing ion clusters within the magnetosphere, even at high latitudes.