Multiple energy-dispersed ion precipitations in the low-latitude auroral oval: Evidence of ExB drift effect and upward flowing ion contribution

The polar-orbiting satellites, Akebono and DMSP F8, have frequently observed energy-dispersed ion precipitation events in the low-latitude auroral oval. The general properties of these precipitations are the following: (1) the characteristic energy decreases with decreasing latitude, (2) the signatures appear simultaneously with the diffuse components of electrons and ions on closed field lines, and (3) multiple (overlapped) signatures of the energy-dispersed ion precipitations are sometimes observed simultaneously in the northern and the southern hemispheres. These precipitation events are classified into two types: type A and type B according to their energy ratios and mass composition. The ratios between the energies of the type A multiple energy-dispersed signatures are constants 1:3(2):5(2):7(2) or 1:2(2) (''odd'' or ''natural'' integer cases). These odd or natural integers indicate the ratios of the flight distances from the source region to the observation point of ion clusters consisting mainly of a single ion species. There are two possible sources: upward flowing ions (UFIs) from the ionosphere and bidirectional ion injection in the equatorial region. The latter model, however, is consistent only with the odd-integer case. The triple energy-dispersed signatures (type B) consist of three ion species (H+, He+, O+), and the ratios of the energies of these signatures always correspond to the mass ratios; that is, E(H+):E(He+):E(O+) = M(H+):M(He+):M(O+) = 1:4:16. This relationship indicates that these multicomposition ion clusters of ionospheric origin have the same flow velocity and the source distance. We conclude that the source of these two types of the energy-dispersed ion precipitations are UFIs ejected from the ionosphere. The ions of UFI origin are velocity-filtered (energy-dispersed), dominated by the E x B drift on closed field lines, and reenter the ionosphere as downward flowing ions. The pitch angle distributions indicate that parallel acceleration through an electrostatic potential producing the UFI beams is the most probable acceleration process.