Bulk Flow Velocity and First-Order Anisotropy of Solar Energetic Particles Observed on the Wind Spacecraft: Overview of Three "Gradual" Particle Events

Abstract

We have developed techniques to calculate bulk flow velocity and first-order anisotropy of solar energetic particles (SEPs) with MeV nucleon-1 energies as recorded on the Wind spacecraft. Using the techniques we selected and analyzed three gradual SEP events having different solar longitudes. Since upstream of interplanetary (IP) shocks during our selected events the interplanetary magnetic field is nearly perpendicular to the solar wind, the diffusive transport of SEPs along the magnetic field line is conveniently decoupled from solar-wind streaming. We present the bulk flow velocity measurements of H, He, O, and Fe ions at different energies. In two of the three events studied, it is seen that the flow directions of heavy ions reverse in sequence, i.e., faster ions reverse their direction earlier. Several hours before the IP shock passage, the bulk flows of all heavy ions become opposite to the proton flow. Thus, in the upstream region we mainly observe shock-accelerated protons that continue to flow away from the shock, while higher rigidity heavy ions predominantly come from strong acceleration near the Sun. The reversed ion direction appears also to involve a reflecting boundary beyond 1 AU, from which higher velocity ions return earlier. The preferred geometry of the selected 2001 September 24 event also allows us to determine the propagating direction of proton-generated Alfvén waves based on flow velocity measurements of heavy ions.