How Do Shock Waves Define the Space-Time Structure of Gradual Solar Energetic Particle Events?

Abstract

We revisit the full variety of observed temporal and spatial distributions of energetic solar protons in “gradual” solar energetic-particle (SEP) events resulting from the spatial variations in the shock waves that accelerate them. Differences in the shock strength at the solar longitude of a spacecraft and at the footpoint of its connecting magnetic field line, curved by solar rotation nominally 55^{circ} to the west, drive much of that variation. The shock wave itself, together with energetic particles trapped near it by self-amplified hydromagnetic or Alfvén waves, forms an underlying autonomous structure. This structure can drive across magnetic field lines intact, spreading proton intensities in a widening SEP longitude distribution. During the formation of this fundamental structure, historically called an “energetic storm particle” (ESP) event, many SEPs leak away early, amplifying waves as they flow along well-connected field lines and broaden the distribution outward; behind the structure, between the shock and the Sun, a “reservoir” of quasi-trapped SEPs forms. Very large SEP events are complicated by additional extensive wave growth that can spread an extended ESP-like trapping region around the Sun throughout most of the pre-shock event. Here SEP intensities are bounded at the “streaming limit,” a balance between proton streaming, which amplifies waves, and scattering, which reduces the streaming. The multiplicity of shock-related processes contributing to the observed SEP profiles causes correlations of the events to be poorly represented by the single peak intensity commonly used. In fact, the extensive spatial distributions of SEPs are sometimes free and sometimes interwoven with the structures of the shocks that have accelerated them. We should consider new questions: Which extremes of the shock contribute most to a local SEPs profile of an event, (1) the shock at the longitude of a spacecraft, (2) the shock sim55^{circ} to the west at the footpoint of the field, or (3) SEPs that have collected in the reservoir? How does the space-time distribution of SEPs correspond with the underlying space-time distribution of shock strength?