Solar energetic particles in the intermediate region of the heliosphere

Abstract

Energetic particle increases in the energy range 4 to 130 MeV observed by the Kiel Electron Telescope (KET) during the in-ecliptic part of the ULYSSES mission between 1 and 5.4 AU differ distinctly from their appearance in the inner heliosphere inside 1 AU. Prompt events are rare, and for most of the events velocity dispersion is lacking. The observed intensity-time profiles show poor time relations with optical solar flares. The data is far better organized by the structure of the interplanetary medium with its shocks, tangential discontinuities, corotating interaction regions, heliospheric current sheet crossings (sector boundaries), and coronal mass ejections. Nine particle events are discussed in detail. Spectral information as well as composition measurements are used to determine the source of the particle increases: solar, interplanetary, and Jovian. The few prompt events can be fitted with diffusive interplanetary propagation models, yielding radial mean free paths in the range λr = 0.05 to 0.15 AU. The shock related events are dominant at these intermediate distances, with particularly high intensities of MeV-electrons and > 30 MeV protons at merged or double shocks. A new feature are slowly-varying populations (SVP) with long time scales neither related to prompt events nor to local shock effects.