The Radial Variation of Suprathermal Particles in Stream Interaction Regions: Comparisons between Solar Orbiter, ACE, and STEREO-A

Abstract With the launch of Solar Orbiter, the intensity and composition of suprathermal particles have been sampled within the inner heliosphere at distances between ∼0.3 and 1 au. This includes a multitude of observations of suprathermal ions associated with stream interaction regions (SIRs) measured by the Solar Orbiter Energetic Particle Detector, with corresponding observations at 1 au from the Ultra-Low-Energy Isotope Spectrometer on the Advanced Composition Explorer mission and the Suprathermal Ion Telescope on the Solar-Terrestrial Relations Observatory-Ahead spacecraft. Comparing observations between these spacecraft enables a statistical view of the radial variations of SIR-associated suprathermal particles, allowing for greater insight into energetic particle transport within the inner heliosphere. The observed radial trends agree well with that found by M. A. I. Van Hollebeke et al. but indicate a smaller modulation of particle spectra due to transport than predicted by L. A. Fisk & M. A. Lee. This possible difference between the observed empirical relationship and theoretical formulation may arise from either sub-Parker spiral topology in the rarefaction region and/or a velocity-dependent, rather than rigidity or explicit species-dependent, diffusion treatment in L. A. Fisk & M. A. Lee.