Examining the Radial Evolution of a Corotating Interaction Region Observed at STEREO-A and MAVEN

Abstract Corotating interaction regions (CIRs) are long-lasting solar wind structures that persist over multiple solar rotations. These structures accelerate particles throughout the heliosphere and can impart significant energy onto planetary ionospheres and magnetospheres. Understanding how CIRs and their associated energetic particles evolve radially with heliocentric distance is of great interest and can give insight into acceleration mechanisms that occur within these structures. CIR solar wind and particle properties have been examined at numerous heliocentric distances but have been largely unexplored at Mars. We examine the properties of a CIR observed over two Carrington rotations by the Solar Terrestrial Relations Observatory (STEREO)-A at 1 au and the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft at 1.5 au. This CIR was observed during near-radial alignment of the spacecraft, allowing for the exclusion of significant longitudinal variations in the CIR’s properties. We find that during both rotations, the scaled solar wind density and interplanetary magnetic field are slightly higher at MAVEN than measurements at STEREO-A, and the CIR compression occurs over similar periods at both spacecraft during each event. Suprathermal particle enhancements are observed at both spacecraft during both passes of the CIR. Variations in the particle spectra between observations indicate suprathermal particle acceleration at both spacecraft, with higher-energy populations observed at 1.5 au compared to 1 au. Spectral anisotropies observed at MAVEN also indicate a combination of local and nonlocal particle acceleration, further demonstrating that the acceleration mechanisms associated with CIRs are complex and evolve with heliocentric distance.