Acceleration of Energetic Ions in Corotating Interaction Region near 1.5 au: Evidence from MAVEN

The dearth of observations between 1 and 3 au limits our understanding of energetic particle acceleration processes in interplanetary space. We present first-of-their-kind observations of the energetic particle acceleration in a corotating interaction region (CIR) using data from two vantage points, 1 au (near Earth) and 1.5 au (near Mars). The CIR event of 2015 June was observed by the particle detectors aboard the Advanced Composition Explorer satellite as well as the Solar Energetic Particle (SEP) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft situated near 1.5 au. We find that a CIR shock can accelerate a significant number of particles even at 1.5 au. During this event the acceleration by the shocks associated with the CIR could cause an enhancement of around two orders of magnitude in the SEP energetic ion fluxes in the ∼500 keV to 2 MeV range when the observations near 1 and 1.5 au are compared. To demonstrate the differences between SEP acceleration in CIR and other impulsive events, we show the energetic ion flux observations during an intense coronal mass ejection period in March 2015, in which case the enhanced SEP fluxes are seen even at 1 au. These observations provide evidence that CIR shock can accelerate particles in the region between Earth and Mars—that is, only within the short heliocentric distance of 0.5 au—in interplanetary space.