  The dearth of observations between 1 au and 3 au limits our understanding of
energetic particle acceleration processes in interplanetary space. We present
the 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 June 2015 was observed by
the particle detectors aboard the Advanced Composition Explorer (ACE) satellite
as well as the SEP (Solar Energetic Particle) 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 CME 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.
