CIRs Observed by MSL/RAD on the Martian Surface

Co-rotating Interaction Regions (CIRs) are recurrent Stream Interaction Regions in the solar wind which are stable transient plasma structures lasting several solar rotations. They can modulate Galactic Cosmic Rays (GCRs) and to some extent result in a modulation of GCR induced secondary energetic particles on the Martian surface. The Mars Science Laboratory/ Radiation Assessment Detector (MSL/RAD) has been measuring the Martian Surface Radiation Environment for four years and observes this modulation effect. We will show that the effect of CIRs can be measured on the Martian surface whose arrival times have been compared with those at Earth. We use ballistic back-mapping as well as a time-shift algorithm to map the plasma properties measured at individual spacecraft locations and times to Mars. We compare these predictions with those of the CCMC ENLIL heliospheric MHD simulations. These results show that the arriving CIRs significantly contribute to dose rate variations (Forbush Decreases) observed by MSL/RAD. These can indicate solar wind plasma properties in the vicinity of Mars which can be used as important constraints for modeling atmospheric response to variations in the solar wind.