Interpreting some properties of CIRs and their associated shocks during the last two solar minima using global MHD simulations

In this study, we investigate some properties of corotating interaction regions (CIRs) during the recent solar minimum (December 2008), and compare them to CIRs observed during the previous minimum (September 1996). In particular, we focus on the orientation of stream interfaces (SIs), which separate wind that was originally slow and dense from wind that was originally fast and tenuous. We find that while the east–west flow deflections imply a systematic tilt of CIRs such that they are aligned with the nominal Parker spiral direction, the north–south flow deflections are much more irregular and show no discernible patterns. Comparison with global MHD model results suggest that this is a consequence of the spacecraft intercepting the equatorward flanks of the CIRs. We also study the solar-cycle variations of CIR-associated shocks over the last cycle, finding that forward (F) shocks tended to occur approximately three times more frequently than reverse (R) shocks, and, moreover, during the recent minimum, there were approximately 3–4 times more R shocks than during the previous minimum. We show that this too is likely due to the orientation of CIRs and Earth's limited vantage point in the ecliptic plane.

► East–west flow deflections imply a systematic tilt of CIRs such that they are aligned with the nominal Parker spiral direction.
► North–south flow deflections are much more irregular and show no discernible patterns.
► Forward (F) shocks tended to occur approximately three times more frequently than reverse (R) shocks.
► During this minimum, there were approximately 3–4 times more R shocks than during the previous minimum.