Effects of background solar wind on the propagation of coronal mass ejection driven shock

Propagation of interplanetary (IP) shocks, particularly those driven by coronal mass ejections (CMEs), is still an outstanding question in heliophysics and space weather forecasting. Here we address effects of the ambient solar wind on the propagation of two CME-driven shocks from Sun to Earth. The two CME-driven shock events (CME03: April 3, 2010 and CME12: July 12, 2012) have the following properties: (1) driven by a halo CME (i.e., source location is near Sun-Earth line), (2) a southern hemispheric CME source location, (3) similar propagation speed (e.g., took ~2 days reach the Earth), (4) occurred in the non-quiet solar period, and (4) leading to a severe geomagnetic storm. What is interesting is that the initial (near the Sun) propagation speed, as measured by coronagraph images, of CME03 was slower (~300 km/s) than CME12, but it took about same time for both events to reach the Earth. According to in-situ solar wind observations from Wind, the CME03-driven shock was associated with a faster solar wind upstream of the shock than the CME12. This is also demonstrated in our global MHD simulations. This study emphasizes the importance of the background solar wind in the propagation of CME-driven shocks. Not only the initial propagation speed near the Sun but also the ambient solar wind speed is the key to timing the arrival of CME events. The present study also demonstrated that global MHD simulations with realistic solar wind inputs is able to precisely predict the arrival of CME events.