Why Coronal Mass Ejections Arrive Differently: Solar Cycle Modulation Through Solar Wind Structure

Abstract Coronal Mass Ejections (CMEs) are large structures of magnetized plasma ejected from the Sun's atmosphere into the heliosphere. The interaction of CMEs with the ambient solar wind during propagation affects arrival time and speed at Earth. Since the solar wind structure changes with the solar cycle, variability in the transit times and arrival speeds of CMEs should also change systematically with the solar cycle. We use a solar wind model, HUXt, to conduct simulations of the same CME propagating through realistic ambient solar wind environments from 1975 to 2024 inclusive. We conduct this experiment with an “average” CME, with an initial speed of 495 km and a full angular width of 37.4, and a “fast” CME, with a speed of 1,070 km and an angular width of 69.8. We find that lower solar activity is associated with increased variability in both transit time to Earth and arrival speed at Earth. For the average CME, the average short‐term variability (i.e., inter‐quartile range within a Carrington rotation) in transit time was 22.4 hr during solar minimum and 16.4 hr during solar maximum. The fast CME demonstrated reduced variability, with corresponding values of 18.2 and 12.5 hr. Similar trends in variability were found for arrival speeds. This suggests that for a given CME, arrival times and speeds at Earth are more predictable at solar maximum than solar minimum. Furthermore, comparing transition phases of the solar cycle, the declining phase was found to be more variable than the rising phase.