Importance of CME Radial Expansion on the Ability of Slow CMEs to Drive Shocks

TL;DR

This study investigates how the radial expansion of slow CMEs influences their ability to drive shocks, highlighting that expansion plays a significant role in shock formation for CMEs under 375 km/s.

Contribution

It provides the first detailed analysis of the impact of CME expansion on shock-driving capability in slow CMEs using two decades of Wind data.

Findings

Approximately half of slow CMEs with shocks are linked to CME expansion.

No significant difference in shock occurrence for slow CMEs between solar cycles 23 and 24.

Slower CME expansion in cycle 24 may explain the unchanged shock frequency.

Abstract

Coronal mass ejections (CMEs) may disturb the solar wind either by overtaking it, or by expanding into it, or both. CMEs whose front moves faster in the solar wind frame than the fast magnetosonic speed, drive shocks. Such shocks are important contributors to space weather, by triggering substorms, compressing the magnetosphere and accelerating particles. In general, near 1 AU, CMEs with speed greater than about 500 km s$^{-1}$ drive shocks, whereas slower CMEs do not. However, CMEs as slow as 350 km s$^{-1}$ may sometimes, although rarely, drive shocks. Here, we study these slow CMEs with shocks and investigate the importance of CME expansion in contributing to their ability to drive shocks and in enhancing shock strength. Our focus is on CMEs with average speeds under 375 km s$^{-1}$. From Wind measurements from 1996 to 2016, we find 22 cases of such shock-driving slow CMEs, and, for about half of them (11 out of the 22), the existence of the shock appears to be strongly related to CME expansion. We also investigate the proportion of all CMEs with speeds under 500 km s$^{-1}$ with and without shocks in solar cycles 23 and 24, depending on their speed. We find no systematic difference, as might have been expected on the basis of the lower solar wind and Alfv\'en speeds reported for solar cycle 24 vs. 23. The slower expansion speed of CMEs in solar cycle 24 might be an explanation for this lack of increased frequency of shocks, but further studies are required.