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Coronal mass ejections (CMEs) may disturb the solar wind either by overtakingit, or by expanding into it, or both. CMEs whose front moves faster in thesolar wind frame than the fast magnetosonic speed, drive shocks. Such shocksare important contributors to space weather, by triggering substorms,compressing the magnetosphere and accelerating particles. In general, near 1AU, CMEs with speed greater than about 500 km s$^{-1}$ drive shocks, whereasslower 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 andinvestigate the importance of CME expansion in contributing to their ability todrive shocks and in enhancing shock strength. Our focus is on CMEs with averagespeeds under 375 km s$^{-1}$. From Wind measurements from 1996 to 2016, we find22 cases of such shock-driving slow CMEs, and, for about half of them (11 outof the 22), the existence of the shock appears to be strongly related to CMEexpansion. We also investigate the proportion of all CMEs with speeds under 500km s$^{-1}$ with and without shocks in solar cycles 23 and 24, depending ontheir speed. We find no systematic difference, as might have been expected onthe basis of the lower solar wind and Alfv\'en speeds reported for solar cycle24 vs. 23. The slower expansion speed of CMEs in solar cycle 24 might be anexplanation for this lack of increased frequency of shocks, but further studiesare required.

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