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

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 Alfvén 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.

15 pages, 4 Figures, 5 Tables, accepted to ApJ

References in corpus (5)

• Anomalous Expansion of Coronal Mass Ejections during Solar Cycle 24 and its Space Weather Implications
• Global and local expansion of magnetic clouds in the inner heliosphere
• The Peculiar Behavior of Halo Coronal Mass Ejections in Solar Cycle 24
• Factors Affecting the Geo-effectiveness of Shocks and Sheaths at 1 AU
• Shocks inside CMEs: A Survey of Properties from 1997 to 2006

Cited by in corpus (6)

• Inconsistencies Between Local and Global Measures of CME Radial Expansion as Revealed by Spacecraft Conjunctions
• Moving solar radio bursts and their association with coronal mass ejections
• Properties of the Sheath Regions of Coronal Mass Ejections with or without Shocks from STEREO in situ Observations near 1 AU
• Forecasting Periods of Strong Southward Magnetic Field Following Interplanetary Shocks
• How flux feeding causes eruptions of solar magnetic flux ropes with the hyperbolic flux tube configuration?
• Magnetic Structure and Propagation of Two Interacting CMEs from the Sun to Saturn