Relationships between coronal mass ejection speeds from coronagraph images and interplanetary characteristics of associated interplanetary coronal mass ejections

Abstract

With an eye toward space weather forecasting and the planned Solar Terrestrial Relations Observatory mission, a combination of Solwind and SMM coronagraph data and Helios‐1 and Pioneer Venus Orbiter interplanetary field and plasma data are used to study statistical relationships between the speeds of coronal mass ejections (CMEs) observed near the Sun and key characteristics of the associated interplanetary disturbances (interplanetary coronal mass ejections (ICMEs)) detected near the ecliptic at ≤ 1 AU. When confident associations can be made between the coronagraph observations and interplanetary observations, a predictable relationship is found between observed coronagraph CME speeds and subsequently observed ICME bulk plasma speeds. Consistent with earlier work, the CMEs, regardless of their speed, produce ICMEs moving at least as fast as the minimum solar wind speed. As a rule, the CMEs observed at speeds below the average solar wind speed produce ICMEs that travel faster than the associated CME, implying acceleration, while CMEs with coronagraph speeds above the average solar wind speed produce ICMEs that travel slower than the associated CME, implying deceleration of initially fast low‐heliolatitude ejecta. A formula is provided for estimating ICME speed from CME speed. As also previously found, faster CMEs tend to produce ICMEs with larger internal magnetic field magnitudes. While the size and occurrence of southward Bz in an ICME are not generally related to the observed CME speed, Bz in the sheath region preceding the ICME shows some positive correlation. These observations confirm that while the occurrence of large interplanetary magnetic field magnitudes and high bulk plasma speeds associated with ICME passage may be predictable from coronagraph‐derived CME speeds, other important ICME features like large‐magnitude southward Bz require other diagnostics and tools for forecasts.