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On 15–17 February 2008, a CME with an approximately circular crosssection was tracked through successive images obtained by the HeliosphericImager (HI) instrument onboard the STEREO-A spacecraft. Reasoning that anidealised flux rope is cylindrical in shape with a circular cross-section,best fit circles are used to determine the radial width of the CME. As partof the process the radial velocity and longitude of propagation aredetermined by fits to elongation-time maps as 252±5 km/s and 70±5°respectively. With the longitude known, the radial size is calculatedfrom the images, taking projection effects into account. The radial width ofthe CME, S (AU), obeys a power law with heliocentric distance, R, as the CMEtravels between 0.1 and 0.4 AU, such that S=0.26 R0.6±0.1. Theexponent value obtained is compared to published studies based onstatistical surveys of in situ spacecraft observations of ICMEs between 0.3and 1.0 AU, and general agreement is found. This paper demonstrates the newopportunities provided by HI to track the radial width of CMEs through thepreviously unobservable zone between the LASCO field of view and Helios insitu measurements

The radial width of a Coronal Mass Ejection between 0.1 and 0.4 AU estimated from the Heliospheric Imager on STEREO