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

Abstract. On 15–17 February 2008, a CME with an approx-imately circular cross section was tracked through succes-sive images obtained by the Heliospheric Imager (HI) instru-ment onboard the STEREO-A spacecraft. Reasoning that an idealised flux rope is cylindrical in shape with a circu-lar cross-section, best fit circles are used to determine the radial width of the CME. As part of the process the radial velocity and longitude of propagation are determined by fits to elongation-time maps as 252±5 km/s and 70±5 ◦ respec-tively. With the longitude known, the radial size is calculated from the images, taking projection effects into account. The radial width of the CME, S (AU), obeys a power law with heliocentric distance, R, as the CME travels between 0.1 and 0.4 AU, such that S=0.26R0.6±0.1. The exponent value ob-tained is compared to published studies based on statistical surveys of in situ spacecraft observations of ICMEs between 0.3 and 1.0 AU, and general agreement is found. This paper demonstrates the new opportunities provided by HI to track the radial width of CMEs through the previously unobserv-able zone between the LASCO field of view and Helios in situ measurements