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Fast coronal mass ejections (CMEs) generate standing or bow shocks as theypropagate through the corona and solar wind. Although CME shocks havepreviously been detected indirectly via their emission at radio frequencies,direct imaging has remained elusive due to their low contrast at opticalwavelengths. Here we report the first images of a CME-driven shock as itpropagates through interplanetary space from 8R_Sun to 120R_Sun (0.5AU), usingobservations from the STEREO Heliospheric Imager (HI). The CME was measured tohave a velocity of ~1000 km/s and a Mach number of 4.1\pm1.2, while the shockfront stand-off distance (Delta) was found to increase linearly to ~20 R_Sun at0.5 AU. The normalised standoff distance (Delta/DO) showed reasonable agreementwith semi-empirical relations, where D_O is the CME radius. However, whennormalised using the radius of curvature, Delta/R_O did not agree well withtheory, implying that R_O was under-estimated by a factor of ~3-8. This is mostlikely due to the difficulty in estimating the larger radius of curvature alongthe CME axis from the observations, which provide only a cross-sectional viewof the CME.

STEREO Direct Imaging of a CME-driven Shock to 0.5AU