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

Fast coronal mass ejections (CMEs) generate standing or bow shocks as they propagate through the corona and solar wind. Although CME shocks have previously been detected indirectly via their emission at radio frequencies, direct imaging has remained elusive due to their low contrast at optical wavelengths. Here we report the first images of a CME-driven shock as it propagates through interplanetary space from 8R_Sun to 120R_Sun (0.5AU), using observations from the STEREO Heliospheric Imager (HI). The CME was measured to have a velocity of ~1000 km/s and a Mach number of 4.1\pm1.2, while the shock front stand-off distance (Delta) was found to increase linearly to ~20 R_Sun at 0.5 AU. The normalised standoff distance (Delta/DO) showed reasonable agreement with semi-empirical relations, where D_O is the CME radius. However, when normalised using the radius of curvature, Delta/R_O did not agree well with theory, implying that R_O was under-estimated by a factor of ~3-8. This is most likely due to the difficulty in estimating the larger radius of curvature along the CME axis from the observations, which provide only a cross-sectional view of the CME.