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

TL;DR

This paper presents the first direct optical images of a CME-driven shock propagating through interplanetary space, revealing its dynamics and testing theoretical models of shock-CME interactions.

Contribution

It provides the first direct imaging of a CME-driven shock from 8 to 120 solar radii, offering new insights into shock stand-off distances and curvature estimations.

Findings

Shock observed from 8 to 120 solar radii.

Stand-off distance increases linearly with distance.

Discrepancy in radius of curvature estimates suggests observational limitations.

Abstract

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.