Stereoscopic analysis of STEREO/SECCHI data for CME trajectory determination

The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) coronagraphs on the twin Solar TErrestrial RElations Observatory (STEREO) spacecraft provide simultaneous views of the corona and coronal mass ejections from two view points. Here, we analyze simultaneous image pairs using the technique of tie-pointing and triangulation (T&T) to determine the three-dimensional trajectory of seven coronal mass ejections (CMEs). The bright leading edge of a CME seen in coronagraph images results from line-of-sight integration through the CME front; the two STEREO coronagraphs see different apparent leading edges, leading to a systematic error in its three-dimensional reconstruction. We analyze this systematic error using a simple geometric model of a CME front. We validate the technique and analysis by comparing T&T trajectory determinations for seven CMEs with trajectories determined by Thernisien et al. (2009) using a forward modeling technique not susceptible to this systematic effect. We find that, for the range of spacecraft separation studied ( ≤ 50 ), T&T gives reliable trajectories (uncertainty < 10 in direction and < 15% velocity) for CME propagating within approximately ±40 of perpendicular to Sun-Earth line. For CMEs close to the Sun-Earth or Sun-Spacecraft lines, T&T is subject to larger errors, especially in the velocity.