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Using data from the Heliospheric Imagers (HIs) onboard STEREO, it is possibleto derive the direction of propagation of coronal mass ejections (CMEs) inaddition to their speed with a variety of methods. For CMEs observed by bothSTEREO spacecraft, it is possible to derive their direction using simultaneousobservations from the twin spacecraft and also, using observations from onlyone spacecraft with fitting methods. This makes it possible to test and comparedifferent analyses techniques. In this article, we propose a new fitting methodbased on observations from one spacecraft, which we compare to the commonlyused fitting method of Sheeley et al. (1999). We also compare the results fromthese two fitting methods with those from two stereoscopic methods, focusing on12 CMEs observed simultaneously by the two STEREO spacecraft in 2008 and 2009.We find evidence that the fitting method of Sheeley et al. (1999) can result insignificant errors in the determination of the CME direction when the CMEpropagates outside of 60deg \pm 20 deg from the Sun-spacecraft line. We expectour new fitting method to be better adapted to the analysis of halo or limbCMEs with respect to the observing spacecraft. We also find some evidence thatdirect triangulation in the HI fields-of-view should only be applied to CMEspropagating approximatively towards Earth (\pm 20deg from the Sun-Earth line).Last, we address one of the possible sources of errors of fitting methods: theassumption of radial propagation. Using stereoscopic methods, we find that atleast seven of the 12 studied CMEs had an heliospheric deflection of less than20deg as they propagated in the HI fields-of-view, which, we believe, validatesthis approximation.

solar physics

Accuracy and Limitations of Fitting and Stereoscopic Methods to  Determine the Direction of Coronal Mass Ejections from Heliospheric Imagers  Observations