The radial speed‐expansion speed relation for Earth‐directed CMEs

Earth‐directed coronal mass ejections (CMEs) are the main drivers of major geomagnetic storms. Therefore, a good estimate of the disturbance arrival time at Earth is required for space weather predictions. The STEREO and SOHO spacecraft were viewing the Sun in near quadrature during January 2010 to September 2012, providing a unique opportunity to study the radial speed ( V rad )‐expansion speed ( V exp ) relationship of Earth‐directed CMEs. This relationship is useful in estimating the V rad of Earth‐directed CMEs, when they are observed from Earth view only. We selected 19 Earth‐directed CMEs observed by the Large Angle and Spectrometric Coronagraph (LASCO)/C3 coronagraph on SOHO and the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)/COR2 coronagraph on STEREO during January 2010 to September 2012. We found that of the three tested geometric CME models the full ice‐cream cone model of the CME describes best the V rad ‐ V exp relationship, as suggested by earlier investigations. We also tested the prediction accuracy of the empirical shock arrival (ESA) model proposed by Gopalswamy et al. (2005a), while estimating the CME propagation speeds from the CME expansion speeds. If we use STEREO observations to estimate the CME width required to calculate the V rad from the V exp measurements, the mean absolute error (MAE) of the shock arrival times of the ESA model is 8.4 h. If the LASCO measurements are used to estimate the CME width, the MAE still remains below 17 h. Therefore, by using the simple V rad ‐ V exp relationship to estimate the V rad of the Earth‐directed CMEs, the ESA model is able to predict the shock arrival times with accuracy comparable to most other more complex models.