Estimating Travel Times of Coronal Mass Ejections to 1 AU Using Multi-spacecraft Coronagraph Data

We study the relationship between the speeds of coronal mass ejections (CMEs) obtained close to the Sun and in the interplanetary medium during the low solar-activity period from 2008 to 2010. We use a multi-spacecraft forward-modeling technique to fit a flux-rope-like model to white-light coronagraph images from the STEREO and SOHO spacecraft to estimate the geometrical configuration, propagation in three-dimensions (3D), and the radial speeds of the observed CMEs. The 3D speeds obtained in this way are used in existing CME travel-time prediction models. The results are compared to the actual CME transit times from the Sun to STEREO, ACE, and Wind spacecraft as well as to the transit times calculated using projected CME speeds. CME 3D speeds give slightly better predictions than projected CME speeds, but a large scatter is observed between the predicted and observed travel times, even when 3D speeds are used. We estimate the possible sources of errors and find a weak tendency for large interplanetary CMEs (ICMEs) with high magnetic fields to arrive faster than predicted and small, low-magnetic-field ICMEs to arrive later than predicted. The observed CME transit times from the Sun to 1 AU show a particularly good correlation with the upstream solar-wind speed. Similar trends have not been observed in E.K.J. Kilpua ( ) Department of Physics, Division of Geophysics and Astronomy, University of Helsinki, Helsinki, Finland e-mail: [email protected] M. Mierla Institute of Geodynamics of the Romanian Academy, Jean-Louis Calderon 19-21, Bucharest-37, Romania 020032 M. Mierla · L. Rodriguez · A.N. Zhukov · M.J. West Solar-Terrestrial Center of Excellence – SIDC, Royal Observatory of Belgium, Av. Circulaire 3, 1180, Brussels, Belgium A.N. Zhukov Skobeltsyn Institute of Nuclear Physics, Moscow State University, 119992 Moscow, Russia N. Srivastava Physical Research Laboratory, Udaipur Solar Observatory, P.O. Box 198, Badi Road, Udaipur 313001, India 478 E.K.J. Kilpua et al. previous studies using data sets near solar maximum. We suggest that near solar minimum a relatively narrow range of CME initial speeds, sizes, and magnetic-field magnitudes led to a situation where aerodynamic drag between CMEs and ambient solar wind was the primary cause of variations in CME arrival times from the Sun to 1 AU.