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It is difficult to measure the true speed of Earth-directed CMEs from acoronagraph located along the Sun-Earth line because of the occulting disk.However, the expansion speed (the speed with which the CME appears to spread inthe sky plane) can be measured by such a coronagraph. In order to convert theexpansion speed to radial speed (which is important for space weatherapplications) one can use an empirical relationship between the two thatassumes an average width for all CMEs. If we have the width information fromquadrature observations, we can confirm the relationship between expansion andradial speeds derived by Gopalswamy et al. (2009a). The STEREO spacecraft werein qudrature with SOHO (STEREO-A ahead of Earth by 87o and STEREO-B 94o behindEarth) on 2011 February 15, when a fast Earth-directed CME occurred. The CMEwas observed as a halo by the Large-Angle and Spectrometric Coronagraph (LASCO)on board SOHO. The sky-plane speed was measured by SOHO/LASCO as the expansionspeed, while the radial speed was measured by STEREO-A and STEREO-B. Inaddition, STEREO-A and STEREO-B images provided the width of the CME, which isunknown from Earth view. From the SOHO and STEREO measurements, we confirm therelationship between the expansion speed (Vexp) and radial speed (Vrad) derivedpreviously from geometrical considerations (Gopalswamy et al. 2009a): Vrad =1/2 (1 + cot w)Vexp, where w is the half width of the CME. STEREO-B images ofthe CME, we found that CME had a full width of 76o, so w = 38o. This gives therelation as Vrad = 1.14 Vexp. From LASCO observations, we measured Vexp = 897km/s, so we get the radial speed as 1023 km/s. Direct measurement of radialspeed yields 945 km/s (STEREO-A) and 1058 km/s (STEREO-B). These numbers aredifferent only by 7.6% and 3.4% (for STEREO-A and STEREO-B, respectively) fromthe computed value.

The Relationship Between the Expansion Speed and Radial Speed of CMEs Confirmed Using Quadrature Observations of the 2011 February 15 CME