  It is difficult to measure the true speed of Earth-directed CMEs from a
coronagraph located along the Sun-Earth line because of the occulting disk.
However, the expansion speed (the speed with which the CME appears to spread in
the sky plane) can be measured by such a coronagraph. In order to convert the
expansion speed to radial speed (which is important for space weather
applications) one can use an empirical relationship between the two that
assumes an average width for all CMEs. If we have the width information from
quadrature observations, we can confirm the relationship between expansion and
radial speeds derived by Gopalswamy et al. (2009a). The STEREO spacecraft were
in qudrature with SOHO (STEREO-A ahead of Earth by 87o and STEREO-B 94o behind
Earth) on 2011 February 15, when a fast Earth-directed CME occurred. The CME
was 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 expansion
speed, while the radial speed was measured by STEREO-A and STEREO-B. In
addition, STEREO-A and STEREO-B images provided the width of the CME, which is
unknown from Earth view. From the SOHO and STEREO measurements, we confirm the
relationship between the expansion speed (Vexp) and radial speed (Vrad) derived
previously 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 of
the CME, we found that CME had a full width of 76o, so w = 38o. This gives the
relation as Vrad = 1.14 Vexp. From LASCO observations, we measured Vexp = 897
km/s, so we get the radial speed as 1023 km/s. Direct measurement of radial
speed yields 945 km/s (STEREO-A) and 1058 km/s (STEREO-B). These numbers are
different only by 7.6% and 3.4% (for STEREO-A and STEREO-B, respectively) from
the computed value.
