Large difference in radial speed between the interplanetary (IP) shock propagation and the solar wind near the Earth.

The solar wind velocity monitor at the Sun-Earth L1 point (SOHO, ACE, DSCOVR) has been used to estimate the arrival time of interplanetary (IP) shocks associated with coronal mass ejections (CMEs) and corotating interaction regions (CIRs).  In this estimate, the radial propagation speed of the IP shock is assumed to be the same as the measured solar wind (proton) speed.  However, near the Sun, the CME front identified by SOHO LASCO imager sometimes propagates at velocity >1000 km/s, being faster than solar wind velocity measured at L1 (e.g., Tokumaru et al., 2006).  Even after considering the deceleration of CME propagation with distance, these two speeds at 1 AU are not guaranteed to be the same.

We compared these speeds: between the solar wind and the radial propagation of the IP shock front.  We used SOHO and ACE spacecraft data for the velocity and IP shock timing at L1, and geomagnetic data (geomagnetic sudden commencement: SC) for the IP shock timing at the Earth.  We examined about 400 IP shock events that are consistent between SOHO and ACE during more than two solar cycles.  We found the following tendency.
(1) The estimated arrival time driven from geomagnetic SC is often quite different from expected arrival time of the IP shock from the L1 velocity measurement. 
(2) The estimated propagation velocity of the IP shock was from 80% to > 200% of the solar wind velocity.
(3) For a majority of the cases, the SC-estimated propagation velocity is slightly faster than the measured solar wind proton velocity, and is rather close to the velocity of the solar wind alpha particle.

The upcoming SMILE mission will give extra dataset for the arrival of the IP shock for further study.