Propagation of interplanetary (IP) shocks near the Earth

Abstract. The solar wind monitor at the Sun-Earth L1 point has been used to estimate the arrival time of interplanetary (IP) shocks associated with coronal mass ejections (CMEs) and co-rotating interaction regions (CIRs). In standard estimates, including NASA/OMNI database, the radial propagation speed of the IP shock is assumed to be the same as the measured solar wind (proton) speed, although these speeds are known to be different in both the shock theory and CME observations. To diagnose the actual error in the arrival time of the IP shocks, we statistically compared the radial propagation speed of the IP shock with the measured solar wind speed at L1. The propagation speed is obtained from the time-of-propagation between the IP shock passage at the L1 monitoring spacecraft (SOHO and ACE) and at the Earth, the latter of which is represented by the geomagnetic sudden commencement (SC). In statistics, we limited to the IP shocks with a clear geomagnetic SC signature and with velocity profiles consistent between SOHO and ACE. During 1998–2022, 375 IP shocks satisfied such conditions. For the solar wind speed, the highest value during 15 min after the IP shock passage observed by the L1 monitoring spacecraft was used. We found the following tendencies. (1) As expected, actual arrival time of the IP shock to the Earth (represented by the geomagnetic SC) is often quite different from the predicted arrival time using the L1 velocity measurement. (2) For a majority of the cases, the geomagnetic SC is observed 0–10 min earlier than the predicted IP arrival time. (3) The speed difference is distributed asymmetrically toward faster propagation, with peak of the distribution about +10 %.