Are all leading shocks driven by magnetic clouds?

Magnetic clouds (MCs) are commonly observed in association with shocks at 1 AU, and many authors have claimed that the leading shocks are driven by MCs, without any direct evidence. In this work we surveyed the relations between MCs and their associated shocks. Using the interplanetary plasma and magnetic field data measured by Wind, we have identified 97 MCs near Earth during 1995 to 2007. Sixty‐two (64%) of the MCs were associated with leading shocks. As Lepping et al. have pointed out, if a leading shock is driven by an MC, the axis of the driver should be approximately perpendicular to the shock normal. We calculated the angle θ between the axis of the MC and its leading shock normal and found that the angle θ for 21 of the 62 MCs deviates from 90° by more than 25°. Seventeen of the 21 MCs also have the following signatures: (1) The MC leading edge moves more slowly than the preceding shock; and (2) the time interval between the preceding shock and the front boundary of the MC is always long, with an average period of 15.4 h. The speed profiles of the other four events revealed that the leading shock was driven not directly by the MC but by the immediately subsequent flow, and their sheath durations are shorter than the time interval between the preceding shocks and the MC front boundaries. Therefore, at least 21 (34%) of 62 leading shocks were not directly driven by MCs.