Interplanetary shocks unconnected with earthbound coronal mass ejections

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[1] An associated study by Howard and Tappin (2005) identified 7 Earthbound forward shocks (of which 3 were geoeffective) which were not connected with any detectable coronal mass ejection activity along the Sun-Earth line. This largely unexplored result lends evidence to the fact that some large interplanetary transients are not detected by coronagraphs. This letter explores two possibilities for the formation of the interplanetary foreward shock, namely Corotating Interaction Regions (CIR) or Erupting Magnetic Structures (EMS). Data from EPAM, SWEPAM and MAG on board ACE provided details of the interplanetary shock and associated energetic ions along the Sun-Earth line, while evidence of magnetic field reorientation at the Sun was investigated using EIT on board SOHO, the GOES network and ground-based Hα and radio telescopes. No evidence was found to associate 6 of the shocks with CIRs, although we were uncertain about one event, and in each case evidence of chromospheric activity at the Sun was detected between the estimated onset time of the transient and the arrival of the shock at ACE. The nature of this surface activity included X-Ray (≥C5.0) and Hα flares, associated Type III and Type II radio bursts and disappearing filaments. These results lead to the proposal that EMS are the likely source of some interplanetary transients.