A large polar crown coronal mass ejection and a ''problem'' geomagnetic storm: April 14-23, 1994

In the 1960s and 1970s, systematic programs to associate geomagnetic storms with solar flares gave rise to a category of ''problem storms'' for which there were no clear associations or for which the associations were with solar activity that seemed too insignificant to properly account for the magnitude of the geomagnetic events. In the last 2 decades the role of filament eruptions and coronal mass ejections (CMEs) in driving geomagnetic activity has received increasing attention, and an answer to the problem storm question has been pieced together. Until recently the inability to view CMEs on the solar disk has impeded a precise demonstration of the inferred relationships. The data from the Yohkoh Soft X ray Telescope (SXT), however, allow the association of soft X ray coronal structures with the aftermath of CMEs. Since CMEs are now believed to be the main cause of aperiodic severe disturbances of Earth's magnetosphere, the ability to distinguish the aftermath of a CME on the disk improves our ability to predict geomagnetic storms. On April 14, 1994, a large scale coronal arcade formation was observed by SXT after the eruption of a long section of the southern polar crown neutral line. Data from the Mauna Loa coronagraph are consistent with this event being the result of a CME that formed in the helmet streamer underlying the heliospheric current sheet. The CME was followed by a severe geomagnetic storm, starting 68 hr later, and was also observed at 3.2 AU and high heliographic latitude by the Ulysses satellite. Since there was no significant flare associated with the eruption, and no significant filament disappearance, the subsequent geomagnetic events are an extreme example of a problem storm and of the great utility of soft X ray imagers for the prediction of space weather. In this paper we have assembled a wide range of data relating to the solar event, the evolution of the region that generated it, and the various observed consequences at Earth and in the interplanetary medium. The global magnetic field changes leading to the CME are outlined, a brief description of the coronal arcade formation is given, and the arcade's association with subsequent events at Earth and the interplanetary disturbance observed at Ulysses is described. We present this case as an example of the advantages to be derived from a comprehensive multidisciplinary approach to studying these potentially disruptive phenomena.