Relationship of halo coronal mass ejections, magnetic clouds, and magnetic storms

Halo coronal mass ejections (CMEs) had been rarely reported in coronagraph observations of the Sun before the Solar and Heliospheric Observatory (SOHO) mission. Since mid-1996, however, the SOHO Large Angle Spectrometric Coronagraph (LASCO) instruments have observed many halo or partial-halo CMEs. A halo CME, especially when associated with solar activity near sun center, is important for space weather concerns because it suggests the launch of a potentially geoeffective disturbance toward Earth. During the post-solar minimum period from December 1996 to June 1997, we found that all six halo CMEs that were likely Earthward-directed were associated with shocks, magnetic clouds, and moderate geomagnetic storms at Earth 3-5 days later. The results imply that magnetic cloud-like structures are a general characteristic of CMEs. Most of the storms were driven by strong, sustained southward fields either in the magnetic clouds, in the post-shock region, or both. We discuss the characteristics of the halo events observed during this period, their associated signatures near the solar surface, and their usefulness as predictors of space weather at Earth.