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

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 Coro nagraph (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 m inimum period from December 1996 to June 1997, we found that all six halo C MEs that were likely Earthward-directed were associated with shocks, magnet ic clouds, and moderate geomagnetic storms at Earth 3-5 days later. The res ults imply that magnetic cloud-like structures are a general characteristic of CMEs. Most of the storms were driven by strong, sustained southward fie lds either in the magnetic clouds, in the post-shock region, or both. We di scuss the characteristics of the halo events observed during this period, t heir associated signatures near the solar surface, and their usefulness as predictors of space weather at Earth.