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D. F. Webb (ISR, Boston College), O. C. St. Cyr (CPI, Naval Research Laboratory), S. P. Plunkett (USRA, Naval Research Laboratory), R. A. Howard (Naval Research Laboratory), B. J. Thompson (SAC, NASA/GSFC)
Halo-type CMEs had been rarely reported in solar coronagraph observations before the SOHO mission. Because of their increased sensitivity and dynamic range, the SOHO LASCO coronagraphs are now observing 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 geoeffective disturbance toward Earth. We present statistical summaries of the LASCO halo CMEs observed during the first 2.5 years of SOHO observations. These include their occurrence rates, speeds and morphology, and comparison of the halo CME population with the general characteristics of all LASCO CMEs during this period.
As a test of the geoeffectiveness of halo CMEs, we also examined a 6-month period just after solar minimum from December 1996 through May 1997 during which halo CMEs and geomagnetic storms occurred at a similar rate of 2-3/month. All 7 of the halo CMEs confidently associated with frontside activity were followed 3-5 days later by interplanetary shocks, magnetic clouds and moderate storms at Earth. In the opposite sense most of the moderate storms during this period were associated with halo CMEs. We will present the results of this study. This work was supported by AFOSR grant AF49620-97-1-0070.
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