How often do halo CMEs occur?
That depends on the phase of the solar cycle. The Sun’s activity is not the same year after year, but waxes and wanes in a cycle that lasts about eleven years on average. For example, the years 1990-1991, and 2000-2001 saw a large amount of activity (solar maximum: many flares and CMEs), whereas 1995-1996 was a time of minimal activity. According to researchers using data from the SOHO solar observatory satellite, 25 Earth-directed halo CMEs were detected during the last eight months of 1997, and now in 2007 we again are in a time of low activity. But the fact that the Sun’s activity is cyclic means that in the next 1-2 years we should expect to see CMEs becoming more frequent.
