There is also a chance of major CMEs from impacts of debris or comet with the sun.
That's a confirmed observation of other comets hitting the sun, much smaller than ISON: http://www.youtube.com/results?search_query=comet+CME&oq=comet+CME&gs_l=youtube.3..0.212.2865.0.3399.9.8.0.1.1.0.136.683.7j1.8.0...0.0...1ac.1.11.youtube.JeaZBUCXUNw
Comets don’t cause CMEs
http://www.isoncampaign.org/karl/sungrazers-and-cmes
When we see events like this, I inevitably (and understandably) receive questions about whether the solar eruption was caused by the incoming comet. We’ve dealt with this many times now, and the answer so far has always been “no”.
Actually we will pass first through ISON”s departure tail then a few days later it’s entrance debris.
What is interesting is that there could be some consequence to this debris field in that it may not burn upon entry into our atmosphere but merely float.
Depending on what is left of the Comet and how large the Tail consumes this breakup could pose interesting consequences to our weather.
Highly hypothetical....but something to watch.
Not to mention that it is going really fast. Now the real danger in regard to the asteroid belt is a comet hitting (long odds) a big asteroid and sending it's debris on an earth intersecting path.