Hi Stephanie,

This is a great question and one that we still struggle with. CME speeds range from the low hundreds km/s to 2000 km/s, so it is difficult to predict just how fast a CME will be. One thing we do to try to predict CME speed is look at how the CME speed is related to other solar phenema visible on the solar disk. For example, we are able to measure how fast an erupting filament is taking off from the disk, and erupting filaments are almost always associated with a CME. We are trying to determine if the faster filaments have faster CMEs or if slower filaments are associated with slower CMEs. We don't know yet if we can correlate the two, but if a filament's speed or acceleration determines how fast its corresponding CME will be, this would give us a predictive tool. Another example is the relationship between flares and CMEs. Studies have been done to see if the most energetic flares are associated with the fastest CMEs. Although these things have not been completely determined, this is the type of research that is currently going on to try to predict when CMEs will occur and how fast they will be ejected. Hopefully we will be able to answer this question with more confidence in a few years.

Holly