DUAL CME: Another CME is en route to Earth. It was launched in our direction four days ago by the eruption of a magnetic filament near the center of the solar disk. This movie from the Solar and Heliospheric Observatory shows the Earth-directed CME almost overwhelmed, visually, by a brighter farside CME headed in the opposite direction.
The impact won't be as effective as the double-blow Earth experienced on Sept. 12th, when two CMEs hit in less than 24 hours. Nevertheless, NOAA forecasters estimate a 50% chance of polar geomagnetic storms on Sept. 17th when the CME arrives. (Note: Yesterday we wrote that the CME would arrive on Sept. 16th, however, a revised analysis of its speed suggests a later arrival.) High-latitude sky watchers should be alert for auroras.