One coronal mass ejection erupted on the then far side of the sun on July 19, 2012. Then two more erupted four days later, and only 15 minutes apart. The resulting collision resulted in the largest geomagnetic storm in the space era. If the storm had been aimed directly at Earth, power grids would most likely been short-circuited and satellites disrupted.
By the time CMEs hit Earth, they normally have the same amount of charged particles Earth receives from the sun daily. According to Ying Liu, a physicist at the National Space Science Center in Beijing, the collisions of the CMEs result in particle streams with five times the speed and 10 times the magnetic field strength of a normal CME.
The discovery could help improve space weather forecasting, and help prepare Earth for better response when extreme space weather occurs.