The solar storm of 2012 was a solar storm involving an unusually large and strong coronal mass ejection that occurred on July 23, 2012. It missed Earth with a margin of approximately nine days, as the equator of the Sun rotates around its own axis with a period of about 25 days.
The coronal mass ejection, as photographed by STEREO
A coronal mass ejection (CME) is a significant ejection of magnetic field and accompanying plasma mass from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established.
Photo from the ISS of aurora australis during a geomagnetic storm on 29 May 2010. The storm was most likely caused by a CME that had erupted from the Sun on 24 May 2010, five days prior to the storm.