On 23 July 2012, two eruptions on the sun known as coronal mass ejections (CMEs) burst from an active patch of sunspots on the far side of the sun, which is monitored by a probe that circles the sun ahead of Earth in the same orbit. Emerging about 15 minutes apart, the CMEs quickly merged into one shock wave of charged particles that washed over the probe’s sensors  just 18.6 hours later (the “snowstorm” of particles seen late in this video), researchers report online today in Nature Communications. Behind the shock wave, charged particles raced along at about 2250 kilometers per second—five times their normal speed at Earth’s distance from the sun—and the magnetic field strength there was more than 10 times that normally seen at Earth’s orbit. If pointed in our direction, such a combination would have produced the strongest geomagnetic storm to have struck Earth in history and could have knocked out satellites and earthbound power grids, researchers say. Fortunately, the event was a bit of a fluke, as it erupted into a region of space where the solar wind and the magnetic field had been weakened by a solar flare 4 days earlier. The observations of these CMEs will help scientists better model space weather and predict potentially calamitous solar storms.
(Video credit: Ying Liu)