http://www.newsweek.com/unexpected-solar-storm-geomagnetic-space-weather-1746527

Earth was buffeted by a huge solar storm late on September 26 through to this morning, and the cause is unclear.

At around 10 p.m. UTC (6 p.m. ET) on September 26, the U.S. National Oceanic and Atmospheric Administration's (NOAA) Space Weather Prediction Center (SWPC) released a space weather alert stating that a geomagnetic K-index of 4 was expected—essentially an indication that Earth's magnetic field was being very mildly affected by some solar activity.