_http://www.spaceweather.com/archive.php?view=1&day=07&month=10&year=2015 said:
SOLAR WIND SPARKS GEOMAGNETIC STORMS: NOAA forecasters estimate an 80% chance of polar geomagnetic storms on Oct. 8th as a high speed solar wind stream buffets Earth's magnetic field. This marks the third day in a row of geomagnetic storms and bright auroras around the Arctic Circle. "Last night we had an awesome display," reports Matti Helin from Finland. "Never had I seen such bright auroras. They lit the whole scenery, and I could have read a magazine with ease." ...
Many readers are familiar with CMEs, billion-ton clouds of gas from the sun which can cause geomagnetic storms when they hit Earth. This event, however, was not caused by a CME. Instead, the culprit is a CIR (co-rotating interaction region). CIRs are boundary zones between slow- and fast-moving solar wind streams. Solar wind plasma piles up in these regions, producing density gradients and shock waves that do a good job of sparking auroras. A CIR hit Earth's magnetic field during the early hours of Oct. 7th, amplifying a storm already in progress. A solar wind stream arriving in the wake of the CIR has kept the storm going through Oct. 8th.