A geomagnetic storm is in progress due to the crossing of a solar sector boundary.

From WSO/Stanford:

The solar wind in the ecliptic plane is organized in such a way that it generally has 2 or 4 sectors per solar rotation (27 days) within which the magnetic field is pointed generally toward or away from the Sun. The surface separating the polarities is called the Heliospheric Current Sheet.

Sector boundaries are the times observed at Earth when the polarity of the interplanetary magnetic field (IMF) reverses. Well defined sector boundaries are those boundaries for which there is good data, the reversal takes place cleanly, and intervals on either side of the boundary have a uniform field direction for ~4 days.

http://wso.stanford.edu/SB/SB.html