April 12, 2011: A G1-class geomagnetic storm is in progress, sparked by a high-speed solar wind stream which is buffeting Earth's magnetic field. High latitude sky watchers should be alert for auroras.

What is a geomagnetic storm?

The Earth's magnetosphere is created by our magnetic field and protects us from most of the particles the sun emits. When a CME or high-speed stream arrives at Earth it buffets the magnetosphere.

If the arriving solar magnetic field is directed southward it interacts strongly with the oppositely oriented magnetic field of the Earth.

The Earth's magnetic field is then peeled open like an onion allowing energetic solar wind particles to stream down the field lines to hit the atmosphere over the poles. At the Earth's surface a magnetic storm is seen as a rapid drop in the Earth's magnetic field strength.

This decrease lasts about 6 to 12 hours, after which the magnetic field gradually recovers over a period of several days.