A plasma cloud left the Sun on Saturday Sept 24. It was not directed straight away to the Earth. When a structure moves very fast through a
medium, a shock in front is formed. A shock has a larger spatial extension than the plasma cloud itself. It was this shock that was noticed by the ACE satellite. This satellite is located near the Earth, on the line between the Sun and the Earth. What passed along ACE, will pass also along the Earth.
The shock and the post-shock sheath compressed the magnetosphere and caused a severe geomagnetic storm. The ground station in Dourbes, Belgium, sent out several advance alerts for enhanced geomagnetic activity.
The worst part of the storm has passed. But another magnetic structure linked with a strong solar event of Sept 25
might arrive at Earth this evening.