On Oct 13, an alert was sent by the Space Weather Forecast Centre in Belgium: 'A shock was detected in the solar wind around 21:22UT October 12 which marks the earlier then expected arrival of the October 9 CME. ...'
In human language: a cloud of solar material bumped into Earth.
How did the story go?
On Oct 10, the forecast centre warned for a cloud that seemed to be directed towards Earth. It was detected by the SOHO spacecraft early on October 9. It looked as a faint cloud in the SOHO pictures and its speed was thought to be small, at most 300 km/s. The forecasters expected only a glancing blow and not before October 13.
This is a SOHO movie of the surroundings of the Sun when the cloud passes. But, the halo is very difficult to see.
It is easier to detect the cloud in difference images showing what changed in between two images:
Late Oct 12, the cloud was only 1 hour away from Earth. A satellite located just in front the Earth detected the cloud when it passed. The magnetic energy of the cloud increased slowly but steady because it interacted with a fast solar wind stream that was present at that moment. An alliance like this can be a recipe for a geomagnetic impact.
The event caught the attention of several scientists and sparked the discussion about stealth solar clouds. The 'launch site' of a plasma cloud can be located on the solar disk. The 'launch site' of stealth solar clouds is sort of mysterious and is apparently absent or very vague. Often, these clouds look faint and give the impression of not being harmful. But when they mingle with other clouds and solar wind structures, they might catch us by surprise.
