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Quote from BBC "A strong solar storm is expected to hit Earth shortly, and experts warn it could disrupt power grids, satellite navigation and plane routes."

How is it that we are aware of a massive solar activity that will hit us before it actually does? Isn't that activity all electromagnetic and therefore travelling at the same speed towards us (light speed)?

Or are there separate elements to a solar storm, some light speed waves and some physical particles slower than light speed?



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Coronal Mass Ejection is indeed a stream of particles that moves at velocities significantly lower than the speed of light. The speed of CME varies between 20km/s to 3200km/s with average velocity around 489km/s.

Thus, observations of the Sun can notice changes in Solar activity hours or even days before CME hits Earth (85 hours for average speed of 489km/s, 13 hours for the extreme velocity of 3200km/s).

More importantly, SOHO spacecraft located at L1 (around 1.5 million kms from Earth on the Sun-Earth line) can detect an incoming CME minutes to hours before it hits Earth. The stream of particles travelling at 489km/s takes around 51 minutes to travel 1.5 million kms, while the radio signal from SOHO only needs around 5 seconds. In the extreme case of CME travelling at 3200km/s, SOHO's warning is only 7 minutes ahead, though.

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By a strange co-incidence BBC has just screened an episode of Horizon that answers your question in great detail. If you're in the UK see http://www.bbc.co.uk/programmes/b01d99vb.html as it's available through the iPlayer. If you're not in the UK I imagine the episode will be findable on the Internet.

In brief, there are a number of observatories and satellites that monitor the sun for coronal mass ejections. Something I found very interesting is an attempt to use helioseismology to spot sunspots forming before the reach the surface. The scientist in question thought it might be possible to give up to a weeks notice, though at the moment it's only two days.

Currently the only really reliable predictions are to see the CME happening and give the 19 hours or so notice that the CME takes to reach the Earth.

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