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How can the Advanced Composition Explorer (ACE) warn of incoming solar storms when is is invested with radiation?

Discovery.com article Solar Storm Warning Satellite on Last Legs states that:

Until the sun's free-flying and highly energetic outbursts, known as coronal mass ejections, hit the ACE spacecraft, forecasters don't know the orientation of their embedded magnetic fields.

I read that it can give 15 minutes in advance warning to Earth of the presence of an incoming solar storm. However to do that, it sends a radio signal which goes at the same speed of the solar storm so it will presumably take 15 minutes as well?

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  • $\begingroup$ There is currently not much we can do to prevent damage from solar energetic particles and high energy photons (e.g., x-rays). We can, however, know ~10-30 minutes in advance whether an incident CME will cause a geomagnetic storm, as WayfaringStranger mentions. $\endgroup$
    – honeste_vivere
    Aug 25, 2016 at 12:10

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Solar flare X-rays travel at light speed, but the solar wind from CME's travels at merely 600 to 2000 kilometers per second. It's the wind that causes aurora and solar storms, depending on the orientation of its magnetic filed. Here's a SOHO view of a CME. It's clear the gas is moving slower than light. You can tell if the earth is in the path of an ejection by how near to symmetrically the gas cloud expands around the sun.

So, to answer your question, the satellite measures magnetic field direction, and can give 15 minutes warning because the CME gas is moving much slower than light.

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