<p>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 <a href="http://en.wikipedia.org/wiki/Solar_wind#Coronal_mass_ejection">solar storms</a>, depending on the orientation of its magnetic filed.
Here's a <a href="http://www.spaceweatherlive.com/en/help/how-do-we-know-if-a-cme-is-earth-directed-and-when-its-going-to-arrive">SOHO view of a CME</a>. 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.</p>

<p>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.</p>

CC BY-SA 3.0
2014-08-01T13:38:25.557Z
2014-08-01T13:38:25.557Z
Wayfaring Stranger
<p>How can the <a href="http://www.srl.caltech.edu/ACE/" rel="nofollow">Advanced Composition Explorer</a> (ACE) warn of incoming solar storms when is is invested with radiation?</p>

<p>Discovery.com article <a href="http://news.discovery.com/space/history-of-space/solar-storm-warning-satellite-120309.htm" rel="nofollow">Solar Storm Warning Satellite on Last Legs</a> states that:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>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?</p>

CC BY-SA 3.0
2016-08-25T12:10:13.053Z
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.
honeste_vivere
<the-sun><artificial-satellite><solar-storm>
How can the Advanced Composition Explorer (ACE) warn of incoming solar storms?
dendini
# How can the Advanced Composition Explorer (ACE) warn of incoming solar storms?

(Asked by: username_0 on 2014-08-01)

How can the [Advanced Composition Explorer](http://www.srl.caltech.edu/ACE/) (ACE) warn of incoming solar storms when is is invested with radiation?

Discovery.com article [Solar Storm Warning Satellite on Last Legs](http://news.discovery.com/space/history-of-space/solar-storm-warning-satellite-120309.htm) 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?

**Tags:** the-sun, artificial-satellite, solar-storm

> **Comments:**
> **username_1**: 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.  

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## 1 answers

### Answer by username_2 on 2014-08-01. Score: 6 | Accepted answer

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](http://en.wikipedia.org/wiki/Solar_wind#Coronal_mass_ejection), depending on the orientation of its magnetic filed.
Here's a [SOHO view of a CME](http://www.spaceweatherlive.com/en/help/how-do-we-know-if-a-cme-is-earth-directed-and-when-its-going-to-arrive). 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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