<p>The following are largely excerpts from an answer I wrote at <a href="https://physics.stackexchange.com/a/258093/59023">https://physics.stackexchange.com/a/258093/59023</a>.</p>

<blockquote>
  <p>What are the main differences between solar flares and coronal mass ejections?</p>
</blockquote>

<p>There are several differences between <a href="https://en.wikipedia.org/wiki/Solar_flare" rel="nofollow noreferrer">solar flares</a> and <a href="https://en.wikipedia.org/wiki/Coronal_mass_ejection" rel="nofollow noreferrer">coronal mass ejections</a> (CMEs), the latter of which involve large amounts (i.e., upwards of billions of tons) of matter (in the form of <a href="https://en.wikipedia.org/wiki/Plasma_(physics)" rel="nofollow noreferrer">plasma</a>) to leave the sun.</p>

<p>A solar flare is really described by the sudden enhancement of <a href="https://en.wikipedia.org/wiki/X-ray" rel="nofollow noreferrer">x-rays</a> from a small region in the solar corona.  They can produce streams of high energy particles, called solar energetic particles or <a href="https://en.wikipedia.org/wiki/Solar_energetic_particles" rel="nofollow noreferrer">SEPs</a> (CMEs can produce SEPs as well), but these are streams of magnetic field-aligned particles propagating away from the sun.  They can have energies up to ~GeV for ions and several ~MeV for electrons, but rarely higher.  Earth's atmosphere does a great job of shielding much of this and the surrounding magnetic field helps to "deflect" some of the incident particles as well.</p>

<p>A CME, on the other hand, expands nearly as fast as it propagates which means their radius of curvature by the time they reach Earth is nearly 1 <a href="https://en.wikipedia.org/wiki/Astronomical_unit" rel="nofollow noreferrer">AU</a>.  They are, in effect, a large "magnetic piston" (sometimes called a magnetic cloud or flux rope etc.) that piles-up plasma on their leading edge and can produce rather strong <a href="https://en.wikipedia.org/wiki/Shock_waves_in_astrophysics" rel="nofollow noreferrer">collisionless shock waves</a>.  The magnetic field orientation of the magnetic piston is very important, as its orientation relative to the <a href="https://en.wikipedia.org/wiki/Earth%27s_magnetic_field" rel="nofollow noreferrer">Earth's magnetic</a> field can result in either a small <a href="https://en.wikipedia.org/wiki/Aurora" rel="nofollow noreferrer">auroral</a> response or a major <a href="https://en.wikipedia.org/wiki/Geomagnetic_storm" rel="nofollow noreferrer">geomagnetic storm</a>.</p>

<blockquote>
  <p>...what causes them...</p>
</blockquote>

<p>Both are thought to ultimately arise from a process called <a href="https://en.wikipedia.org/wiki/Magnetic_reconnection" rel="nofollow noreferrer">magnetic reconnection</a> -- a process that results in a reconfiguration of the magnetic field topology and transformation of magnetic energy into bulk particle kinetic energy.</p>

<p>In the case of flares, reconnection results in jets of highly energetic electrons and protons that slam into the upper solar atmosphere resulting in x-rays produced by <em>thick target</em> <a href="https://en.wikipedia.org/wiki/Bremsstrahlung" rel="nofollow noreferrer">Bremsstrahlung</a> radiation.</p>

<p>In the case of CMEs, reconnection results in a release of a massive amount of plasma often confined within a <a href="https://en.wikipedia.org/wiki/Magnetic_cloud" rel="nofollow noreferrer">magnetic cloud</a>.</p>

<blockquote>
  <p>...what sort of damage can they cause...</p>
</blockquote>

<p>See my previous answers at <a href="https://physics.stackexchange.com/a/258093/59023">https://physics.stackexchange.com/a/258093/59023</a>, <a href="https://physics.stackexchange.com/a/214509/59023">https://physics.stackexchange.com/a/214509/59023</a>, and <a href="https://physics.stackexchange.com/a/149199/59023">https://physics.stackexchange.com/a/149199/59023</a>.</p>

<blockquote>
  <p>...what are the differences in their composition?</p>
</blockquote>

<p>As I explained above, flares are just enhancements in x-rays so there is no composition.  CMEs generally have different charge state and heavy ion composition than the nominal <a href="https://en.wikipedia.org/wiki/Solar_wind" rel="nofollow noreferrer">solar wind</a>.</p>

CC BY-SA 3.0
2016-08-02T18:18:31.203Z
2016-08-02T18:18:31.203Z
2017-04-13T12:40:15.377Z
honeste_vivere
<p>Specifically, what causes them, what sort of damage can they cause, and what are the differences in their composition? </p>

<solar-flare><coronal-mass-ejection>
What are the main differences between solar flares and coronal mass ejections?
David Freitag
# What are the main differences between solar flares and coronal mass ejections?

(Asked by: username_0 on 2013-09-25)

Specifically, what causes them, what sort of damage can they cause, and what are the differences in their composition?

**Tags:** solar-flare, coronal-mass-ejection


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## 1 answers

### Answer by username_1 on 2016-08-02. Score: 3

The following are largely excerpts from an answer I wrote at <https://physics.stackexchange.com/a/258093/59023>.

> What are the main differences between solar flares and coronal mass ejections?

There are several differences between [solar flares](https://en.wikipedia.org/wiki/Solar_flare) and [coronal mass ejections](https://en.wikipedia.org/wiki/Coronal_mass_ejection) (CMEs), the latter of which involve large amounts (i.e., upwards of billions of tons) of matter (in the form of [plasma](https://en.wikipedia.org/wiki/Plasma_(physics))) to leave the sun.

A solar flare is really described by the sudden enhancement of [x-rays](https://en.wikipedia.org/wiki/X-ray) from a small region in the solar corona. They can produce streams of high energy particles, called solar energetic particles or [SEPs](https://en.wikipedia.org/wiki/Solar_energetic_particles) (CMEs can produce SEPs as well), but these are streams of magnetic field-aligned particles propagating away from the sun. They can have energies up to ~GeV for ions and several ~MeV for electrons, but rarely higher. Earth's atmosphere does a great job of shielding much of this and the surrounding magnetic field helps to "deflect" some of the incident particles as well.

A CME, on the other hand, expands nearly as fast as it propagates which means their radius of curvature by the time they reach Earth is nearly 1 [AU](https://en.wikipedia.org/wiki/Astronomical_unit). They are, in effect, a large "magnetic piston" (sometimes called a magnetic cloud or flux rope etc.) that piles-up plasma on their leading edge and can produce rather strong [collisionless shock waves](https://en.wikipedia.org/wiki/Shock_waves_in_astrophysics). The magnetic field orientation of the magnetic piston is very important, as its orientation relative to the [Earth's magnetic](https://en.wikipedia.org/wiki/Earth%27s_magnetic_field) field can result in either a small [auroral](https://en.wikipedia.org/wiki/Aurora) response or a major [geomagnetic storm](https://en.wikipedia.org/wiki/Geomagnetic_storm).

> ...what causes them...

Both are thought to ultimately arise from a process called [magnetic reconnection](https://en.wikipedia.org/wiki/Magnetic_reconnection) -- a process that results in a reconfiguration of the magnetic field topology and transformation of magnetic energy into bulk particle kinetic energy.

In the case of flares, reconnection results in jets of highly energetic electrons and protons that slam into the upper solar atmosphere resulting in x-rays produced by *thick target* [Bremsstrahlung](https://en.wikipedia.org/wiki/Bremsstrahlung) radiation.

In the case of CMEs, reconnection results in a release of a massive amount of plasma often confined within a [magnetic cloud](https://en.wikipedia.org/wiki/Magnetic_cloud).

> ...what sort of damage can they cause...

See my previous answers at <https://physics.stackexchange.com/a/258093/59023>, <https://physics.stackexchange.com/a/214509/59023>, and <https://physics.stackexchange.com/a/149199/59023>.

> ...what are the differences in their composition?

As I explained above, flares are just enhancements in x-rays so there is no composition. CMEs generally have different charge state and heavy ion composition than the nominal [solar wind](https://en.wikipedia.org/wiki/Solar_wind).


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