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Wikipedia has this to say about solar particle events:

A solar particle event (SPE), or "proton storm," occurs when particles (mostly protons) emitted by the Sun become accelerated to very high energies either close to the Sun during a solar flare or in interplanetary space by the shocks associated with coronal mass ejections.

Okay, so they seem to have to do with both solar flares and CMEs. And this page on part of the Goddard Space Flight Center website says:

The super-heated electrons from CMEs move along the magnetic field lines faster than the solar wind can flow. Rearrangement of the magnetic field, and solar flares may result in the formation of a shock that accelerates particles ahead of the CME loop.

So is that what a proton storm is, a piece of a coronal mass ejection that was accelerated beyond the rest of the CME by interaction with a solar flare? How does that work?

I used to think that SPEs were CMEs hitting Earth, but that doesn't seem to be the case, or at least only sometimes. After all, why have two terms if they are the same thing? I'm especially interested in it because of the highly variable time it takes for an SPE to reach the Earth. Under the right conditions, they move almost at the speed of light - there was one like that documented in 2005. So the way storms develop, and how that affects their speed, makes a big difference to how people and spacecraft need to be protected against them.

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  • $\begingroup$ Do you think the good people at Astronomy SE may know the answer to this? $\endgroup$ Jul 5 '15 at 11:02
  • $\begingroup$ @Rikki-Tikki-Tavi I went and checked and haven't found anything. I could have asked there, but it is an area that overlaps with Space Exploration SE, and since this is really where i spend my time, i asked here. (At some point i'd like to spend time there too, but not right now. I can't spare any more time.) $\endgroup$
    – kim holder
    Jul 5 '15 at 14:33
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    $\begingroup$ Terminology is a bit fuzzy here. To my understanding, SPE is but a measured effect of proton flux at different energy levels, but it can be caused by a number of things. So if a flare causes an acceleration of charged ions in CME or solar wind, it will create a bow shock moving at greater speed than the rest of the particle density that was unaffected by flare's charge or even decelerated. That then causes detection of a whole range of different eV level particles, first the flare itself at the highest order, then the bow shock of sped up CME's mass, and eventually the rest of the CME. $\endgroup$
    – TildalWave
    Jul 5 '15 at 14:52
  • $\begingroup$ Can this be migrated to Astronomy? $\endgroup$
    – Muze
    Apr 9 '19 at 22:03
  • $\begingroup$ You may find some of these links (in this link) helpful: physics.stackexchange.com/a/549422/59023 $\endgroup$ Jan 19 '21 at 23:35
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How do solar flares and coronal mass ejections create proton storms?

The proper answer to this questions fills multiple books and they all still end with lots of questions, but I will try to give a summary of what they say.

First, solar flares are just the localized enhancement of electromagnetic (EM) radiation (usually dominated in UV and x-ray bands) while coronal mass ejections (CMEs) are the eruption of large blobs of plasma from the solar corona.

So a solar flare does not generate particles directly. The processes that generate a solar flare, e.g., magnetic reconnection (I wrote a more detailed explanation of this phenomena at https://physics.stackexchange.com/a/559759/59023), do, however, accelerate charged particles. Similar processes are thought to be responsible for release CMEs too.

A CME has an addition mechanism to accelerate particles, i.e., a shock wave. Charged particles in space plasmas can get energized by all sorts of processes from reconnection to shocks to electromagnetic waves (e.g., see discussion in the following answer https://physics.stackexchange.com/a/618127/59023).

You may ask how do shocks accelerate particles. I wrote some detailed answers at https://physics.stackexchange.com/a/252885/59023 and https://physics.stackexchange.com/a/344703/59023 but the short answer is that charged particles are controlled by electric and magnetic fields and shocks in space plasmas have both in large quantities and magnitudes.

So is that what a proton storm is, a piece of a coronal mass ejection that was accelerated beyond the rest of the CME by interaction with a solar flare?

A proton storm is just a large enhancement (somewhat subjective definition of what qualifies as large) in energetic (~10s of keV to >MeV energies) protons measured by in situ spacecraft. These are closely related to solar energetic particles (SEPs).

What causes these depends on the situation. Sometimes these result from the same phenomena that caused a solar flare while other times it appears that a CME shock caused the events.

How does that work?

See my above comments and links about charged particle acceleration.

I used to think that SPEs were CMEs hitting Earth, but that doesn't seem to be the case, or at least only sometimes. After all, why have two terms if they are the same thing?

Well, don't give scientists that much credit. We have lots of redundancy in jargon on some topics and it takes a lot of effort to whittle this down because scientists, especially the older ones that created the multiple terms, tend to be stubborn ;)

Joking aside, you are correct that these are two different phenomena but often they are linked by common sources or one can cause the other (as I said above).

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