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Reading an 2005 article by Fraser Cain called Smallest Ever Coronal Mass Ejection I became interested in the exact definition of CME. It seems like a simple question - but I did not manage to find the answer to the question myself. Is there a smallest possible coronal mass ejection (CME)? Is there a theoretical limit or is it a matter of observing capabilities?

Note: I left intentionally open whether "small" refers to mass, size, duration, or any other parameter.

The above mentioned article claims that the CME in the square in the following picture is actually the smallest CME: enter image description here

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B--rian
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    This is a really nice question! There is a smallest typhoon on Earth because there is a threshold in their definition when a tropical cyclone's sustained wind speed crosses a threshold. Definition of terminology is a third possibility here, in addition to physics and observational limits. – uhoh May 19 '21 at 00:08
  • @uhoh What do you suggest to do about my tag cme which I created unknowningly? Could you please suggest a tag-synonym to [tag:coronal-mass-ejection]? I am missing reputation for that. – B--rian May 20 '21 at 21:29
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    I'm just waking up and my coffee has not kicked in yet so I'm not sure what you mean. What exactly does "suggest a tag-synonym" mean? Is there a button to push for that? I mean, I could suggest "sun-fire-air-shooter" which is XKCD simple writer-approved but I don't think that's what you mean by suggesting a tag synonym. update: Oh, I know, I think I've never done it but there is a feature to do that for folks who have sufficient rep associated with one of the tags. I'll check now, coffee kicking in... – uhoh May 20 '21 at 21:40
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    Nope, no can do. The rather dizzying instructions that I can't understand state "Users with more than 1250 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms. Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2." I shouted "damn the instructions, full speed ahead" and tried, but got the error message "Creating a tag synonym requires 5 score in this tag." – uhoh May 20 '21 at 21:44
  • https://i.stack.imgur.com/cDWel.png – uhoh May 20 '21 at 21:44

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No, as ‘coronal mass ejection’ is defined ambiguously and refers to an event that is not physically bound to some exact limit and therefore cannot have an undisputable lowest limit value, in the mathematical sense, in any parameter.

For example, the Oxford dictionary states:

Coronal Mass Ejection. Noun - Astronomy.

An event in which a large cloud of energetic and highly magnetized plasma erupts from the solar corona into space, causing radio and magnetic disturbances on the earth.

Clearly, any limit, if fixed to some exact value of mass, flux, duration, etc., would have to be fixed arbitrarily.

As a thought exercise, consider if someone came along and presented you such a ‘lower limit’.

You, or a third party, can respond with: “well what about the situation where an event occurs just one electron (or one Planck second, etc.) below such a limit? Would that be considered a CME or not?”

B--rian
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M. Y. Zuo
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    Welcome to astronomy SE! Thanks for your contribution. I fully understand your line of argument, but I believe there might be indeed a physical lower limit for a CME, below which some different phenomena occurs, somewhat analogous to whether a Funnel cloud will start a full-fledge tornado or not. – B--rian Jun 02 '21 at 09:56
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    @B--rian That is an interesting idea, perhaps there is some inflection point for CME's, a sharp discontinuity so to speak. Perhaps oneday that will be discovered! – M. Y. Zuo Jun 20 '21 at 22:24
  • There are lower limits as there are phenomena called flux ropes that are not CMEs in the solar wind, but have similar magnetic topology. However, the cutoff is not really clear and people argue about this sort of things at meetings all the time. – honeste_vivere Aug 31 '21 at 14:25