[Loops] thermal nonequilibrium and thermal instability

Wed Nov 27 09:00:16 MST 2019

Thanks, Hugh!

I'll reply to the entire listserve, as I believe you intended this for community discussion.

Although coronal rain was first discussed in the context of flares, the term has been co-opted by the broader community to include cold condensations forming anywhere in the corona. I would add that flare coronal rain is NOT caused by thermal nonequilbrium. TNE applies when the heating is (quasi)steady, which of course is not the case in flares. Flare coronal rain is more like a proper instability. After the evaporation phase, the densities in a flare loop are much higher than equilibrium values (as given by scaling laws, for example). A large enough density perturbation can lead to localized runaway cooling that proceeds faster than the usual flare draining. 

I, too, have struggled with the term "heating." Sometimes we take it to mean increasing temperature, and sometimes we take it to mean the addition of energy (which does not change the temperature if it is balanced by an equal amount of cooling, i.e., removal of energy). I think it would be difficult to stop using "heating" and "cooling" for energy addition and removal processes (radiation, thermal conduction, etc.). Perhaps others have a different opinion.

Thanks again,
Jim

> -----Original Message-----
> From: Hugh Hudson <Hugh.Hudson at glasgow.ac.uk>
> Sent: Wednesday, November 27, 2019 9:19 AM
> To: Klimchuk, James A. (GSFC-6710) <james.a.klimchuk at nasa.gov>
> Cc: (Loops at solar.physics.montana.edu) <Loops at solar.physics.montana.edu>
> Subject: [EXTERNAL] Re: [Loops] thermal nonequilibrium and thermal instability
> 
> Jim, thanks, very clear and helpful as we eager readers of your papers have come to
> expect. Just a couple of quibbles:
> 
> Quibble No. 1. “Coronal rain” has traditionally been a phenomenon associated with
> “loop prominence systems”, “sporadic coronal condensations”, and many another
> term, but basically “flares”. I’ll read Patrick’s paper on the synchronization of raining
> strands next, but this synchronization might be less coincidental in a flare situation.
> 
> Quibble No.2. It seems to be customary in this community to use the terms “heating”
> and “cooling” to be volumetric. You don’t define your terms in this nice paper until Eq.
> 1, but that’s in the Appendix. I think the physics would be clearer if people made
> “heating” synonyous with “time rate of change of temperature”, and “cooling” the
> same way. This is not only more correct terminology, but also closer to the
> observable (T).
> 
> Hugh
> 
> > On Nov 26, 2019, at 9:55 PM, Klimchuk, James A. (GSFC-6710)
> <james.a.klimchuk at nasa.gov> wrote:
> >
> > Hi Loops friends,
> >
> > Attached is a paper just accepted by Solar Physics entitled "The Distinction
> Between Thermal Nonequilibrium and Thermal Instability," a topic which has caused
> some confusion.
> >
> > Abstract:
> > For some forms of steady heating, coronal loops are in a state of thermal
> nonequilibrium and evolve in a manner that includes accelerated cooling, often
> resulting in the formation of a cold condensation. This is frequently confused with
> thermal instability, but the two are in fact fundamentally different. We explain the
> distinction and discuss situations where they may be interconnected. Large-
> amplitude perturbations, perhaps associated with MHD waves, likely play a role in
> explaining phenomena that have been attributed to thermal nonequilibrium but also
> seem to require cross-field communication.
> >
> > Best wishes,
> > Jim
> >
> > ********************************************************************************
> > James A. Klimchuk
> > NASA Goddard Space Flight Center
> > Solar Physics Lab, Code 671
> > Bldg. 21, Rm. 158
> > Greenbelt, MD  20771
> > USA
> >
> > Phone:  1-301-286-9060
> > Fax:      1-301-286-7194
> > E-mail:  James.A.Klimchuk at nasa.gov
> > Homepage:  http://science.gsfc.nasa.gov/sed/bio/james.a.klimchuk
> >
> > No endorsement by NASA is implied for any correspondence related to my official
> role in professional organizations.
> >
> > ********************************************************************************
> >
> >
> <tne_vs_instability19_accepted.pdf>______________________________________
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