[Loops] loops and thermal nonequilibrium

Mon Dec 7 08:52:41 MST 2009

Hi Hugh,

     Thanks also to you for your comments.  Harry (thanks again!) is completely correct that my use of the term "nanoflare" simply means an impulsive energy release on a magnetic strand.  There are many possible mechanisms that have this property, including waves.  In fact, I do not know of a single mechanism that does not have this property under realistic conditions!  Having said that, if the impulsive bursts repeat sufficiently rapidly on a given strand, the plasma conditions will not be greatly different from quasi-static.
     I'm confused by your suggestion that "flow" is the same as "thermal nonequilibrium."  Flows can happen for many reasons, with varying effects on the loop properties.  As I indicated in my response to Leon, steady flows cannot reproduce most observed warm loops, and neither can the flows associated with thermal nonequilibrium.
     Regarding nanoflares being a purely theoretical construct with no observational support, all I can say is that impulsive sub-resolution energy bursts are the only known way to explain the observed properties of warm loops (fan loops at the perimeters of active regions, of the type Leon has mentioned, may have an alternate explanation?).  If you can point to a rigorous paper that offers another explanation, please let me know.
     As Harry stated, quasi-steady heating seems to be a valid explanation of hot loops.  Impulsive heating (with long intervals between repeats in a given strand) may also be a valid explanation for these loops, although a lack of cospatial warm emission in the predicted quantities might rule this out.  We still need to work out the numbers.
     I was interested to hear about Helen's EIS results and look forward to learning more.  As you know, RHESSI observations are consistent with there being a weak super-hot component at the levels found by XRT and predicted by the nanoflare models.  Kuen Ko and her colleagues at NRL found that the sensitivity of EIS to Ca XVII (one of the strongest hot lines in the EIS spectrum) is inadequate to prove or disprove nanoflares.  The intensities predicted by the models are too weak.
    I'm not sure what to make of your comment about the low-temperature boundary in the models apparently negating the results in your view.  We never claim to model the chromosphere in enough detail to compare with chromospheric observations.  Our goal is to study the corona, and our model accurately treats the energetic and dynamic interaction between the corona and chromosphere (i.e., evaporation and condensation).  Do you have some reason to believe that our treatment of the chromosphere adversely affects the coronal solution?  If so, please explain in detail.

Thanks again for your interest,
Jim

From: loops-bounces at solar.physics.montana.edu [mailto:loops-bounces at solar.physics.montana.edu] On Behalf Of Hugh Hudson
Sent: Saturday, December 05, 2009 6:20 PM
To: A mailing list for scientists involved in the observation and modeling of solar loop structures
Subject: Re: [Loops] loops and thermal nonequilibrium

Dear Jim

I have some comments. It looks like a nice piece of work, but as usual the conclusion that only nanoflares can help is hopelessly quixotic. You set up a strawman competing mechanism (why call it "thermal nonequilibrium" when the one-syllable alternative "flow" works as well?) and reject it on the basis of time scales. Who knows about the time scales of the driver of the flows?

The model itself seems highly questionable. I had cited its predecessor in trying to explain the "chewy nougat" observation of X-rays from prominences (http://adsabs.harvard.edu/abs/1999ApJ...513L..83H) but realized later on that the model has a very artificial low-temperature boundary and is thus not compelling. It is surprising to me that a decade later this has not changed, especially since it is just this boundary region in which the heating may be taking place.

To be perfectly clear about nanoflares: I do not think that there is any convincing observational evidence that they exist. It is a purely theoretical construct.

Cheers

Hugh

p.s. at the Hinode meeting just concluded, a nice poster by Helen Mason made it very clear that EIS spectra emphatically do not agree with the various XRT suggestions of a high-temperature component in the loop DEM. The discrepancy is an order of magnitude.

On 4 Dec 2009, at 14:04, Klimchuk, James A. (GSFC-6710) wrote:

Dear Loops Friends,

    If you are interested, the attached paper shows that coronal loops cannot be explained by thermal nonequilibrium.  The results appear to rule out the widespread existence of coronal heating that is both highly concentrated low in the corona and steady or quasi-steady (slowly varying or impulsive with a rapid cadence).  Comments are welcomed.

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
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E-mail:  James.A.Klimchuk at nasa.gov<mailto:James.A.Klimchuk at nasa.gov>
Home page:  http://hsd.gsfc.nasa.gov/staff/bios/cs/James_Klimchuk.html

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<klimchuk_submitted.pdf>
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