[Loops] FW: summaries on nanoflare debates in "coronalloopworkshops"

Wed Mar 11 07:55:08 MDT 2009

Hi Robertus,

    Thanks for sending David's presentation.   He heard about our
listserve discussion and sent me the paper that goes with it.  I in turn
sent him the following two messages and am waiting for his reply.

Cheers,
Jim

Hi David,

    Yes, we've been having an e-mail discussion within the loops
community, and Robertus mentioned that you had ruled out nanoflare
heating.  Thanks for the paper and presentation.  I've only had a chance
to glance at them, but they look very interesting.  
    Do you stand by your claim?  As you know, it is extremely difficult
to measure event energies, especially for small events.  I've had some
discussions with Arnold Benz recently, and he feels strongly that we do
not have accurate enough information to place reliable constraints on
the problem, i.e., to determine whether nanoflares are important based
on energy distributions.  Clare Parnell tells me the same thing.  Also,
I would note that recent studies by the Max Planck group of small events
observed by SUMER give power laws steeper than 2, supporting the
nanoflare idea.
    Nanoflares of the type I believe exist are so small that they cannot
be measured directly, and there is a danger in extrapolating "observed"
energy distributions to lower values.  The physics of large eruptive
flares is probably very different from the physics of small confined
flares and nanoflares.  A recent study by Yashiro et al. finds that
flares that are not associated with CMEs have a power law steeper than
2.
    I'm very interested in your feedback.

Best wishes,
Jim

Hi again,

     I was just looking at Section 5.1 of your paper (slide 29 in the
presentation), where you discuss lower limits to the nanoflare energy
based on the arguments of Markus (1999).  I'm afraid I see some problems
with those arguments.  

     First, Markus' scaling laws are P~T^3 and L~T.  This gives
E_th~PL~T^4, not T^6. 

     Second, the coronal pressure is not set by the chromospheric
pressure.  Rather, the chomosphere will extend to a height at which its
gravitationally stratified pressure matches that of the corona, which is
set by the heating.  Nonetheless, I agree that there is practical lower
limit to the coronal pressure produced by a nanoflare.

     Third, the cross sections of the strands that are heated by
nanoflares are probably very small.  For example, a typical TRACE loop
contains several tens of elemental flux tubes as observed in the
photosphere.  If energy release occurs at the interfaces of these tubes,
as seems very likely, then the nanoflare-heated strands could have a
diameter of 10 km or less.

     The bottom line is that 10^23 erg is not a valid lower limit to the
energy of nanoflares.

Cheers,
Jim

-----Original Message-----
From: Robert von Fay-Siebenburgen [mailto:robertus at sheffield.ac.uk] 
Sent: Monday, March 09, 2009 4:16 PM
To: Klimchuk, James A. (GSFC-6710)
Subject: Re: [Loops] summaries on nanoflare debates in
"coronalloopworkshops"

jim,

see attached:),
cheers,
r.

On Mon, 9 Mar 2009, Klimchuk, James A. (GSFC-6710) wrote:

> Valery and Robert,
>
>
>
>    Thanks for bringing up waves.  I have two quick points.
>
>
>
> 1.  The internal plasma structure of a loop (multi-stranded or not) is
> determined by the heating and cannot be decoupled from the wave
> dissipation (unless the wave is energetically insignificant).  See
> Section 4.3 of the attached.
>
>
>
> 2.  There may be different opinions of what is meant by "nanoflare."
> Some people define it to be an identifiable point-like brightening
that
> probably occurs in a tiny bipole.  Most loops modelers define it to be
> an impulsive energy release in a long and unresolved magnetic strand.
> It cannot be directly observed.  With this definition, even resonant
> wave absorption produces nanoflare heating (also explained in Sections
> 4.3 and 4.2).  I'm very curious about David Berghmans' talk and have
my
> doubts about the conclusions!
>
>
>
> Cheers,
>
> Jim
>
>
>
> ________________________________
>
> From: loops-bounces at solar.physics.montana.edu
> [mailto:loops-bounces at solar.physics.montana.edu] On Behalf Of
> Nakariakov, Valery
> Sent: Saturday, March 07, 2009 9:02 AM
> To: A mailing list for scientists involved in the observation and
> modeling ofsolar loop structures; A mailing list for scientists
involved
> in the observation and modelingofsolar loop structures
> Subject: Re: [Loops] summaries on nanoflare debates in
> "coronalloopworkshops"
>
>
>
> Hi Robertus, hi all,
>
>> however, both hinode and very recent rosa observations have shown the
> omnipotent presence of mhd waves ... that have the necessary poyting
> flux to heat the lower
> atmosphere or corona. so, it is not just reconnection!
>
> There are two important aspects, in my opinion:
> (a) even if the Poynting flux is sufficient, there also should be
> sufficiently effective mechanisms for the wave energy dissipation at a
> reasonable height. The efficiency of both phase mixing and RA is
> determined by the steepness of the transverse structuring of the
> waveguiding plasma structures.
>
> Structuring is definitely an unknown parameter, as you've pointed out:
>> if (IF) we want to get closer to loop fine structure
> (multi-thread vs whatever) or uncover the operating heating function,
a
> very promising and currently already available way is of the one by
> magneto-seismology.
>
> I agree. As I have mentioned in this forum before, there are several
> coronal seismological indications of the subresolution structuring of
> the corona. But, what is important is not only the spatial scale of
> structuring, but also the gradients of the plasma parameters.
>
> (b) The amplitudes of the observed transverse waves are sufficiently
> high to produce a significant change of the magnetic field in the
> vicinity of the potential reconnection
> sites. Hence, the waves can effectively contribute to reconnection,
and
> then the separation of "wave-based" and "reconnection-based"
mechanisms
> for heating seems to be a bit too artificial.
>
> All the best,
>
> Valery
>
>
>
>
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