So many sage comments!

The Parker problem seems now to me too abstract to apply to real observations, for a couple of other reasons. First, at least for flares, I think it is wrong to think of the footpoint perturbations as "random" - the magnetic field is arguably following coherent marching orders from deep in the convection zone. So if the mathematics of "random" is important, that would be a flaw in the concept. Second, the analysis of coronal current systems really must involve consideration of what species carries the currents, and how the conductivity tensor varies through the medium. This seems to be the language of the magnetosphere, where people can directly sample the medium. So I'd relegate the Parker problem to being, well, interesting but abstract.

Hugh

On Mar 5, 2009, at 1:09 PM, Petrus Martens wrote:

Marco et al.,

this is an interesting conversation. My tuppence: parker nanoflares  
work precisely in unipolar reasons, as it is the tangential stresses  
which are relaxed in the original parker scenario.

Agreed, that is certainly what Parker described.  The problem I have with
the Parker scenario is:  why loops?  Why not a uniform distribution of
nanoflares throughout the AR?  "Why loops?" also applies to any mechanism
that places the origin of the heating in the upper chromosphere or below.

Stick and slip reconnection along separators, a scenario that Dana
Longcope has been working on for years, answers the "Why loops?" question
seamlessly.  I am aware of course that observational studies have so far
found no clear relation between separators and loops, except in flaring
loops.  This may be a resolution issue, or an issue related to the large
amount of horizontal flux missed in magnetograms.  Or there is no such
relation, in which case we are back where we started.

Cheers,

Piet




  The presence of multiple polarities
changes this in a way which has never been quantitatively estimated.
I would like to point you to the 2008/07 papers by Rappazzo and myself  
which I believe contain the only scalings for the Parker flt scenario
which have numerical simulation confirmation.

@ARTICLE{2008ApJ...677.1348R, author = {{Rappazzo}, A.~F. and {Velli},  
M. and {Einaudi}, G. and {Dahlburg}, R.~B. }, title = "{Nonlinear  
Dynamics of the Parker Scenario for Coronal Heating}", journal =  
{\apj}, archivePrefix = "arXiv", eprint = {0709.3687}, keywords =  
{Magnetohydrodynamics: MHD, Sun: Corona, Sun: Magnetic Fields,  
Turbulence}, year = 2008, month = apr, volume = 677, pages =  
{1348-1366}, doi = {10.1086/528786}, adsurl = {http://adsabs.harvard.edu/abs/2008ApJ...677.1348R 
}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

On Mar 5, 2009, at 7:31 AM, Harry Warren wrote:

Markus et al.,

It is unambiguous that there is a lot of dynamic activity going on  
in the
transition region. However:

. . . a significant fraction of the Sun's magnetic flux closes at low
heights (several Mm), so it is unclear how much of this activity is  
related
to what we see in the corona.

. . . my impression of SOT magnetic field measurements is that  
active region
plage (where many active region loops are rooted) is unipolar. It is  
hard to
see how small-scale reconnection in the transition region could be  
important
here. I could be wrong about this! I'm sure that an SOT person could  
provide
a more definitive answer.

Best wishes,

Harry

//  
---------------------------------------------------------------------
// Harry P. Warren             phone : 202-404-1453
// Naval Research Laboratory   fax   : 202-404-7997
// Code 7673HW                 email : hwarren@nrl.navy.mil
// Washington, DC 20375        www   : http://tcrb.nrl.navy.mil/~hwarren
//  
---------------------------------------------------------------------


Markus J.
Aschwanden
Sent: Thursday, March 05, 2009 10:16 AM
To: Hugh Hudson; A mailing list for scientists involved in the  
observation
and modeling of solar loop structures
Subject: [Loops] summaries on nanoflare debates in "coronal loop  
workshops"

Dear Hugh,

Jim Klimchuk sent a great summary of the current status of a  
nanoflare model
around,
so it deals with all the PROs. An outside reader who wants to hear  
both
sides might be
interested to hear also the CONs, which you could find in the ApJ  
Letter
entitled
"An observational test that disproves coronal nanoflare heating  
models"
(Aschwanden
2008, ApJ 672, L135). The arguments made therein do not dismiss  
nanoflares
altogether, but come to the conclusion that they occur in the  
transition
region,
rather than in the corona (as Parker originally suggested).

Cheers,
Markus


On Mar 3, 2009, at 7:32 AM, Klimchuk, James A. (GSFC-6710) wrote:


Dear loops friends,

     Here is a paper I wrote for the Hinode II conference  
proceedings.  In
it, I try to do three things:  (1) review the arguments leading to the
conclusion that warm EUV loops must by bundles of strands heated by  
storms
of nanoflares (the flow chart some of you have asked about); (2)  
reconcile
the isothermal/multi-thermal debate in terms of the duration of the
nanoflare storm; and (3) address the possibility that loops can be  
explained
by thermal nonequilibrium.  Comments are welcomed.

Thanks,
James A. Klimchuk

____________________________________________
Dr. Markus J. Aschwanden
Solar & Astrophysics Laboratory
Lockheed Martin Advanced Techology Center
Org. ADBS, Bldg. 252
3251 Hanover St., Palo Alto, CA 94304, USA
Phone: 650-424-4001, FAX: 650-424-3994
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