[Loops] loops and thermal nonequilibrium

Mon Dec 7 09:28:11 MST 2009

Hi Leon,

     A couple quick points of clarification....

Spiros and I were comparing models with the same peak temperature.  In that case, shorter heating scale heights (greater footpoint concentration) produce higher densities because the amplitude of the heating must be increased to achieve the same peak temperature.  You and Amy found the same thing.

I need to qualify my statement about the pressure scale height.  The presence of a steady flow requires that the pressure scale height be smaller than the local gravitational scale height (at the local temperature).  A smaller scale height means a larger pressure gradient, and you need a super-hydrostatic pressure gradient to accelerate the plasma in the upflowing leg and decelerate the plasma in the downflowing leg.  Otherwise, it would be in force balance with gravity and the loop would be static.  Things can get complicated when you compare static and steady flow loops because they have different temperature profiles, so the pressure gradient at a given position in the loop can be bigger in one or the other.

Cheers,
Jim

> -----Original Message-----
> From: loops-bounces at solar.physics.montana.edu [mailto:loops-
> bounces at solar.physics.montana.edu] On Behalf Of Leon Golub
> Sent: Monday, December 07, 2009 9:11 AM
> To: A mailing list for scientists involved in the observation and modeling of
> solar loop structures
> Subject: Re: [Loops] loops and thermal nonequilibrium
> 
> Jim,
> 
> We were indeed modelling the long, relatively faint loops seen in the
> plage regions surrounding sunspots, which are exactly where EIS is
> seeing the flows. It's been known for a long time (the Palermo people
> did such modelling 20 years ago) that flows in coronal loops drop the
> density ("When you start a flow going, the loop disappears.") or
> alternatively, if you see the loop it means the density is enhanced. I'm
> not sure why your modelling failed, but we were able to reproduce the
> scale height quite well. Having the flow (in either direction) extends
> the emission scale height quite a bit beyond the hydrostatic value.
> 
> As you know, the AIA on SDO will have far more extensive temperature
> coverage than TRACE does. There is a puzzle right now in that EIS sees
> the flows at higher temperatures than we saw in TRACE. I think that this
> topic is going to be a major one in the coming years.
> 
> Leon
> 
> 
> Klimchuk, James A. (GSFC-6710) wrote:
> > Wow, I'm surprised, and pleased, at the interest this paper has generated!
> Let me first respond to Leon's comment.  As Harry said (thanks!), in order to
> get the extreme excess densities that are observed in most warm loops, the
> footpoint heating needs to be so concentrated that no equilibrium exists
> (which strictly speaking is different from an instability).  Hence, thermal
> nonequilibrium.  A few years back, Spiros P. and I addressed your suggestion
> with Amy that asymmetric heating and steady flows might explain the
> observations.  Our modeling showed that the density enhancement was
> adequate to explain some loops, but the scale height is too small and the
> filter-ratio temperature profile is far too structured (paper attached).  We
> thus rejected this explanation.   Sorry!
> >
> > Thanks for your comment,
> > Jim
> >
> >> -----Original Message-----
> >> From: loops-bounces at solar.physics.montana.edu [mailto:loops-
> >> bounces at solar.physics.montana.edu] On Behalf Of Harry Warren
> >> Sent: Sunday, December 06, 2009 6:19 PM
> >> To: A mailing list for scientists involved in the observation and modeling of
> >> solar loop structures
> >> Subject: Re: [Loops] loops and thermal nonequilibrium
> >>
> >>
> >> Leon,
> >>
> >> As I recall, your paper with Amy relied on footpoint heating, which does
> >> lead to higher apex densities and flatter temperature ratios. The observed
> >> densities near 1 MK are so high, however, that the loops become
> >> thermodynamically unstable. Also, the high speed EIS flows are typically
> >> seen in faint regions and are not associated with the types of loops that
> >> Jim is attempting to model.
> >>
> >> Harry
> >>
> >>
> >> On 12/5/09 3:16 PM, "Leon Golub" <golub at cfa.harvard.edu> wrote:
> >>
> >>> Jim,
> >>>
> >>> Amy and I addressed the issues of excess density, flat temperature
> >>> profiles and large scale height about 10 years ago. Having flows of
> >>> 30-40 km/sec, as is observed in TRACE and now verified by EIS, solves
> >>> these problems quite nicely. So there is a viable mechanism other than
> >>> nanoflares.
> >>>
> >>> Leon
> >>>
> >>>
> >>> 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
> >>>>
> >>>> Fax:      1-301-286-7194
> >>>>
> >>>> 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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> >> --
> >> // ---------------------------------------------------------------------
> >> // Harry P. Warren             phone : 202-404-1453
> >> // Naval Research Laboratory   fax   : 202-404-7997
> >> // Code 7673HW                 email : hwarren at nrl.navy.mil
> >> // Washington, DC 20375        www   : http://tcrb.nrl.navy.mil/~hwarren
> >> // ---------------------------------------------------------------------
> >>
> >>
> >>
> >>
> >>
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> --
> ______________________________________________________________
> ________________
> Leon Golub
> Smithsonian Astrophysical Observatory
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