[Loops] loops and thermal nonequilibrium
Petrus Martens
pmartens at cfa.harvard.edu
Tue Dec 22 08:51:34 MST 2009
Leon and Tongjiang,
At the AGU Bart De Pontieu showed me very interesting data that
indicate persistent blue-shifts at the basis of fan-like bundles of
loops, in addition to "puffs" going up the legs of the same loops.
The interpretation seems obvious and is consistent with the
previous emails. I have little to add to this since I am not so
familiar with the data, but this will be interesting, and fairly
easy in my opinion, to simulate with hydro codes.
What is important to me is that the signatures of oscillations
that we detect with our feature finding codes could indicate any of
the options discussed here: pure waves, puffs, oscillatory flows.
So it is important that we label the output of our oscillation code
correctly, not just as the detection of pure waves.
Is it lot of trouble to put together a web-site showing some
of the best examples of the phenomena discussed here? That would
certainly help modelers.
Cheers,
Piet
Leon Golub wrote:
> Dear Gordon and Piet,
>
> All I can contribute to this discussion is the observation that the
> flows are steady and continual, lasting for at least several days if not
> longer. The individual strands may come and go, but the overall
> structure of the fan only changes slowly. Meanwhile, the flows go on and
> on. My measurement (unpublished, sorry to say - long story) is that the
> flows increase in velocity, reaching up to 140 km/sec by the time they
> get to heights of ~10^5 km. After that they become too faint to see.
>
> These are very long closed loops, about as near to being open structures
> as they can be and still be closed. I suspect that the material cools as
> it flows and it comes down at transition region temperatures. Going up,
> it's at ~1 MK in TRACE (Fe IX/X and Fe XII), probably hotter as seen in
> XRT. AIA should be able to see the cooler material (Fe VIII at 131A) and
> tell us if it's coming down.
>
> So the puzzle in my mind is why we don't see the instabilities that you
> folks insist should be present. There are, of course, lumps in the flow;
> if there weren't then we would not be able to tell that anything is
> moving. But those start at the footpoints and move upward - they are, in
> fact, what we measure to determine the velocities.
>
> Cheers,
>
> Leon
>
>
> Gordon Petrie wrote:
>> Dear Piet,
>>
>> If I understand the steady isothermal theory correctly, the scale
>> height can become unbounded wherever the flow approaches the sonic
>> point, v -> c_s, because of a v^2-c_s^2 factor. Beyond a (generally
>> different) critical point, v=sqrt(g_s Z_0) where g_s is the solar
>> surface gravitational acceleration and Z_0 a length scale of the flux
>> tube spatial expansion, mass conservation demands that the density
>> invert.
>>
>> Best regards,
>>
>> Gordon
>>
>> On Mon, 07 Dec 2009 10:11:07 -0700
>> Petrus Martens <pmartens at cfa.harvard.edu> wrote:
>>> Gordon,
>>>
>>> I agree with your comment. In fact, it is fairly easily shown
>>> directly from the momentum equation that flows up to the sound speed
>>> increase the pressure scale height by at most a factor two, when v~c_s,
>>> much less when v<c_s because the flow effect scales as (v/c_s)^2.
>>>
>>> A really original solution as to why observed loops can be overdense
>>> near their apexes was presented by Craig Deforest in 2007. If the
>>> unresolved strands that make up the observed loop increase in cross-
>>> section from footpoints to apex an observer would conclude that the
>>> loop is overdense and has a scale height much larger than what follows
>>> from the loop temperature. In reality there is simply more emitting
>>> volume near the apex. Of course one has to explain why strands increase
>>> in cross-section, while the loops that they collectively form appear not
>>> to, but Craig shows some nice images in his paper that seem to support
>>> his suggestion.
>>>
>>> There are ways to verify this from observations.
>>>
>>> Cheers,
>>>
>>> Piet
>>>
>>>
>>>
>>> Gordon Petrie wrote:
>>>> Dear All,
>>>>
>>>> In a basic model of steady, isothermal hydrodynamic flow
>>>> (http://adsabs.harvard.edu/abs/2006ApJ...649.1078P), steady flows
>>>> can only affect hydrostatic scale heights under exceptional
>>>> conditions. These states have fast flows approaching the sound
>>>> speed and form a small part of the solution space right next to
>>>> unphysical regimes with density inversions. If significantly many
>>>> loops really are of this kind, it would be an interesting problem
>>>> explaining why. On the other hand, it's clear from rho*V*A why
>>>> steady flows decrease densities.
>>>>
>>>> Best regards,
>>>>
>>>> Gordon
>>>>
>>>> On Mon, 07 Dec 2009 09:11:07 -0500
>>>> Leon Golub <golub at head.cfa.harvard.edu> wrote:
>>>>> 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
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>> **************************************************************
>>>>>>> ***************
>>>>>>>>> ***
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> ------------------------------------------------------------------------
>>>>>>>>> _______________________________________________
>>>>>>>>> Loops mailing list
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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
>>>>> 60 Garden Street
>>>>> Cambridge, MA 02138
>>>>> 617 495 7177
>>>>> FAX 496 7577
>>>>> lgolub at cfa.harvard.edu
>>>>> ______________________________________________________________________________
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>>>
>>>
>>> --
>>> --------------------------------------------------------------------
>>> Piet Martens Tel: 617-496-7769
>>> Center for Astrophysics Fax: 617-496-7577
>>> 60 Garden Street, MS 58 Cell: 617-999-0353
>>> Cambridge, MA 02138 pmartens at cfa.harvard.edu
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--
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Piet Martens Tel: 617-496-7769
Center for Astrophysics Fax: 617-496-7577
60 Garden Street, MS 58 Cell: 617-999-0353
Cambridge, MA 02138 pmartens at cfa.harvard.edu
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