[Loops] loops and thermal nonequilibrium
Hugh Hudson
hhudson at ssl.berkeley.edu
Mon Dec 7 11:55:23 MST 2009
Another geometrical point is that the line-of-sight column density
increases at loop top if you look at it out-of-plane at all. And then
another geometrical effect is that if it is not a planar loop, then
that twist can augment the line-of-sight effect. The twist,
incidentally, can be present even in a potential field. The "loop top"
and "above-the-loop-top" brightenings observed by SXT need to have
these considerations.
Hugh
On 7 Dec 2009, at 10:42, Craig DeForest wrote:
> Piet, Jim,
>
> Jim is of course right that unresolved structure makes the excess
> density problem worse, though at this point I've lost track of just
> how bad it really is.
>
> In fact, in that 2007 paper I cited some of the discrepancies
> between density-sensitive line pair spectral measurements of
> density, and corresponding EUV measurements using relative
> photometry - the discrepancies seem to go in the right direction
> (i.e. that spectral diagnostics yield higher densities than one
> would expect from simple EUV photometry).
>
> The whole point of the geometric argument I was making then is that
> large bundles of active region threads expand much more rapidly than
> the threads themselves, in current EUV images. Assuming that the
> image threads come from structures that expand at the same rate as
> the bundle neatly solves the hydrostatic scale height problem. It
> is not necessary to assume that loop strands are expanding faster
> than the body of the loop.
>
> Jim and Marcello published a very nice analysis indicating that (at
> least) they're not being stupid, but IMHO there are still too many
> uncertainties in instrument performance to know whether loop strands
> are indeed unresolved and expanding rapidly. I haven't followed up
> in the literature, because I think higher resolution (maybe from Hi-
> C) is required to nail down that argument on one side or the other.
> (I won't expand this note still further by elaborating).
>
> Best,
> Craig
>
> On Dec 7, 2009, at 11:22 AM, Klimchuk, James A. (GSFC-6710) wrote:
>
>> Hi Piet,
>>
>> Craig's nifty idea is a possible explanation for the larger-than-
>> hydrostatic scale heights (although as you say, how can the
>> unresolved strands expand faster than the loop), but unfortunately
>> it does NOT explain the excess densities. In fact, it makes the
>> problem worse! We find that warm loops are over dense when we use
>> a filling factor of unity to get the density from the observed EM
>> and loop diameter: n = sqrt(EM/(df)). Taking f<1 makes the loops
>> even more over-dense.
>>
>> Jim
>>
>>> -----Original Message-----
>>> From: loops-bounces at solar.physics.montana.edu [mailto:loops-
>>> bounces at solar.physics.montana.edu] On Behalf Of Petrus Martens
>>> Sent: Monday, December 07, 2009 12:11 PM
>>> To: A mailing list for scientists involved in the observation and
>>> modeling of
>>> solar loop structures
>>> Subject: Re: [Loops] loops and thermal nonequilibrium
>>>
>>> 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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>>>>
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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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