[Loops] loops and thermal nonequilibrium
Craig DeForest
deforest at boulder.swri.edu
Mon Dec 7 11:42:55 MST 2009
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
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>
>> **************************************************************
>>>>>> ***************
>>>>>>>> ***
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> ------------------------------------------------------------------------
>>>>>>>>
>>>>>>>>
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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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