[Loops] loops and thermal nonequilibrium
Leon Golub
golub at cfa.harvard.edu
Mon Dec 21 00:43:30 MST 2009
Dear Tongjiang,
This is exactly the "argument" I've been having for the past ten years,
starting with Nakariakov in 1999. I've been waiting for the Doppler data
since then, because where I see flows, other people see waves. I do
think that the EIS results are now showing higher velocities of some
tens of km/sec, and in TRACE the flows do start out slow at the
footpoints, so there's no inconsistency with what you're saying about
the speeds. There is a difference in that EIS sees the upflows at
somewhat higher temperatures than we do with TRACE, and XRT sees them
too, meaning higher T. I'm hoping that AIA will help sort this out.
What I can tell you is that when we see waves in the corona, they are
quite clearly periodic and there's no doubt about it. Just look at any
movie (with high time cadence) taken above sunspots: it looks like a
loudspeaker pulsing away. These fans are different, and if you look
closely at your time-distance plots, you'll see the lower portions
curving upward, as the flows accelerate on their way up.
Leon
Tongjiang Wang wrote:
> Dear Leon,
>
> However, those quasi-periodic intensity perturbations (or lumps) along
> fan-like loops may not be signatures of steady flows but just the
> acoustic waves (see my recent paper, Wang et al. 2009, 503, L25). So you
> don't need to worry about any instability caused by flows.
> In this paper, I reported the first Doppler shift measurements
> of these moving features with Hinode/EIS, which show the amplitudes
> of only several km/s in Fe XII, but not tens of km/s as expected
> if they are episodic flows of 100-120 km/s, also in this case the
> Doppler shift at the footpoints is less than 7 km/s. Therefore, I wonder
> the moving blobs seen in XRT are wave features (they are actually periodic
> with period about 10-12 min), while steady flows seen in EIS near the
> footpoint of dark regions are not possible detected by imaging
> observations as you pointed.
>
> Best Regards
>
> Tongjiang
>
> -------------------
> Tongjiang Wang
>
> NASA GSFC - Code 671
> Bldg 21 - RM 177B
> Greenbelt, MD 20771
>
> Tel. 301-286-6575
> Fax. 301-286-1617
>
> On Sat, 19 Dec 2009, 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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>>> > > > > > > _______________________________________________
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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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>>> > Center for Astrophysics Fax: 617-496-7577
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