[Loops] loops and thermal nonequilibrium
Ignacio Ugarte Urra
iugarte at ssd5.nrl.navy.mil
Wed Dec 23 14:14:38 MST 2009
Hi all,
I would just want to add to Leon's comment on cool downflows, that
EIS/Hinode in its wide slit mode can make movies in a couple of
transition region lines (Mg VI 269.0, Si VII 275.3) and we have in fact
observed and recently reported
(http://adsabs.harvard.edu/abs/2009ApJ...695..642U) downflows of 100km/s
at those temperatures.
A short movie not included in the paper:
http://tcrb.nrl.navy.mil/~iuu/out/offlimb/20071219_eis_slot_offlimb.mpeg
Cheers,
Ignacio
--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
~ Ignacio Ugarte-Urra ~
Naval Research Laboratory. Code 7673U.
4555 Overlook Ave SW. Washington, DC 20375
Contractor. Spain.
phone: (+1) 202 404 1779
email: iugarte at ssd5.nrl.navy.mil
http: http://tcrb.nrl.navy.mil/~iuu
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
On Sat, 2009-12-19 at 08:03 -0500, 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 My measurement (unpublished, sorry to say - long story) is that the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
> >>>>>>>> Loops at solar.physics.montana.edu
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> >>>>>>> _______________________________________________
> >>>>>>> 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
> >>>>>> //
> >>>>>> ---------------------------------------------------------------------
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> _______________________________________________
> >>>>>> Loops mailing list
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> >>>>>> https://mithra.physics.montana.edu/mailman/listinfo/loops
> >>>>> _______________________________________________
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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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> >
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