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 My measurement (unpublished, sorry to say - long story) is that the flows increase in velocity, reaching up to 140 km/sec by the time they get to heights of ~10^5 km.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@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@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@solar.physics.montana.edu [mailto:loops- > bounces@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@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@nasa.gov > mailto:James.A.Klimchuk@nasa.gov > > > Home page: > > > http://hsd.gsfc.nasa.gov/staff/bios/cs/James_Klimchuk.html > > > > > > > > > > > > > ************************************************************** > *************** > > > *** > > > > > > > > > > > > > > > ------------------------------------------------------------------------ > > > _______________________________________________ > > > Loops mailing list > > > Loops@solar.physics.montana.edu > > > https://mithra.physics.montana.edu/mailman/listinfo/loops > > _______________________________________________ > > Loops mailing list > > Loops@solar.physics.montana.edu > > https://mithra.physics.montana.edu/mailman/listinfo/loops > -- > // > --------------------------------------------------------------------- > // Harry P. Warren phone : 202-404-1453 > // Naval Research Laboratory fax : 202-404-7997 > // Code 7673HW email : hwarren@nrl.navy.mil > // Washington, DC 20375 www : > http://tcrb.nrl.navy.mil/~hwarren > // > --------------------------------------------------------------------- > > > > > > _______________________________________________ > Loops mailing list > Loops@solar.physics.montana.edu > https://mithra.physics.montana.edu/mailman/listinfo/loops _______________________________________________ Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
-- ______________________________________________________________________________ Leon Golub Smithsonian Astrophysical Observatory 60 Garden Street Cambridge, MA 02138 617 495 7177 FAX 496 7577 lgolub@cfa.harvard.edu ______________________________________________________________________________ _______________________________________________ Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
--
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@cfa.harvard.edu
Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops