Leon, why not look at the yellow line with one of these AO systems? They are more than 2X better than TRACE or XRT, and soon there will be ~1.5 m apertures. That would be faster than getting a rocket or satellite built.

Hugh

On Jan 8, 2009, at 1:03 PM, Leon Golub wrote:

Dear Serge,

It is useful to be reminded of the history and of the ground-based data. 
  Still, I don't think that this contradicts my assertion. There is some 
evidence from density diagnostics, from analysis of cooling loops in the 
different EUV wavelengths, and from the SOT C IV data, that finer 
structures exist in the hot corona below the level we currently can see 
in the EUV or soft x-ray. I'm afraid that the only way to settle this is 
to fly a higher resolution telescope, going at least 2X better than what 
we currently have. (Indeed, as Jean-Francois notes, EUI may do that on 
Solar Orbiter. But I'd very much like to see this done - even if it's 
only a snapshot on a sounding rocket - sooner.)

Leon


Serge Koutchmy wrote:
Dear Leon, Helen, Markus, Hugh, Jean-Francois, Joan et al.,
Regarding the fuzziness (also called the diffuseness, the lack of 
resolution, the impression of smearing- I am not using here any 
impressionistic terminology, just because colors are not involved in 
your business), it could be interesting as well to recall some 
historical observations and, apparently, basic facts. If you think this 
is outside the scope of your Space and/or other research, please delete 
this message and accept my apologize.
Loops (like the corona) were first observed, more than a century ago, 
during solar total eclipses, in W-L. and we thought that we understand 
at least a part of their physics (like being in hydrostatic equilibrium 
based on their radial density variations) when assuming they are 
stationnary.
Lyot coronagraphs permitted to easily image the inner corona in emission 
lines and hundred of papers were written on loops. Lines were classified 
in 3 classes (excellent representative are for class I: the red line of 
FeX; for class II: green line of FeXIV and for class III: CaXV yellow 
line which is seen, by the way, all the time in active regions- see the 
routine measurements of SPO).
Classes correspond to roughly temperatures differing by a factor 2, 
starting with the red line at 1 MK, almost without overlap in space 
(uncorrelated). When the first good (photographic) images were obtained, 
immediately the fuzziness of the Yellow line was noticed and more 
importantly, that the FeXIV is fuzzier than the simultaneously obtained 
FeX line images was established. I was at the Pic du Midi observatory in 
1971-72 when these images were taken by J-L Leroy and we had rather hot 
discussions on their interpretation. We decided it is probably due to 
instrumental problems, including seeing problems.
Again and after in the 80ies, thousands of FeXIV and FeX images were 
taken with the former "one-shot" coronagraph of SPO, with improved 
resolution (more than a solar cycle covered !) and discussing about that 
with Ray Smartt and many others, we could not exclude an instrumental 
effect. What was noticed is that interacting loops were seen more often 
in FeXIV, and higher in the corona.
Later, came the Norikura team, boosted by the success of the Yohkoh SXR 
imager. They again confirm this famous effect. Let me give you the exact 
reference of a paper already written in the "Space age" in ApJ: 
Ichimoto, K. Hara, H. Takeda, A. et al 1995, ApJ 445, 978 and extract 
the interesting statement made p. 979: "...because the 3 emission lines 
(FeX; FeXIV and CaXV) are observed with the same optical system, we 
beleive that the diffuseness of the hot components compared with the 
cool components IS A FIRM RESULT."
Everybody knows of course the wonderful Trace 171A images of the 1 MK 
component of the corona, with loop systems, which were definitely better 
of quality (sharper) than what is obtained in hotter lines, using THE 
SAME instrument. Even FINER loops are seen in much cooler lines, like 
the Hapha or Lyman alpha line, starting from the drawings made in 1870 
ies (yes, not 1970 ies), and after, photographic images taken at 
SacPeak, rocket images from TRC, etc. and more recently CCD images and 
movies taken with the La Palma SST and Hinode.
Everything written tells us that the impression we get from an image is 
subject to a false interpretation when instrumental parameters are not 
taken into account. Resolution is not a matter of just pixel size. It is 
mainly a matter of signal/noise ratio over the feature you consider and 
even the signal alone is "noisy", just because the lack of photons; this 
is at least my understanding after observing during more than 40 Years 
and you can disagree.
Now coming back to the W-L observations let me notice that:
1- The hotter is the loop, the higher it is (see also the V-R scaling 
law). The heating makes the temperature higher when going radially 
outwardly up to 0.3 to 1 solar radius from the surface. The important 
factor is that the radial gradient is higher in cooler loops compared to 
hotter loops. It is of course what the hydrostatic law shows (on W-L 
images). The scale height of cool loops (FeX; 171A etc) is roughly 50000 
km; it is 100 000 for FeXIV. This is what you get analyzing W-L images 
assuming the loop iso-thermal and computing the radial gradient. Of 
course the higher is the gradient, the sharper is the image.
2- Talking now about the transverse gradients, the interplay between the 
magnetic pressure and the gaz pressure should be considered. The beta is 
decreasing when going up to the heights of hot loops (and further 
opening up due to the wind, see eclipse images). It makes transverse 
gradient weaker for hot loops. Accordingly, cool loops will look sharper.
This is instrumental effect.

In this naive analysis made by a lambda eclipse/coronagraphic observer, 
I do not see nothing wrong in having hotter loops fuzzier and I do not 
see the urgent need to introduce an additional filling factor, as far as 
we use good images giving large aspect ratio of loops. What is needed is 
a more sophisticated diagnostic, like the spectroscopic diagnostic which 
would include the line profile analysis to measure the Doppler-Fizeau 
effect. In addition we need the temporal resolution to look at 
transverse waves of shorter periods... there is a bright future for a 
new groundbased large aperture coronagraph which would permit the access 
to these diagnostics. Improving the W-L and visible monochromatic 
imaging in Space (inner corona; fast imaging; higher resolution; 
polarization) with an ASPIICS space mission which produces artificial 
eclipses during several Years could be the ultimate. For the moment, I 
guess SDO with its 4K coronal imagers of improved resolution will bring 
a lot and I am sure it will indeed bring a complete renewal of the 
topic. This is what you told us and let's hope it will be launched in 2009.

Thank you for reading this up to the end (almost). Let me wish you all 
the best for 2009 and use the attached season's greeting coronal image 
for that, including the caption I put after.
(apologize to those of you who already got it).
S.
-----------------------------------------

BEST WISHES FOR A HAPPY 2009 NEW YEAR !

Enclosed is a seasons’greetings compressed composite that we did using
some new observations from the Pic du Midi Observatory CLIMSO
instrument: a CaII-K line disk and a coronagraphic H-alpha image, same
scale, but obtained 2 s later, in the very late afternoon. No activity
during this surprisingly extended solar minimum. However here the solar
disk was near the W horizon, at the time when the daily Madrid to Zurich
Airbus 320 was passing by… It is the 1^st time the infamous aircraft
contrails are imaged in “emission”, using grazing incidence rays near
the H-alpha line. Would it be solid particles producing additional
pollution ?

We believe some new type of propagating waves was revealed using these
unique coronal observations, although a definite Doppler-Fizeau
signature is missing. Note that the heating process due to their
dissipation in the Earth atmosphere was not taken into account to
explain the not less infamous global warming because waves possibly also
accelerate the rather cool Mistral (a well known wind in the South of
France)...

Take care.





Leon Golub a écrit :
Helen et al.,

I can't let this go uncommented. Even though this paper was accepted 
(I know, I was the editor) I disagree with the conclusion
that hot loops are fuzzier. Yes, that's what you see, but it is
also explainable if the hot parts of the AR have many fine threads
(unresolved at present resolution) with a large filling factor.
That as the conclusion Joan and I came to from analyzing 284A
data.

Anyway, this won't be settled until we fly an imager with higher
resolution. We're proposing one this year and I'm hoping we get
the chance to clear this up, finally.
Cheers,

Leon
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