It is useful to be reminded of the history and of the ground-based data.
in the EUV or soft x-ray. I'm afraid that the only way to settle this is
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
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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