[Loops] EIS AR Loops - Tripathi et al, 2009, ApJ/ some historical comments etc.

Thu Jan 8 05:40:56 MST 2009

Bonjour Serge, et bonne année,

Je ne lis pas tous ces mails sur les loops, mais merci pour
ta superbe image du Pic !

Amicalement,
Guillaume

Le Jeu 8 janvier 2009 11:56, Serge Koutchmy a écrit :
> 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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>> https://mithra.physics.montana.edu/mailman/listinfo/loops
>>
>>
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>

_________________________________________________

   Guillaume Aulanier
   Coordinateur du Pôle de Physique Solaire

   Observatoire de Paris, LESIA
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   email:  guillaume.aulanier at obspm.fr
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