Cosmic perspective during twilight time

[Christian Buil]

Spectral phenomena during an observation of nova Scorpii 2015 - at twilight time and at very low elevation above horizon for France (dec. -35°). The sources distance are very different on these spectra. The sodium doublet atmospheric airglow is well visible...



Christian B

[Joan Guarro Flo]

Salut Christian:

Parfois, nous donnons une bonne leçon de maître...

Félicitations, Joan.

[Christian Buil]

Merci Joan! And now, twilight at sun down toward the zenital direction (less light pollution). Note presence of aurora [O I] lines:



Christian B

[Paul Luckas]

Very nice. Here's another cosmic perspective - this time of the star Achernar - taken during the day last November while setting up a new Lhires III, centred around 4655Å:

Sol_Achernar.png (69.65 KiB) Viewed 6133 times

Paul

[Paolo Berardi]

Thanks for sharing Christian, very instructive poster and wonderful ambience during dawn time! Several telluric lines frequently interfere with our spectra (but also help to check the wavelength calibration!).

Paul, a star spectrum during daylight? Fantastic!

Paolo

[Olivier Thizy]

Hello,


excellent results!

I have an astrophysical question. I understood from EPFL astrophysics on-line course that aurora emission line were produced because electrons and protons particles sent by the Sun were captured by Earth magnetosphere and concentratred around polar poles... and those particles were exciting by shock the high altitude atmosphere (80km-250km depending on which line), creating those beautiful aurora and those emission lines seen in your spectra at night.

But the sodium flash is also the 80-100km high atmosphere excited creating this sodium flash; but it's only visible at some point of time when the Sun is shining at ~90° of the Zenith of the observing location.

What specific process is exciting those sodium atoms and creating a temporary flash in the sodium emission lines?


Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/

[Hubert Boussier]

Bonjour,

"What specific process is exciting those sodium atoms and creating a temporary flash in the sodium emission lines?"

Juste une remarque sur l'expression "temporary flash" qui m'avait un peu dérouté la première fois que j'ai vu ce phénomène présenté lors d'un stage spectro à l'OHP.

Je crois que le sodium de la haute atmosphère n'est pas excité seulement pendant les 10 minutes au moment du coucher de soleil, il doit l'être pendant tout le temps où le soleil éclaire la haute atmosphère. Simplement, quand le ciel est éclairé on doit faire des poses très courtes pour ne pas saturer le capteur et on ne voit pas l'émission du Na, puis au fur et à mesure que la luminosité du ciel baisse, on peut poser de plus en plus long et on fini alors par voir la lumière du doublet du Na.
Il serait intéressant de noter les temps de pose sur les images que Christian à faites. J'ai fait une fois cette manip, les premières poses étaient de 2s, puis juste avant le flash je posais 33s, au moment du flash sodium les poses étaient de 60s pour terminer à 120s.

Pour ce qui est du phénomène d'excitation, j'avais trouvé une publication assez ancienne de A. Kastler Physica XII, n°9-10 (12/1946) dans laquelle étaient proposés deux mécanismes : résonance optique ou photoluminescence excitée par les UV du soleil. Un troisième mécanisme mixte : excitation optique par les UV du soleil d'un niveau supérieur, puis réémission de lumière par étapes successives.

Depuis, les savants ont résolu cette question, il semble que cela soit un phénomène de résonnance, c'est dit dans la référence suivante :
Ann. Phys. Fr., Vol. 10, N°6 1985, pp. 705-716
DOI: 10.1051/anphys:01985001006070500

Recherches sur le sodium atmosphérique
J. Blamont

CNRS, Service d'Aéronomie, B.P. n° 3, 91370 Verrières-le-Buisson, France



Abstract
The sodium flash discovered in 1938 as a strong feature present in the spectrum of the light scattered by the atmosphere during twilight could be caused either by the dissociation of NaO molecules, or by the optical resonance of free Na atoms. Bricard and Kastler demonstrated that optical resonance is the excitation mechanism by measuring both the linewidth of the emission with absorption cells containing sodium atoms (1944) and the polarization of the doublet (1949). This proved that free Na atoms could exist for a long time in the atmosphere around 90 km of altitude. The consequences of this work are briefly examined : a) The sodium clouds ejected by rockets gave a large wealth of data on the atmosphere from 80 to 500 km (winds, temperature, turbulence parameters) ; the baryum ions clouds are still used for the study of magnetospheric and interplanetary plasmas. b) The absorption technique has been transposed to the study of hydrogen around the Earth and in the interplanetary medium, with the discovery of the local interstellar wind ; the use of optical resonance for refined spectroscopic measurements has led to the first measurement of the Einstein red shift on the sun and to the recent development of the study of solar oscillations, which constitutes the modern tool for the probing of the sun down to its core.


N'étant plus en activité, je n'ai pu avoir accès qu'au résumé, je n'ai pas cherché plus loin...

[Martin Dubs]

Hello Olivier,

Hubert has it explained in great detail. I would like to add one thing. The sodium at this altitude is mostly from the disintegration of meteors, which usually evaporate at around this altitude of 80 to 100 km. The sodium is usually one of the prominent lines in the meteor spectrum. Visible light at the sodium transition is a very efficient excitation mechanism and is used also at the large observatories for creating an artificial guide star for adaptive optic telescope guiding:http://www.eso.org/public/images/vlt-brunier-nuit/. The observation that sodium (and other alkali metals) absorbs at the same wavelength if illuminated by sunlight as it emits if heated by a flame goes back to Bunsen and Kirchhoff in their pioneering study of the solar and man made spectra, which showed that the sun is made from the same elements as the earth. The reason for that comes from the fact that the alkali atoms have their resonance transition from the groundstate in the visible spectrum, unlike other elements with their lowest transition far in the UV (H, He, metals etc.)

Regards, Martin