Emission lines of the mesospheric sodium layer

[Bajastro]

On 3-rd of September, just before sunset, I set up my equipment, aimed the telescope towards the zenith, set the spectrograph on the double sodium line and every now and then ran a series of photos, adjusting the exposure time (30-120 s) and gain. At the end it was so dark that the gain had to be increased almost to "what the factory gave".

I was not sure if we would be able to register anything at the resolution of 1800 l/mm holographic diffraction grating, slit with 40 μm wide. I also tracked the height (the depth of the sun below the horizon). I read in some publications that when the Sun is about 8° below the horizon, mesospheric sodium layer is in emission near the zenith above the observation site.

Two disappearing absorption lines and emission lines appearing in their place were recorded:

And an animation with the given position of the Sun relative to the horizon:

The capabilities of the printed in 3D technology my Low Spec spectrograph are amazing.

[andrew s]

Nice work, well done.

Regards Andrew 

[markse68]

That’s very interesting- what is it about the suns position that causes this flash?  Quick google suggests this sodium layer and other metals up there are from meteors burning up! And it’s used by astronomers as an artificial star excited by lasers to enable atmospheric distortion correction! Very cool

[robin_astro]

18 hours ago, markse68 said:

what is it about the suns position that causes this flash?  

The emission is caused by resonant scattering of sunlight off sodium atoms when in direct sunlight. With the sun above (or just below) the horizon,  the bright sunlight (scattered off lower levels of the atmosphere) dominates the sky spectrum so the sodium D lines appear in absorption.  After the sun sets as seen from the ground, the higher layers of  the atmosphere are still illuminated by the sun so we see the emission from the sodium atoms. 

Cheers

Robin

Edited September 14, 2020 by robin_astro
clarification