Atmospheric sodium spectroscopy

There is a layer of atmosphere with sodium at around 80km above us which is excited by our Sun and makes an emission spectrum. Peter Schlatter gave me the idea to observe this phenomenon back at OHP few years ago.

During the day, it is not visible as the solar spectrum is too bright. During the night, this layer is not excited by solar rays But there is a transition phase when the Sun is ~6° below horizon. The sky spectrum switch from solar spectrum to night spectrum and the sodium appears in emission from this layer during a short period (around 15-20 minutes).

Here are the spectra obtained with the echelle spectrograph:

As time goes (from bottom to the top), the absorption is filled up and raise in emission to get lost into the noise as the solar spectrum disapear and the night is ready for some more stellar spectropscopy!