CDAP: A portable CCD-based daytime airglow photometer for investigations of ionosphere-thermosphere phenomena

A photometric technique to obtain daytime optical airglow emissions is described. Airglow emissions occur naturally due to photochemical and chemiluminescent processes taking place in the upper atmosphere. Airglow emissions that occur in the daytime are referred to as dayglow. It is a challenge to detect these dayglow emissions as the solar scattered background against which they occur is extremely bright. Thus, it is required that high spectral resolution measurements are carried out to delineate the difference between the dayglow signal and the solar scattered background brightness. In the method being described here, a low-resolution Fabry-Perot etalon is used as a high spectral resolution filter to obtain the dayglow signal. The spectral resolution achieved by this instrument is 0.026 nm at 630.0 nm, which is sufficient to separate the signal and neighbouring spectral regions. The present technique builds up on the success of employing such methods in the past and brings in innovations that make it reliable, rugged, and suitable for unattended field operations. The details of the technique and the new elements brought in are described in this paper.