The Faraday filter-based spectrometer for observing sodium nightglow and studying atomic and molecular oxygen associated with the sodium chemistry in the mesopause region

Abstract

This paper describes the operational principles, design and field testing of a new, compact, Faraday filter-based spectrometer to measure the D 2 (589.158 nm) to D 1 (589.756 nm) intensity ratio of the sodium nightglow. This work was motivated by the observations of Slanger et al. (2005) who reported an annual variation in D 2/D 1 with values ranging from 1.2 to 1.8. Their proposed explanation, a modified Chapman mechanism, requires that the intensity ratio is related to the concentration ratio of atomic oxygen [O] to molecular oxygen [O 2]. Our method of measuring D 2/D 1 utilizes narrowband Na vapor Faraday filters, which can yield observations on the fractional contributions of the two chemical pathways of the modified Chapman mechanism. Since delineation of the two chemical pathways requires a spectral resolution of 0.0002 nm, this is not possible with any other existing instrument.