Abstract

Na and most other mesospheric species suffer from two major shortcomings, low density and low saturation intensity. The Na density in the mesosphere is typically 10³ - 10⁴ atoms per cm³. Moreover this density is spread over about 100 velocity classes with a natural width of 10 MHz giving a total Doppler width of 1 GHz for each line. The D₂ line is split by 1.772 GHz into a doublet ignoring other hyperfine splittings of the order of 10 MHz. The doublet is often treated as a single line with 3 GHz FWHM. The Doppler-broadened Na cross section is about 2.7 (-12) cm2 but the low density results in only about 1.5% unsaturated absorption across the entire mesosphere (depth ≈ 10 km).

© 1996 Optical Society of America