Exploring High Altitude Beacon Concepts Other Than Sodium

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).