Figure 1

(a) Energy level diagram of the sodium D2 line, showing D2a transitions that can be produced by irradiation with circularly polarized light inducing ∆M = 1 absorptive transitions. The numbers between the ground and excited states or the widths of the arrows indicate the relative transition strengths. (b) Doppler-broadened hyperfine structure of the sodium D2 transition at mesospheric temperatures near 190 K. The splitting is almost entirely due to the splitting in the 32S1/2 ground state.