Hanle effect in laser-induced fluorescence and Na and Fe resonance scattering lidars

The nature of the Hanle effect on laser-induced fluorescence (LIF) remains subtle and physically ambiguous. By associating the Hanle effect with the linearly superposed phase-locked excited substates induced by laser-like coherent light, this paper attempts to demystify its underlying physics. The resulting LIF radiation leads to angular distribution of radiation, whose detail depends on the quantum structure of the target atoms. Three simple quantum radiators are used to illustrate the fact that LIF could result in distributions ranging from that of spontaneous emission to that of a classical dipole oscillator. The radiation patterns of six Na ${{\rm D}_2}$D₂ and five Fe transitions of interest are presented. The nonunity backward enhancement $q$q-factor of a transition alters its contribution proportionally to the received lidar signal; thus, it results in temperature and wind errors if this factor is ignored in data processing of Na and Fe Doppler lidars as well as of the Fe Boltzmann lidar; these errors are shown to be less than 1 K and 1 m/s.