Comparisons between NaD ₂ (589  nm), Fe (372 and 386 nm), and He( ³ S) (1083  nm) laser-induced fluorescence lidars: influence of the Hanle effect and single-photon measurement uncertainty

Inthis paper, we systematically compare the bias caused by ignoring the Hanle effect in retrieving atmospheric density <i>N</i>, temperature <i>T</i>, and line-of-sight wind <i>V</i> by four laser-induced fluorescence (LIF) lidars. Under a typical condition in the mesopause (<i>T</i>=200<i>K</i>,<i>V</i>=0<i>m</i><i>s</i>^-1) for <i>N</i><i>a</i><i>D</i>₂ (589 nm), Fe (372 nm), and Fe (386 nm) and in the thermosphere (<i>T</i>=1000<i>K</i>,<i>V</i>=0<i>m</i><i>s</i>^-1) for <i>H</i><i>e</i>(³<i>S</i>) (1083 nm) lidars, we find an overestimate in <i>N</i> of 8.3%, 8.7%, 16%, and 17%, respectively. Though no bias exists in the retrieved <i>T</i> and <i>V</i> for the two Fe lidars, the bias of ignoring the Hanle effect is (-0.28<i>K</i>, 0.70 m/s) for <i>N</i><i>a</i><i>D</i>₂ (589 nm) lidar and (-19.6<i>K</i>, -24.6<i>m</i>/<i>s</i>) for <i>H</i><i>e</i>(³<i>S</i>) (1083 nm) lidar. We also evaluated the associated single-photon measurement uncertainties (<i>δ</i><i>T</i>_1<i>S</i> and <i>δ</i><i>V</i>_1<i>S</i>) and find them to be (506 K, 429 m/s), (451 K, 252 m/s), (494 K, 247 m/s), and (3847 K, 2560 m/s), respectively. When divided by the received signal-to-noise ratio (SNR) of a measurement, these quantities are related to the corresponding measurement uncertainties (<i>δ</i><i>T</i>_{<i>a</i><i>i</i><i>r</i>}<i>a</i><i>n</i><i>d</i><i>δ</i><i>V</i>_{<i>a</i><i>i</i><i>r</i>}) as (2<i>δ</i><i>T</i>_1<i>S</i><i>S</i><i>N</i><i>R</i>><i>δ</i><i>T</i>_{<i>a</i><i>i</i><i>r</i>}≥<i>δ</i><i>T</i>_1<i>S</i><i>S</i><i>N</i><i>R</i> and 2<i>δ</i><i>V</i>_1<i>S</i><i>S</i><i>N</i><i>R</i>><i>δ</i><i>V</i>_{<i>a</i><i>i</i><i>r</i>}≥<i>δ</i><i>V</i>_1<i>S</i><i>S</i><i>N</i><i>R</i>), where the equality holds when the received background is zero.