LAUSR

Future space missions (e.g., ASIM and TARANIS) are soon to be launched to observe transient luminous events (TLEs) from a nadir-viewing geometry. The mission GLIMS already performed observations of TLEs from a nadir-viewing geometry on board the International Space Station. Although this observation geometry is of first interest to study TLEs, it makes the determination of some quantities, such as streamer altitudes, very difficult. In this study, we propose a method to estimate the altitude of downward propagating sprite streamers using a spectrophotometric approach. Using a plasma fluid model, we simulate sprite streamers at different altitudes and quantify their optical emissions in the Lyman-Birge-Hopfield (LBH) (∼100–260 nm), the first positive (1PN 2) (∼650–1070 nm), and the second positive (2PN 2) (∼330–450 nm) bands systems of molecular nitrogen and the first negative (1NN + 2) (∼390–430 nm) bands systems of N + 2. The estimation of associated ratios allows to trace back the electric field in the streamer head as well as the altitude at which the streamer is propagating owing to different dependencies of quenching processes on the air density. The method takes into account the nonsteady state of the populations of some excited species and the exponential expansion of the streamer. The reported results could potentially be used for all TLEs but is of special interest in the case of column sprites or at the early stage of carrot sprites.