Multispectral Optical Diagnostics of Lightning from Space

Abstract

We present spectroscopic diagnostic methods that allow us to estimate the gas and the electron temperature in emerged lightning stroke channels (from thunderclouds) observed by the photometers and cameras of the Atmosphere Space Interaction Monitor (ASIM). We identify the species (molecules, atoms and ions) producing light emission in different wavelengths, and how the blue (337 ± 2 nm), red (777.4 ± 2.5 nm) and ultraviolet (180–230 nm) optical emissions captured by ASIM photometers change as a function of the temperature in the lightning stroke channel. We find good agreement between the light curves of the emerged lightning observed by ASIM and the synthetic ones obtained from calculated spectra. Our results suggest that (i) early stage (high temperature > 20,000 K) emerged lightning strokes at high altitude can contribute to the optical signals measured by the PH2 photometer (180–230 nm), (ii) intermediate stage (mid temperatures, 6000–21,000 K) emerged lightning strokes can produce 777.4 nm near-infrared radiation (observable by PH3) exhibiting higher intensity than PH1 observable N2 SPS between ∼6000 K and ∼8000 K, and than ion optical emissions (336.734 nm and 337.714 nm) between ∼16,000 K and ∼21,000 K, (iii) from ∼16,000 K to 35,000 K, neutral oxygen 777.4 nm radiation and ion emissions at 336.734 nm and 337.714 nm can be simultaneoulsy observed but 777.4 nm dominates only between ∼16,000 K and ∼21,000 K, (iv) the availability of detections with a narrow 0.5 nm gap filtered photometer (336.75–337.25 nm), with the same or better sensitivity than PH1 in ASIM-MMIA but with a central wavelength at exactly 337.0 nm (the strongest N2 SPS transition), would give access to the late stage of lightning strokes (emerged or not) when temperatures are between 8000 K and 5000 K (or lower for a photometer with better sensitivity than PH1 in ASIM-MMIA) when the production of nitrogen oxides (NOx) and hydroxyl radicals (OH) maximizes.