High-speed spectroscopy of lightning-like discharges: evidence of molecular optical emissions

Abstract

High speed spectra (between ∼ 380 nm and ∼ 800 nm) of meter long lightning-like discharges recorded at 672000 fps and 1400000 fps (with 1.488 μs and 0.714 μs time resolutions and 160 ns exposure time) show optical emissions of neutral hydrogen, singly-ionized nitrogen, oxygen and doubly-ionized nitrogen which are similar to those found in natural lightning optical emissions. The spectra recorded in the near ultraviolet-blue range (380 - 450 nm) and visible-near infrared (475 - 793 nm) exhibited features of optical emissions corresponding to several molecular species (and emission bands) like CN (Violet bands), N2 (Second Positive System), N +2 (First Negative System), C2 (Swan band) and CO (Quintet and Å ngström bands). Molecular species can be formed at regions of the lightning-like channel where the gas temperature would be milder and / or in the corona sheath surrounding the heated channel. We have quantified and compared electron densities and temperatures derived from different sets of neutral and ion line emissions and have found different sensitivities depending on the lines used. Temperatures derived from ion emissions are higher and change faster than those derived from neutral emissions.