Ultra-fast time-resolve spectroscopy of lightning-like discharge with galius

Resumen

On 17th August 1961, the first time-resolved spectra of an individual lightning flash was obtained (Salanave,1961). This opened a new era of time-resolved lightning spectroscopy. A few years later, Orville (1968a, b, c, d) recorded the first time-resolved spectra of individual lightning strokes between the cloud and the ground with a time resolution of 5 μs (a lightning flash is formed by several strokes). These spectra showed the dynamics of temperature and electron density up to 50 μs. Recently, doubly ionized nitrogens were first found in spectra of triggered lightning return strokes with a time resolution of 1.5 μs (Walker and Christian, 2017, 2019). Quantifications from these spectra also revealed high temperatures up to 40000 K and high electron densities up to 10^19 cm^-3. These high values could be explained by the short exposure time (1.44 μs) of the instrument and its ability to record in the early time of the discharges. This moves us to explore lightning spectra within even shorter timescale and higher recording speeds. In this thesis, we will present a study on time-resolved spectroscopy at micro and sub-microsecond timescales of lightning-like discharges with a new instrument developed in our group at IAA-CSIC, named GALIUS. GALIUS - GrAnada LIghtning Ultrafast Spectrograph - is a portable, ground-based spectrograph intended to carry out spectral analysis of natural lightning or artificial lightning. GALIUS can record spectra up to 2.1 million frame-per-second (Mfps), from the near-ultraviolet to the near-infrared thanks to four interchangeable grisms - a grism is a combination of a prism and a grating arranged in such a way that light at a chosen central wavelength passes straight through. Each grism is customized and designed to record spectra at certain spectral ranges with different speeds: grism R1 allows to record spectra in the near ultraviolet-blue (380 - 450 nm) at 672000 fps; grism R2 allows recording at the same speed but in the visible-near infrared (475 - 795 nm); grism R3 allows images at 2.1 Mfps in a short visible region (645 - 665 nm), centered in the H alpha line (656.5 nm). Finally, grism R4 allows to record spectra within the near-infrared (770 - 805 nm) at 1.4 Mfps. Details of the configurations and characteristics of each grism were described in the article published in Applied Optics with title GALIUS: An ultrafast imaging spectrograph for the study of lightning (Passas et al, 2019). In total, GALIUS has 22 configurations which we designed for the different experimental setups both in the laboratory and in the field. Initially, GALIUS was tested with small (4 cm) sparks generated by an electrostatic generator at IAA-CSIC, Granada. After that, GALIUS was moved to Tarrasa (Barcelona) to work with relatively long (1 m) electrical discharges produced by a 2.0 MV Marx generator in the Switching and Lightning modes operated at 800 kV. Spectra of these sparks were carefully analyzed to identify optical signatures of chemical species and to quantify key magnitudes (temperature, electron density, electrical conductivity ...). Results of these studies will be presented in different chapters of the thesis corresponding to each of the papers published. Our first results are about the analysis of optical emission spectra from lightning-like electrical discharges recorded at 2.1 Mfps with grism R3. These are the first time-resolved spectra obtained with high-speed framing cameras at sub-microsecond timescales. Quantifications from these spectra revealed evidence of non-LTE conditions right behind the shock front in post-trigger sub-microsecond times. We published these results in a paper in Geophysical Research Letters with title 'Sub-microsecond Spectroscopy of Lightning-Like Discharges: Exploring New Time Regimes' (Kieu et al., 2020). At lower recording speeds, GALIUS was capable of filming lightning-like discharges from the near ultraviolet-blue to the near-infrared with grisms R1, R2, R4. We found evidence of molecular optical emissions from CO, CN, C2, N2, and N_2^+. This finding may open the door to identify and quantify lightning NO production by using high-speed optical emission spectroscopy. From the emissions of different spectral lines, electron concentrations and gas temperatures were estimated by different methods and compared with each other. These results were published in a paper in Journal of Geophysical Research Atmospheres with title 'High-speed spectroscopy lightning-like discharges: evidence of molecular optical emissions', (Kieu et al., 2021). References: 1. Kieu, T. N., Gordillo-Vazquez, F. J., Passas, V. M., Sánchez, J., and Pérez‐Invernón, F. J. (2021). High-speed spectroscopy of lightning-like discharges: evidence of molecular optical emissions. Journal of Geophysical Research: Atmospheres, 126(11):e2021JD035016. 2. Kieu, N., Gordillo‐Vázquez, F.J., Passas, M., Sánchez, J., Pérez‐Invernón, F.J., Luque, A., Montanyá, J. and Christian, H., (2020). Submicrosecond spectroscopy of lightning‐like discharges: Exploring new time regimes. Geophysical Research Letters, 47(15), p.e2020GL088755 3. Passas-Varo, M., Sánchez, J., Kieu, T.N., Sánchez-Blanco, E. and Gordillo-Vázquez, F.J., (2019). GALIUS: an ultrafast imaging spectrograph for the study of lightning. Applied optics, 58(29), pp.8002-8006. 4. Orville, R. E. (1968a). A high-speed time-resolved spectroscopic study of the lightning return stroke: Part I. A qualitative analysis. Journal of the Atmospheric Sciences, 25(5):827-838. 5. Orville, R. E. (1968b). A high-speed time-resolved spectroscopic study of the lightning return stroke: Part II. A quantitative analysis. Journal of the Atmospheric Sciences, 25(5):839-851. 6. Orville, R. E. (1968c). A high-speed time-resolved spectroscopic study of the lightning return stroke. Part III. A time-dependent model. Journal of Atmospheric Sciences, 25(5):852-856 7. Orville, R. E. (1968d). Spectrum of the lightning-stepped leader. Journal of Geophysical Research, 3(22):6999-7008. 8. Salanave, L. E. (1961). The optical spectrum of lightning. Science, 134(3488):1395-1399. 9. Walker, T. D. and Christian, H. J. (2017). Triggered lightning spectroscopy: Part 1. A qualitative analysis. Journal of Geophysical Research: Atmospheres, 122(15):8000-8011. 10. Walker, T. D. and Christian, H. J. (2019). Triggered lightning spectroscopy: Part 2. Aquantitative analysis. Journal of Geophysical Research: Atmospheres,124(7):3930-3942.