We present spectra from 400 to 900 nm of a bolt from the blue stepped leader on July 19, 2015 including novel observations from 400 to 600 nm. The spectra of the luminous steps near the tip of the leader look qualitatively similar and contain singly ionized nitrogen lines. The presence of these ionized lines indicate currents and temperatures comparable to peak return stroke. The entire channel illuminates over the final 1.3 ms before attachment, but does not contain evidence of ionization. This neutral spectrum is qualitatively similar to the return stroke spectrum after peak current. This may indicate increasing potential and current along the channel prior to attachment.
We have examined fine-structure mixing between the rubidium 5 2 P 3/2 and 5 2 P 1/2 states along with quenching of these states due to collisions with methane gas. Measurements are carried out using ultrafast laser pulse excitation to populate one of the Rb 5 2 P states, with the fluorescence produced through collisional excitation transfer observed using time-correlated single-photon counting. Fine-structure mixing rates and quenching rates are determined by the time dependence of this fluorescence. As Rb(5 2 P ) collisional excitation transfer is relatively fast in methane gas, measurements were performed at methane pressures of 2.5 − 25 Torr, resulting in a collisional transfer cross section (5 2 P 3/2 → 5 2 P 1/2 ) of (4.23 ± 0.13) × 10 −15 cm 2 . Quenching rates were found to be much slower and were performed over methane pressures of 50 − 4000 Torr, resulting in a quenching cross section of (7.52 ± 0.10) × 10 −19 cm 2 . These results represent a significant increase in precision compared to previous work, and also resolve a discrepancy in previous quenching measurements. arXiv:1812.09466v1 [physics.atom-ph]
Methane is a prevalent greenhouse gas with potent heat trapping capabilities, but methane emissions can be difficult to detect. Hyperspectral imagery is an effective method of detection which can be used to locate methane emission sources, as well as provide accountability for reaching emissions reduction goals. Because of methane's absorption features, both shortwave infrared (SWIR) and longwave infrared (LWIR) hyperspectral sensors have been used to accurately detect methane plumes. However, surface, environmental, and atmospheric background conditions can cause methane detectability to vary, and there have not been previous studies which evaluate this variability over a wide range of conditions. To assess this variation, this trade study compared methane detectability for two airborne hyperspectral sensors: AVIRIS-NG in the SWIR and HyTES in the LWIR. We modeled methane plume detection under a wide range of precisely known conditions by making use of synthetic images which were comprised of MODTRAN-generated radiance curves. We applied a spectral matched filter to these images to assess detection accuracy, and used these results to identify the conditions which have the most significant impact on detectability in the SWIR and LWIR. We then computed the specific boundaries on these conditions which make methane most detectable for each instrument; these novel results explore methane detectability over a broader range of conditions and sensors than previous studies. This trade study and methodology can aid decision making about which sensors are most useful for various types of methane emission analysis, such as leak detection and emission rate quantification.
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