Temporal and radiometric statistics on lightning flashes observed from space with the ISUAL spectrophotometer

Abstract

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT-2 satellite was launched in 2004 and records transient luminous events (TLEs). ISUAL is an instrument composed of an imager and a multichannel spectrophotometer which is able to measure the brightness from far ultraviolet to near infrared for TLEs and lightning flashes. Several types of TLEs have been identified, classified, and documented between 2004 and 2014. However, no statistical analyses on lightning have already been realized. In this paper, we focus on lightning flashes which did not induce TLEs. Statistics about brightness or temporal dynamic are presented from fitted waveforms from far ultraviolet to near infrared, after having corrected for atmospheric effects for each spectrophotometer channels. We demonstrate that the lightning/satellite distance and the altitude of the lightning need to be taken into account. The lightning waveforms from each spectrophotometer channel could be modeled as an exponential pulsed function which the shape could be explained by the photon propagation in the cloud. The median values at 777.4 nm are 1.63 × 10 À6 W m À2 for the maximum brightness, 5.85 × 10 4 J for the total energy at the source, 414 μs for the full width half maximum, 620 μs for the full width at quarter of maximum, and 170 μs for the risetime. Results are compared with previous experiments performed from visible to near-infrared domains. The median temporal variables for near-infrared waveforms are shorter than the values of the UV waveforms.

Spectral Observations of Optical Emissions Associated With Terrestrial Gamma‐Ray Flashes

“…We assume a cloud top composition of water ice droplets with typical values r c = 15, 20 µm and n c = 2.5 × 10 8 m −3 (Dye et al, 2007;Ursi et al, 2019) while also accounting for the direction from the source to the detector relative to zenith. The altitude is estimated by assuming the cloud tops are at the tropopause (Splitt et al, 2010;Ursi et al, 2019) and that the tropopause altitude follows equation (2) of Offroy et al (2015).…”

“…Whereas only 12% of TGFs are associated with EIPs (Lyu et al, 2016), we find that almost all of them are followed by strong optical pulses and some with pre-activity starting many ms earlier (Figure 3d). The pulses have a higher blue-to-red ratio (Figure 3c) and longer durations than lightning without identified TGFs (Offroy et al, 2015;Christian & Goodman, 1987;Adachi et al, 2016), suggesting that they are a special type of current surge.…”

Simultaneous observations of optical lightning from space and LF band lightning waveforms from the ground

“…Based on previous studies, the absolute optical intensity from the first return strokes is on the order of 10 6 –10 9 W with a wavelength of 0.4–1.1 µm [ Guo and Krider, ]. Offroy et al [] and Kirkland et al [] showed that the median optical intensity of the return strokes is 2.5 × 10 8 W using satellite observation data from the ISUAL. Considering the difference in wavelength, the result indicates a reasonable agreement.…”

“…Using space‐ and ground‐based measurements, Kirkland et al [] showed that the pulse widths for return strokes, detected by the National Lightning Detection Network, are inversely proportional to pulse peak power. The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT‐2 satellite has also been used to observe TLEs and lightning emissions [ Adachi et al, , ; Frey et al, ; Offroy et al, ]. The system detects TLEs in the Earth's limb direction.…”

Spectral Observations of Optical Emissions Associated with Terrestrial Gamma-Ray Flashes

“…We assume a cloud top composition of water ice droplets with typical values r c = 15, 20 µm and n c = 2.5•10 8 m -3 (Dye et al, 2007;Ursi et al, 2019) while also accounting for the direction from the source to the detector relative to zenith. The altitude is estimated by assuming the cloud tops are at the tropopause (Splitt et al, 2010;Ursi et al, 2019) and that the tropopause altitude follows equation 2of Offroy et al (2015).…”