Modeling elves observed by FORMOSAT‐2 satellite

Abstract

[1] The ISUAL experiment on the FORMOSAT-2 satellite has confirmed the existence of ionization and Lyman-Birge-Hopfield (LBH) band emissions in elves. In this paper, an in-depth study of the ISUAL recorded elves was carried out. Numerical simulation results of elves based on an electromagnetic finite difference time domain (FDTD) model of the emissions between 185-800 nm and of their spatial-temporal evolution are presented. To account for the effect of atmospheric attenuation, three major attenuation mechanisms: O 2 , O 3 , and molecular Rayleigh scattering are considered. Validations of the electromagnetic FDTD model were conducted in three ways: by comparing the calculated and observed photon fluxes in the ISUAL spectrophotometric channels, by directly comparing the simulated and observed morphologies of elves, and by comparing the computed photon counts of the ISUAL Imager based on the derived peak currents for two elve-associated NLDN (National Lightning Detection Network) cloud-to-ground discharges (CGs) with those recorded by the ISUAL Imager. In all three ways, very good agreement was achieved.

ISUAL Imager and far-ultraviolet spectrophotometer degradation

“…Although the parent lightning is also in the FOV of the SP1, the lightning emission in the FUV band is difficult to transmit through the atmosphere from 10 km altitude to FORMOSAT-2 (~890 km) (Chang et al 2010). According to the elves simulation presented by Kuo et al (2007), a relationship exists between the time-integrated N 2 1P photon flux and the time-integrated SP1 FUV photon flux. Hence, we can estimate the standard total photon flux in SP1.…”

“…3). The light source used here is elves because the elves optical intensity was well described (Kuo et al 2007). We estimated the calibrated SP4 photon fluxes for the elves by assuming the negligible optics degradation (lenses and filters) and by considering the PMT sensitivity degradation.…”

“…Elves is induced by the electromagnetic pulses emitted by an intense lightning (Inan et al 1991) and was first observed by Fukunishi et al (1996). Both the theoretical and experimental results show that the time-integrated photon flux of elves in both the N 2 first positive (1P) band and N 2 Lyman-Birge-Hopfield (LBH) bands depends on the lightning peak current (Barrington-Leigh and Inan 1999; Kuo et al 2007;Chang et al 2010;Chen et al 2014). Owing to the proximity between the elves and the causative lightning, the spectrophotometric observations of the elves N 2 1P optical emissions are always contaminated by the lightning emissions except for the elves triggered by lightning located behind the Earth's limb.…”

“…The far-ultraviolet (FUV) emissions emitted by transient luminous events (TLEs) originated in the Lyman-Birge-Hopfield (LBH) band of nitrogen molecule N 2 . From the ISUAL data analyses, it is known that elves Kuo et al 2007Kuo et al , 2008Chang et al 2010), halos Kuo et al 2008), sprites (Kuo et al 2005Liu and Pasko 2005;Liu et al 2006), and gigantic jets (Kuo et al , 2009Chou et al 2010) are accompanied by FUV emissions. However, blue jets without the FUV emission were also reported (Chou et al 2010) as the longer life time of the N 2 LBH band upper state results in quenching at the lower altitudes.…”

“…A new standard light source is needed for calibrating the Imager and SP1. Due to the wellinvestigated emission intensities of elves (Kuo et al 2007) and their high occurrence rate ; elves can be used as a calibrator. This paper presents a method to estimate the degradation rate using the elves emissions as a calibrator, and the ISUAL Imager sensitivity degradation as well as the SP1 is also discussed.…”