The Imager for Sprites and Upper Atmospheric Lightning (ISUAL)

Abstract

The Imager for Sprites and Upper Atmospheric Lightning (ISUAL) was the first specifically dedicated instrument to observe lightning‐induced transient luminous events (TLE): sprites, elves, halos, and gigantic jets from space. The Imager is an intensified CCD system operating in the visible wavelength region with a filter wheel to select from six positions with filters. The Imager has a 5° × 20° (vertical times horizontal) field of view. The spectrophotometer (SP) is populated with six photometers with individual filters for emissions from the far ultraviolet to the near infrared. An array photometer with two channels operating in the blue and red provides altitude profiles of the emission over 16 altitude bins each. The Associated Electronics Package (AEP) controls instrument functions and interfaces with the spacecraft. ISUAL was launched 21 May 2004 into a Sun‐synchronous 890 km orbit on the Formosat‐2 satellite and has successfully been collecting data ever since. ISUAL is running on the nightside of the orbit and is pointed to the east of the orbit down toward the limb. The instrument runs continuously and writes data to a circular buffer. Whenever the SP detects a sudden signal increase above a preset threshold, a trigger signal is generated that commands the system to keep the data for about 400 ms starting from ~50 ms before the trigger. Over its lifetime of ~11 years the system recorded thousands of TLE and also successfully observed aurora and airglow.

The ASIM Mission on the International Space Station

“…They include elves (Fukunishi et al 1996), gigantic jets (Pasko et al 2002), trolls, gnomes and pixies (Lyons et al 2003), and blue glimpses . Global observations of TLEs have most notably been taken by the ISUAL instrument (Imager of Sprites and Upper Atmospheric Lightning) on the FormoSat-2 polar orbiting satellite, viewing the Earth's atmosphere towards the limb (Frey et al 2016). The observations allowed for studies of the statistics and distribution of TLEs (Chen et al 2008), and for single event studies of their dynamics using a complement of array photometers (e.g.…”

“…If an event is likely to be a TLE, the spatial and temporal structure of emissions will aid in the interpretation of the kind of TLE was observed. The higher temporal and spatial resolution of the ASIM measurements relative to past observations (Blanc et al 2004;Sato et al 2015;Frey et al 2016) are expected to make this task simpler and the conclusions more robust. Some specifications of the MMIA sensors are given in Table 2 and in Chanrion et al (2019).…”

Observations of Blue Discharges Associated With Negative Narrow Bipolar Events in Active Deep Convection

“…The ISUAL AP consists of two modules with each comprising 16 vertical spaced channels between 350–470 nm (blue) and 530–650 (red) to record temporal and spatial photometric data. The sampling rate of AP is 20 kHz during the initial 20 ms and decreases to 2 kHz thereafter until 240 ms after the trigger time (Chern et al, ; Frey et al, ).…”

Variations of the 630.0 nm airglow emission with meridional neutral wind and neutral temperature around midnight

“…where µ D is the quantum yield of O( 1 D), which is about 1-1.3 (Torr and Torr, 1982); γ is the rate coefficient of Reaction (R1) (St.-Maurice and Torr, 1978); k 1 , k 2 and k 3 are the rate coefficients of O( 1 D) quenched by N 2 , O 2 and O, respectively (Langford et al, 1986;Streit et al, 1976;Sun and Dalgarno, 1992); and A 1D and A 2D are the transition coefficients (Froese-Fischer and Saha, 1983). The formulas for the rate coefficients (Vlasov et al, 2005) are listed in Table 1 (Huba et al, 2000).…”