4.1. Investigations into the physical development of lightning discharges
Analyses of optical and combined optical/RF emissions from lightning have provided a wealth of insights on processes involved in the lightning discharge [Suszcynsky et al. , 2000; Thomas et al., 2000; Ushio et al. , 2002; Noble et al. , 2004;Østgaard et al. , 2013; Bitzer , 2017; Peterson et al. , 2017; Peterson and Rudlosky , 2019; Zhang and Cummins , 2020]. Hence, the capability of ISS LIS to detect the optical fingerprint of lightning on a global scale with a relatively high detection efficiency makes for a unique dataset to compare with optical, RF, and other measurements of lightning and related atmospheric electrical phenomena.
Current and planned space-based missions to investigate lightning and upper atmospheric electrical phenomena include measurements across several parts of the electromagnetic spectrum that complement those obtained with ISS LIS. One of these is the Atmosphere-Space Interactions Monitor (ASIM), which is an instrument suite developed by the European Space Agency and installed on the ISS in April 2018 [Neubert et al. , 2019]. The imager onboard ASIM has a 42x slower frame rate than LIS, but it has a 10x greater pixel resolution at nadir than the LIS camera [Chanrion et al ., 2019]. Furthermore, ASIM is capable of measuring light intensity across its FOV about 200x faster than LIS. This complementary nature of ASIM and LIS enables resolving the spatial and temporal components of lightning in more detail.
Both instruments detected a lightning flash on 7 February 2019, at 1941 UTC over Madagascar (Fig. 12). During its 150-ms duration, the flash illuminated three frames of the ASIM camera, while LIS detected nine groups (i.e., collections of adjacent events in the same frame; c.f., Section 2). Figure 12 highlights one of these frames from the ASIM camera, along with the LIS events and temporal evolution of LIS groups and ASIM’s 777.4-nm photometer. Each of the three LIS groups detected during this ASIM camera frame had a corresponding relative peak irradiance measured by the ASIM photometer. Although ASIM’s photometer was able to detect these distinct flash subcomponents, LIS was used to locate where in ASIM’s photometer FOV they occurred, which is especially important for separating multiple source regions within active thunderstorm complexes. One of the strokes illuminated 36 pixels of the LIS camera for 2 ms, and the higher-resolution ASIM camera observations revealed the narrowest part of the illumination at cloud top to be roughly half a LIS pixel wide (~2 km).
As shown above, when sub-pixel-sized lightning sources trigger ISS LIS detections, ASIM can be used to infer more information about the true spatial extent of that discharge. Also, curved channels within one or more pixels and dark regions between emitting parts of a cloud may not be resolved by LIS. Instead, the intensity measured by LIS would be proportional to the seemingly more active part of the cloud, obscuring whether the respective source was wider and less intense or small and bright [Zhang and Cummins , 2020].
Satellite missions such as ASIM and the upcoming Tool for Analysis of Radiation from lightning and Sprites (TARANIS) [Lefeuvre et al., 2008], which focus on investigating transient luminous events (TLEs) and terrestrial gamma ray flashes (TGFs), will be compared with observations from ISS LIS for calibration and validation of their optical instruments and used to better determine the spectral fingerprint of these upper atmospheric electrical phenomena. Additionally, LIS observations are being compared with ground-based electric field measurements from Marx meter arrays in Panama [Bitzer et al. , 2013; Zhu et al. , 2020] to investigate signatures of TGFs measured at the ground to their optical characteristics observed in space. Marx meter arrays in Alabama, Panama, and Argentina also are being compared with ISS LIS to observations to further investigate the extent of optical emissions by narrow bipolar events [Jacobson et al. , 2013; Rison et al. , 2016;Liu et al. , 2018].