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].