Optical properties of the shallow and exposed lightning discharges observed by ASIM

The Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS) observes lightning and Transient Luminous Events (TLEs) above the thunderstorm clouds. ASIM includes three photometers that sample at 100 kHz and two cameras that image at 12 frames per second. The photometers measure part of the far ultraviolet (FUV) and middle ultraviolet (MUV) band at 180 – 300 nm, a line of the second positive system of N₂ at 337nm (blue) and an atomic oxygen line at 777.4 nm (red). The cameras measure in the blue and red bands of the photometers with a spatial resolution on the ground around 400 m × 400 m. When ASIM is in a nadir-viewing configuration, photometer signals in the blue and red are sometimes associated with coincident UV signals, indicating that the UV photons originated from lightning discharges at cloud altitudes and not from the TLEs at higher altitudes. Here, we analyse the optical properties of these events by combining data from ASIM, the global lightning network GLD360, Lightning Mapping Arrays (LMAs) and NEXRAD radars. Of the 12 cases identified with such data coverage, 5 are Cloud-to-Ground (CG) and 7 are Intra-Cloud (IC) lightnings. The lightning leaders are located nearby the cloud top boundaries or partly exposed outside the cloud. Both the CG and IC lightnings are associated with the exposed lightning leaders. The 5 CG lightnings are identified as the “bolts from the blue”, and the 7 IC lightnings are “cloud-to-air” lightning. The altitudes of the sources vary from 5 km to 7 km for the CG lightnings and from 7 km to 15 km for the IC lightnings. The optical properties for the events, such as their irradiance, rise time and duration in the different optical bands are summarized and discussed. The results provide information that allows to estimate the global occurrence of “bolts from the blue” and “cloud-to-air” lightning.