Analysis of Blue Corona Discharges and Lightning observed by the ASIM and GLM Space Missions

Blue corona discharges are a special form of lightning that has been observed by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS). They are millisecond-duration flashes, that appear in the upper regions of the thunderclouds, at times reaching into the stratosphere above. They are flashes of so-called cold streamers, and are thought to be the optical equivalent of Narrow Bipolar Events (NBEs) observed in radio signals.

The first part of the thesis analyses optical measurements from ASIM along with satellite and radar measurements, of a tropical multi-cellular storm in Australia. The storm consists of cells in different convection phases with regions of dissipating and developing clouds. The environments allow us to identify the characteristics of clouds that generate these phenomena. Focusing on discharges at the very top of the clouds, we find that 14 events are created in the highest cloud tops of both regions, in the dissipating region at the top of a few discrete convective cells that remain. The discharges have been poorly documented in the past due to the challenging viewing geometry of ground observations of cloud
tops. However, with the analysis of ASIM data, taken from the 400 km orbit of the ISS, it appears that they are associated with convective cells, even in the collapsing cells. In addition, the analysis shows that they correlate with currents of both polarities (mostly negative) and magnitudes of 9-144 kA, as detected by ground based networks. 7 out of the 14 events carry currents of a magnitude that trigger elves in the ionosphere. Finally, based on the observed currents and the radar data, their connection with NBEs is also discussed.

During the past few years, a new generation of meteorological satellites have been launched to geostationary orbit by the USA and China. They carry, for the first time, lightning cameras that track lightning activity over large regions of the earth. Europe is planning to launch a similar instrument in 2022 on the MTG, the Meteosat Third Generation satellite (MTG-LI). ASIM includes a camera that works on the same principle as the lightning cameras on the new meteorological satellites. It is looking down on the earth as the other instruments, but is in the low orbit of the ISS that gives a much higher spatial resolution and larger sensitivity.

The second part of the thesis compares simultaneous measurements by ASIM and the geostationary lightning mapper (GLM) on the US GOES-16 satellite. Three storms were chosen strategically, one in the center of the GLM field of view (FOV) and two towards the edge, in order to investigate the dependence of quality of the GLM data on the viewing angle to the storms. Furthermore, the two storms were located within the overlap region of the FOV of GLM and the future MTG-LI with the perspective of using these results as inputs for the MTG applications. The best agreement is found when lightning is close to the nadir-pointing axis of both instruments. Here, GLM measures about twice the cloud top energy as ASIM. When lightning is off axis (for both instruments), the correspondence of the cloud top energies decrease as does the number of mutual events (detection efficiency). The observed differences between ASIM and GLM are attributed to differences in their spatial resolution and to the cloud structure that may obscure emissions viewed at large angles.