The ASIM mission main scientific objective is to study the correlation between Transient Luminous Events (TLE) and Terrestrial Gamma Ray Bursts (TGF) which occur int eh upper atmoshpere above thunderstorms. This is achieved by the two instruments, one optical instrument for observing TLEs and one X and Gamma Ray instrument for observing TGFs. The two instruments implement advanced triggering algorithms for the identification of these event, and also cross-trigger each other to capture both optical and gamma ray data for all events which are potential candidates of TLE/TGFs. The full set of science goals for ASIM is outlined by ESA:
Transient luminous events are electromagnetic energy discharges in the upper atmosphere, well above the normal lightning we usually observe from ground. TLEs include sprites, blue jets, and elves (and more), the most common of which are the sprites. TLEs occur above thunderstorms which means that lightning activity will be on-going in the lower atmosphere. ASIM mission life will be minimum two years.
Terrestrial gamma ray flashes are high energetic discharges in the Earth atmosphere, but the origin of events is unclear. This is why ASIM aims to study the correlation between the TGF with lightning and TLE.