TGF location in the FOV of ASIM from MXGS/MMIA imaging and WWLLN events and zenith angle variations in TGF flux, spectral hardness and LE-HE pulse delay

Abstract:

After two years of ASIM observations we now have ~700 TGFs archived, from which ~400 have a FOV location from either MXGS imaging, MMIA cameras, WWLLN/VAISALA events, or combinations thereof. In this presentation we will make an overview of ASIM imaging location methods, their accuracy, and the variation with zenith angle of TGF parameters such as frequency, flux, LE spectral index, HE/LE pulse arrival delay time, and HE/LE hardness derived from MXGS imaging.

MXGS imaging returns not only a most probable TGF location, but also its LE flux and a HE scatter background, from which an analogy to hardness ratio can be determined, and this allows us to infer how far from the vertical a TGF might be beamed. This was introduced in a poster for AGU2019, but there were not enough data points to make any clear inference after only one year of observations, and we can now show there are clear deviations which can only be explained by a significant beam angle from the vertical, useful to underpin current theories of TGF origin.

MXGS performance is greatly reduced by the small CZT detector area (~1000 cm2) available, making it something of a pathfinder mission, and we present a new mission design, with much larger area, which will enable the accurate location of nearly all TGFs as opposed to the 1/3 available via lightning event locations, and the limited FOV of optical cameras.