Energy spectrum from single TGFs detected by ASIM

Terrestrial Gamma-ray Flashes (TGFs) are short bursts of high energy photons associated with lightning flashes in thunderstorms. The Atmosphere-Space Interactions Monitor (ASIM) was launched in April 2018 and is the first space instrument specifically designed for TGF detection. We will focus on data from the High Energy Detector (HED), which is a part of the Modular X- and Gamma-ray Sensor (MXGS) of ASIM. HED is sensitive to photons with energies from 300 keV to > 30 MeV.

There are still unresolved questions regarding the production of TGFs. The energy spectrum of TGFs, together with Monte Carlo simulations, can provide answers to production altitude and beaming geometry of the TGFs. Constraints have already been set on the production altitude and beaming geometry using other spacecraft and radio measurements. Some of these studies are based on cumulative spectra of a large number of TGFs (e.g. Dwyer and Smith 2005), which smooth out individual variability. Spectral analysis of individual TGFs has been carried out up to now for Fermi TGFs only, showing spectral diversity (Mailyan et al. 2016). Crucial key factors for individual TGF spectral analysis are a large number of counts, energy range extended to several tens of MeV, a good energy calibration, knowledge and control of any instrumental effects affecting the measurements.

Thanks to ASIM's large effective area and lower orbital altitude, single TGFs detected by ASIM have much more count statistics than observations from other spacecraft capable of detecting TGFs. During the first 10 months, ASIM detected more than 60 TGFs, out of 215, with more than 100 counts. This allows for a large sample for individual spectral analysis.

The goal is to put much stricter constraints on production altitude and beaming geometry, by comparing Monte Carlo simulations to energy spectra from single ASIM TGFs. We will present the dataset and method, including correction for instrumental effects, and preliminary results.