Modeling and constraining a gamma-ray glow observed at 20 km altitude during the FEGS/ALOFT campaign

Slides of the presentation held during the 2018 AGU Fall Meeting in Washington, D.C. . See full information here.

The associated journal article is available here.

Abstract :

In the spring of 2017 the «GOES-R Validation Flight Campaign» was undertaken with an ER-2 aircraft over the continental United States. The plane, flying at a cruise altitude of 20km, had a scientific payload designed to detect optical signals, electric fields and gamma rays from thunderstorms.

On May 8, 2017, the on-board gamma-ray spectrometers could detect a gamma-ray glow event lasting for about 4 minutes over Colorado. It was detected by several spectrometers, including three BGO-based detectors, similar to the ones on-board of the ASIM instrument docked to the international space station. The glow showed a gamma-ray count increase on top of the background of 10 to 40 %, which was rather unexpected from such high altitude (20 km).

We present a detailed analysis and modeling of this glow event, by building a model based on the GEANT4 toolkit [1] (for particle propagation and detector simulation), associated with the PARMA code [2] (for cosmic ray generation). We evaluate two hypotheses for the glow generation:

- A: large scale (tens of kilometers) electric field starting from the top of the cloud (13 km) towards higher altitudes, exponentially decaying.

- B: kilometer scale constant electric field inside the thundercloud (8 to 13 km altitude) with positive or negative polarity.

We conclude that some of the type B configurations can give good spectrum fits and compatible photon flux increases, but require a polarity that is incompatible with the cloud charge structure deduced from other measurements (LMA, E-field antennas). Type A scenarios can also reproduce compatible increase in gamma-ray flux and good spectral fits, but require electric fields above the thundercloud that are several times larger than the ones measured during the event.

References:

[1] Allison, J., K. Amako, J. Apostolakis, H. Araujo, P. A. Dubois, M. Asai, G. Barrand, R. Capra, S. Chauvie, R. Chytracek, et al. (2006), Geant4 developments and applications, IEEE Transac- tions on Nuclear Science, 53, 270–278, doi:10.1109/TNS.2006.869826.

[2] Sato, T. (2016), Analytical model for estimating the zenith angle dependence of terrestrial cosmic ray fluxes, PLOS ONE, 11(8), 1–22, doi:10.1371/journal.pone.0160390