Modeling aircraft-based detections of 511 keV background enhancements observed in thunderstorms.

Poster presented during the AGU 2017 Fall Meeting, (Wednesday, 13 December 2017)

In January 2016, an A340 aircraft equipped with a dedicated in-flight lightning detection system ILDAS ( flew through thunderstorms at 12 km altitude over Northern Australia. ILDAS includes two high performance LaBr3 based gamma-ray detectors. It detected strong enhancements of the 511 keV background line (each lasted for 0.5 - 1.0 s and total duration over 15 s) in correlation with intensive electrical activity. Such events seem quite similar to the 2009 "Positron clouds" observations of the Airborne Detector for Energetic Lightning Emissions (ADELE) reported in [Dwyer et al. 2015].

Using the GEANT4 toolkit and a model of the aircraft that includes the gamma-ray detectors, we probe different hypotheses to try to explain the observed 511 keV enhancement :

  •  Positron clouds of several sizes
  •  RREA, with a favorable geometrical configuration
  •  Neutron induced radioactivity

Conclusions :

  • The 511 keV enhancements are associated with static discharges from the aircraft.
  • The observed spectrum below 511 keV is consistent with almost only positron annihilation, but cannot reproduce the higher energy part.
  • Annihilation area is likely of the size of the aircraft or close
  • Combination of photons from Relativistic Runaway Electron Avalanche (RREA) + positrons may work.
    • With RREA alone and a favorable geometrical configuration:
      • Not more than ~ 20 % positron to photon ratio
      • Even this requires high Electric fields (1.4 times RREA threshold)
  • Neutron induced radioactivity may explain some of the observed features, but setting up the models correctly is challenging (3 models -> 3 different results)