Polarization of TGFs and X-rays from natural and rocket-triggered lightning and its association with source geometry

Terrestrial gamma-ray flashes (TGFs) are bright bursts of gamma rays produced by thunderstorms, most often observed by spacecraft in the low-Earth orbit. Two important properties of TGFs are their source altitudes and the width and direction of the gamma-ray beam. Unfortunately, it has been difficult to disentangle these properties using single point spacecraft measurements, which has hampered attempts to constrain TGF models. X-ray and gamma-ray polarization measurements of astrophysical sources has been of interest for many decades, and new space missions are being proposed to make such measurements. This raises the question, do TGFs and the related X-ray emissions from lightning have observable gamma-ray or X-ray polarization, and, if so, what would this polarization tell us about their source? In this work, the REAM Monte Carlo code, which has been used to model TGFs for many years, has been modified to record the linear polarization of X-rays and gamma rays. The predicted polarization and fluence of TGFs and X-rays from lightning are presented as a function of source altitude and beam geometry. It is found that mid-altitude TGFs, near of just above the tops of thunderclouds, could have polarizations as high as 10%, but because only a few tens of counts are typically detected by spacecraft in low-Earth orbit from TGFs, detecting this polarization level is unlikely. It is also found that low-altitude TGFs ( 3.5 km) could produce gamma ray polarizations up to about 5% on the ground, which may be detectable since the number of counts detected during ground-level TGFs can be large. For both TGFs seen from space and ground-level TGFs, the dominant process for producing the polarization is found to be Compton scattering. On the other hand, avalanches of X-rays produced from rocket-triggered lightning measured on the ground showed a maximum polarization of 13% which is also due to Compton scattering. This level of polarization seems detectable by polarimeters on the ground at radial distances up to 1 kilometer.