AbstractTerrestrial gamma‐ray flashes (TGFs) are bright bursts of gamma rays produced by thunderstorms, typically observed by spacecraft in the low‐Earth orbit. Unfortunately, it has been difficult to disentangle the source altitude and the width and direction of the gamma‐ray beam using single point spacecraft measurements, which has hampered attempts to constrain TGF models. Polarimetry of astrophysical sources has been of interest for many decades, which raises the question: Do TGFs and X‐rays from lightning have observable polarization, and if so, what would this polarization tell us about their source? REAM Monte Carlo code has been modified to record the linear polarization of X‐rays and gamma rays as a function of source altitude and beam geometry. It is found that polarization degree of a 20‐km narrow beam of TGF is substantially different from a 15‐km‐wide beam, which could be used to constrain the source geometry of TGFs. However, due to the low fluence of these events in space, detecting this level of polarization would be challenging. It is also found that low‐altitude TGFs (source at 3.5 km) produce polarizations up to about 8%; however, detectors need to be very close to the source region. Furthermore, very low altitude ground‐level TGFs and X‐rays showed a maximum polarization of 13% on the ground, of which the TGF's fluence was large enough for polarimetry. In addition, polarization reached its maximum further away from the z axis as the TGF's beam broadened. The dominant mechanism of the polarization was found to be Compton scattering.
