On the Linear Polarization of TGFs and X‐Rays From Natural and Rocket‐Triggered Lightning and Its Association With Source Geometry

Abstract

Terrestrial 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.

Science with the Daksha High Energy Transients Mission

“…These rates are easily detectable in Daksha, where individual MEPs will have an average of 0.03 counts in 10 µs bins, and are designed to handle might higher peak count rates. TGFs are expected to have a few percent degrees of polarization [176] which varies with source altitude. Daksha will be able to put stringent observational constraints on the polarization of the brightest TGFs.…”

Upward Lightning at the Gaisberg Tower: The Larger‐Scale Meteorological Influence on the Triggering Mode and Flash Type

“…In this context, another type of UL triggered by a rocket which is connected to the ground via wire has been comprehensively investigated. The benefit of predictability of rocket‐triggered over tower‐initiated UL has led to various studies conducted in the US (e.g., Bagheri et al., 2019 ; Kereszy et al., 2022 ; Miki et al., 2005 ; Winn et al., 2021 ) and in China (e.g., Fan et al., 2021 ; Jiang et al., 2020 ; Li et al., 2021 ). The properties of tower‐initiated and rocket‐triggered UL have been found to be very similar.…”