X‐ray Signatures of Lightning‐Induced Electron Precipitation

Abstract

We present detailed numerical modeling to predict expected X-ray signatures from lightning-induced electron precipitation (LEP) events. Simulations include the transionospheric propagation of the lightning electromagnetic pulse; ray tracing through the magnetosphere, including Landau damping; wave-particle interactions resulting in loss cone fluxes; and Monte Carlo simulation of the electron precipitation process, including bremsstrahlung photon production and propagation to a balloon observing platform. Modeling is included to show that our simulated events are consistent with subionospheric very low frequency observations of LEP. Simulations show that these events should be observable from balloon-based platforms. The modeled LEP event signatures, including X-ray flux, spectrum, and pulse shape, are then compared with observations of energetic X-ray pulses from the Balloon Array for Radiation-belt Relativistic Electron Losses 1U balloon taken in January 2013 and are shown to be highly consistent with the observations. The pulses were observed near L = 4, while the balloon was off the coast of Antarctica at about 70 • S, 5-20 • W. Correlations with GOES imagery and global lightning data, together with analysis of the temporal and spectral signatures of these pulses, led to the initial suggestion that these events could be signatures of LEP. However, for only a fraction of these events could a causative lightning discharge be identified in global lightning data sets. This poor correlation may be partially attributed to causative strokes being missed by the global lightning networks; nonetheless, we discuss other possible explanations for these events.

Chemical Response of the Upper Atmosphere Due to Lightning‐Induced Electron Precipitation

“…Note that precipitation fluxes are strongly dependent on the assumption of the background fluxes and pitch angle distributions; the most important part of pitch angle distribution is the region near the loss cone angle; the uncertainty brought by the assumption of pitch angle distribution has been previously discussed in Marshall, Xu, Sousa, et al. (2019); our results were validated using Van Allen Probes data, as reported in Marshall, Xu, Sousa, et al. (2019).…”

“…The details of this code, as well as the most recent updates, can be found in Sousa (2018). In short, a standard WIPP-LEP simulation includes four steps (Bortnik, 2004;Marshall, Xu, Sousa, et al, 2019;Sousa, 2018): (a) The electromagnetic pulse (EMP) energy emitted by the return stroke current of a lightning discharge is calculated and mapped to the base of the ionosphere at 100 km altitude, (b) We calculate the attenuation of lightning-emitted VLF waves during their propagation through the lossy ionosphere (100-1,000 km altitude) using the VLF attenuation curves Helliwell, 1965), (c) Starting from 1,000 km altitude, we propagate each frequency component in the XU ET AL.…”

“…However, there exist large variations in the precipitation flux, ionization production, and occurrence rate of LEP events depending on the peak current of the parent lightning discharge, as well as the season, location, and intensity of thunderstorm activity (Sousa, 2018). For example, Figure 1 shows the comparison of ionization production between the mean LEP ionization production used by Rodger et al (2005); Rodger et al (2007), and our more recent LEP modeling results (Marshall, Xu, Sousa, et al, 2019 (Marshall, Xu, Sousa, et al, 2019) using the wave-induced particle precipitation code (Bortnik, 2004;Lauben et al, 1999;Sousa, 2018) and the mean LEP ionization production (Rodger et al, 2005(Rodger et al, , 2007. The colored lines show the altitude profile of ionization production at L = 2 by a lightning source with a peak current of 100 kA at 35 magnetic latitude.…”

“…This code explicitly simulates from first principles the entire LEP process from the source lightning discharge to precipitation fluxes in the upper atmosphere. This model framework has been extensively used to analyze LEP-associated VLF measurements (e.g., Inan et al, 2010;, and more recently calibrated using X-ray measurements by the Balloon Array for Radiation-belt Relativistic Electron Losses (BARREL) during possible LEP events (Marshall, Xu, Sousa, et al, 2019). In the first 5 s, the mean LEP ionization production used by Rodger et al (2007) (possibly corresponding to a peak current smaller than 100 kA) is on average one order of magnitude lower than that produced by this simulated 100-kA lightning discharge (Marshall, Xu, Sousa, et al, 2019).…”

A Generalized Method for Calculating Atmospheric Ionization by Energetic Electron Precipitation

“…As a built‐in feature, this model outputs height‐resolved energy deposition, and ionization production is then derived from energy deposition by assuming that an average energy of ∼35 eV is needed to produce an ion‐electron pair (Rees, 1989, p. 40). We emphasize that this model has been validated in the past few decades in a variety of studies, including gamma‐ray emissions produced by lightning discharge (Lehtinen et al, 1999), interaction of a beam of relativistic electrons with the atmosphere (Marshall et al, 2014), bremsstrahlung effects in EEP (Xu et al, 2018; Xu & Marshall, 2019), and lightning‐induced electron precipitation (Marshall et al, 2019).…”