Lightning-induced relativistic electron precipitation from the inner radiation belt

The Earth's radiation belts, consisting of inner and outer belts with varying energetic particle populations, are dynamic environments shaped by numerous acceleration, loss, and transport mechanisms. Microbursts, rapid bursts of energetic electrons precipitating into the atmosphere, are key loss mechanisms, predominantly observed in the outer belt and attributed to interactions with whistler mode chorus waves. However, microbursts also occur in the inner belt and slot region, driven by lightning-generated whistlers, a phenomenon known as LEP. While LEP has been observed at 10s-100s keV, this study provides direct evidence of MeV-energy LEP, highlighting the intricate link between terrestrial weather and space weather. The upper energy limit of electrons in the inner radiation belt is a subject of ongoing debate, with some studies showing minimal MeV electron flux during certain solar cycles. This study directly addresses this by presenting evidence of MeV electrons precipitating from this region, primarily influenced by lightning activity and subsequent geomagnetic activity.

Previous research has extensively documented microbursts in the outer radiation belt, primarily linked to wave-particle interactions with chorus waves. LEP at lower energies (10s-100s keV) has been observed in the inner belt and attributed to lightning-generated whistlers. However, direct in-situ measurements of MeV electron microbursts driven by lightning have been lacking. Indirect inferences of MeV electron LEP have been made through ground-based VLF receivers, but the ionospheric recombination time limits the temporal resolution of these measurements, obscuring the rapid evolution of the electron packets. Modeling and limited lower-energy in-situ measurements suggest that LEP events exhibit repeated periodic signatures resulting from the bouncing motion of electrons in the magnetosphere. The concept of "bouncing packets," where consistently spaced peaks of electrons are repeatedly observed with decaying amplitude, highlights the dynamic interaction between wave-particle interactions and magnetic field lines. This study directly addresses the absence of direct observation of MeV LEP events in the inner radiation belt.

This research utilized data from the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX) satellite's Heavy Ion Large Telescope (HILT) instrument. The study focused on a decade of SAMPEX data (1996-2006), specifically analyzing the 20 ms count rate data to identify microbursts. Two algorithms were employed: an adapted version of an existing algorithm to detect microbursts and a novel algorithm developed by the authors to identify bouncing packets. The adapted algorithm, originally designed to find microbursts, was modified to detect lower-amplitude events. The authors' algorithm identified and selected specific intervals, added buffers for searching and identified peaks (local maxima) with a minimum prominence to minimize false positives. The algorithm prioritized intervals exhibiting at least four consistently spaced peaks, imposing stringent criteria on event length and the number of unique count rate values to improve accuracy and prevent false positives. Data were filtered to select only events from the inner radiation belt (L-shell ≤ 3). Background trends were removed from each identified event to analyze the shape of the microburst. The identified events were then characterized by their L-shell, magnetic local time (MLT), number of peaks, minimum peak spacing, and characteristic electron energy. Geomagnetic storm activity was assessed using the Dst index, Kp index, and AE index. The study also incorporated data from the National Lightning Detection Network (NLDN) to correlate MeV microbursts with lightning activity, focusing on events over the contiguous United States to compare to events observed over Africa. Statistical analyses (two-population t-tests) were performed to determine the statistical significance of correlations between microburst events and geomagnetic activity and lightning strikes.

The study identified 45 bouncing microburst events at L-shells below 3. These events were predominantly observed on the nightside of the magnetosphere, consistent with the expected characteristics of LEP. The detected bouncing packets exhibited diverse shapes, including crown (increasing-then-decreasing), decreasing, increasing, and other profiles. The events were clustered around L = 2, lasted between 0.96 and 2.9 seconds, and the calculated characteristic electron energy ranged from 323 keV to 7.81 MeV. The study found that microburst events frequently occurred during periods of higher geomagnetic activity (lower Dst index). A significant correlation was established between lightning activity (as recorded by NLDN) and microbursts over the Americas, while this correlation was absent over Africa. Specifically, three out of seven events over the United States showed strong correlation with lightning strikes within a few seconds of the electron microburst onset. Analysis revealed a statistically significant increase in causative lightning for events observed over the Americas, further supporting the lightning-induced nature of the observed MeV microbursts. The temporal proximity of some events, suggesting correlated source events like lightning strikes within the same storm, further strengthens the correlation. The location of these events were largely clustered over the South Atlantic Anomaly due to the weaker magnetic field in that area. Finally, these MeV microburst events tended to occur shortly after periods of heightened geomagnetic activity when MeV electrons were present at low L-shells. This suggests geomagnetic storms contribute to a temporary source of MeV electrons in this region, making them susceptible to scattering by lightning-generated whistlers.

This research provides the first direct evidence of lightning-driven precipitation of MeV electrons, significantly advancing our understanding of LEP. The findings highlight the previously uncharacterized spatial variability of LEP microbursts, offering new insights into wave-particle interaction processes. The observed diverse shapes of the microbursts (crown, decreasing, increasing) may be attributed to the spatial extent and variability of precipitation patches, suggesting that the spacecraft's trajectory through the event influences the observed temporal profile. The relatively infrequent observation of MeV LEP events, despite the common occurrence of high-amperage lightning, could be explained by factors such as the rarity of events that fill the slot region with MeV electrons, the short duration of these precipitation events, and the spatial localization of the precipitation patches. The study's stringent criteria likely led to undercounting, indicating that further refinement of detection methods could increase the number of events identified. Future studies could utilize worldwide lightning networks and high-time resolution measurements from missions like IMPAX or RADICALS to obtain a more complete picture of the lightning-microburst relationship.

This study provides groundbreaking evidence of lightning-induced precipitation of MeV electrons from the inner radiation belt, significantly expanding our understanding of LEP. The findings reveal a previously unknown coupling between terrestrial weather and space weather, underscoring the complexity of near-Earth space dynamics. Future research should focus on improving detection efficiency, incorporating global lightning data, and conducting high-resolution measurements to fully characterize the relationship between lightning and inner radiation belt microbursts. This work holds implications for understanding the lifetimes and behavior of high-energy particles near Earth and for mitigating the risks these particles pose to space assets and human spaceflight.

The study's analysis of lightning correlation was limited by the NLDN's geographical coverage, primarily focusing on the contiguous United States and the Caribbean. The stringent criteria for microburst identification in the algorithms used likely resulted in some events being overlooked. Furthermore, variations in the shapes of the microburst events require further exploration to understand how the spacecraft's trajectory and the spatial characteristics of the precipitation patches interplay to create such diverse temporal profiles.