Influence of the South Atlantic Anomaly on Low-Earth Orbit Lightning Observations
Authors: Clark Austin , Fung Shing , Buzulukova Natasha , Lang Timothy , Kosar Burcu 
Published: March 2024

We present spatiotemporal patterns of interference in low-Earth orbit (LEO) lightning observations within the South Atlantic Anomaly (SAA) and compare them to the patterns obtained from modeled and in situ observations of particle energies and fluxes. In particular, the high sampling frequency of the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) allowed mapping of the SAA to reveal its seasonal and diurnal variability. Overall, lightning observation interference patterns in the SAA have been observed to vary with solar cycle, to lag in peak timing between solar cycle and maximum interference, and to drift longitudinally, similarly to the generally known behaviors of the SAA. However, the seasonality of the SAA from TRMM LIS differed from that of the higher altitude Optical Transient Detector (OTD) and previous studies, e.g., Schaefer et al. (2016), which have shown a bimodal seasonality compared to the more unimodal pattern from TRMM LIS. This is indicative of other potential influences on the interference patterns at the TRMM LIS altitude of 400km.The LIS and OTD were optical lightning instruments operating in low-Earth orbit from 1995-2023. These packages were designed to observe the near-infrared (777.4-nm) channel, which is emitted by lightning and scattered by the clouds to produce a detectable signal, with a charge-coupled device (CCD) array. Radiation in the low-Earth environment would also impact the sensors and lead to false detections. Much of this noise could be filtered out given the typically random spatiotemporal distribution of radiation, especially when compared to the more organized lightning signal. Recently, however, spatiotemporal patterns have been noted in some portions of these lightning records within the SAA. The trapped radiation in the SAA yields far more false detections than anywhere else for these instruments, and occasionally this increased volume of noise can overwhelm the sensors&#8217; processing capabilities and render it unable to intake new data, effectively blinding it. The blinded state is known as a First-In First-Out (FIFO) buffer overflow and is recognizable as a reduction in the time that the instrument was operating nominally over a given region. Although this effect also occurred with large and intense thunderstorm systems, it was an order of magnitude more common in the SAA, leading to reductions in nominal observation time of up to 80% (per-month for LIS; lifetime for OTD). Tracking the nominal observation time and the FIFO buffer flagging reported by the science data has allowed for the SAA patterns of interference to be extracted, as lightning itself is unaffected by the SAA. The differences in altitude and inclination angle between the OTD and the LIS on TRMM and the International Space Station (ISS) allow for interesting inter-instrument comparisons within these patterns as well, including coincidence with other instruments aboard the ISS during the ISS LIS lifetime.Schaefer, R.K., Paxton, L.J., Selby, C., Ogorzalek, B.S., Romeo, G., Wolven, B.C., and Hsieh, S.Y., 2016: Observation and modeling of the South Atlantic Anomaly in low Earth orbit using photometric instrument data. Space Weather, 14, 330&#8211;342. doi: 10.1002/2016SW001371.