A Global Analysis of Pre-Earthquake Ionospheric Anomalies

Local ionospheric density anomalies have been reported in the days prior to major earthquakes. This global study statistically investigates whether consistent ionospheric anomalies occur in the 24 hours prior to earthquakes across different regions, magnitudes, temporal and spatial scales. We match earthquake data to Total Electron Content (TEC) data from 2000-2020 at a higher resolution and cadence than previous assessed. Globally, no significant, consistent anomaly is found. Regionally, statistically significant ionospheric anomalies arise in the 12 hours prior to earthquakes with $p \leq 0.01$ following Wilcoxon tests. For the Japanese region we find a median negative ionospheric anomaly of around 0.5 TECU between 3 and 8 hours before earthquakes. For the South American region, the median TEC is enhanced by up to ~ 2 TECU, between 7 and 10 hours before an event. We show that the results are robust to different definitions of the ''local'' region and earthquake magnitude. This demonstrates the promise of monitoring the ionosphere as part of a multimodal earthquake forecasting system.