  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.
