Characteristics of Potential Ionospheric Anomalies Prior to the M7.4 April 2, 2024 Taiwan Earthquake Identified With Multiple Observations

In this letter, we investigated the potential spatiotemporal characteristics of ionospheric anomalies before the Taiwan earthquake occurred on April 2, 2024, by utilizing the total electron content (TEC) and electron density (
${N}_{e}$
) observations. These anomalies of global ionospheric map (GIM)-TEC, global positioning system (GPS)-TEC, tomographic
${N}_{e}$
, and Swarm
${N}_{e}$
were identified with the GIM, the GPS, and Swarm satellites, respectively. After eliminating the influence of solar and geomagnetic activities, positive anomalies were exhibited by both GIM-TEC and GPS-TEC parameters on March 27 and 29, followed by negative anomalies from April 1 to 3, indicating a noticeable spatiotemporal overlap. The negative
${N}_{e}$
anomaly 2 h before the shock at the height of 450 km was consistently confirmed by tomographic
${N}_{e}$
and Swarm
${N}_{e}$
. The spatiotemporal features of increased intensity and expanded range of ionospheric anomalies on April 2 coincided precisely with the meta-instability phase before the mainshock.