The ROC Curve Examination on Traveling Ionospheric Disturbances in FORMOSAT‐7/COSMIC‐2 IVM Ion Density Triggered by the 15 January 2022 Tonga Volcanic Eruption.

The receiver operating characteristic (ROC) and the area under the curve (AUC), initially developed for signal processing and psychology, are a test for assessing the performance of a binary classification problem at varying threshold values. The ion density (Ni) observed by FORMOSAT‐7/COSMIC‐2 is used to study traveling ionospheric disturbances (TIDs) triggered by the 15 January 2022 Tonga volcanic eruption. We examine parameters of Ni, differential Ni, and standard deviation Ni (STD_Ni) in January 2022, simulate TID wavefronts traveling with various speeds from 10 to 1,000 m/s, and apply the ROC curve to globally identify the significance of TIDs in STD_Ni triggered by the volcanic eruption. ROC and AUC results show that in addition to TIDs related to tsunami/tropospheric Lamb waves and a series of fast‐moving waves with propagation speeds of 180–350 and 450–600 m/s, respectively, those long‐lasting low‐speeds less than 70 m/s and high‐speeds about 690, 860, and 990 m/s meet 95% statistical significance, which confirms TIDs being detected. These show that the FORMOSAT‐7/COSMIC‐2 ion density can be used to globally detect various TIDs triggered by the Tonga volcanic eruption. The ROC test results also show a potential use case for detecting other geophysical signals in future applications. Plain Language Summary: Large‐scale natural phenomena such as volcanoes and tsunami induce prominent traveling atmospheric disturbances (TADs) which also simultaneously and locally influence the above ionosphere activating prominent traveling ionospheric disturbances (TIDs). To assess the significance of such TIDs, we treat the TIDs as a series of signals, and the receiver operating characteristic (ROC) and the area under the curve (AUC), developed to address binary classification problems, are used to evaluate these signals at varying threshold values. The 15 January 2022 Tonga volcanic eruption triggers a series of TADs, TIDs, and tsunami waves propagating around the globe. Measurements of the ion density (Ni) by IVM (ion velocity meter) onboard FORMOSAT‐7/COSMIC‐2 (F7C2) are used to identify TIDs triggered by the volcanic eruption. We examine TIDs of F7C2 Ni, differential Ni, and standard deviation (STD) of Ni in January 2022, and find that TIDs associated with the tsunami waves, tropospheric Lamb waves, and high‐speed waves within 450–600 m/s triggered by the Tonga volcanic eruption on the globe are very prominent on the eruption day. In addition, TIDs with long‐lasting low‐speeds less than 70 m/s and high‐speeds about 690, 860, and 990 m/s can be identified in the ROC curves and AUC indices. Key Points: The FORMOSAT‐7/COSMIC‐2 ion density globally observes various traveling ionospheric disturbances triggered by the Tonga volcanic eruptionThe receiver operating characteristic (ROC) test can detect traveling ionospheric disturbances and potentially other geophysical signalsDisturbances with low‐speeds of 10–70 m/s, tsunami, tropospheric Lamb wave, and high‐speeds of 490/690/860/990 m/s are identified [ABSTRACT FROM AUTHOR]