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 (N_i) 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 N_i, differential N_i, and standard deviation N_i (STD_N_i) 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.