Three largest geomagnetic superstorms by SYM-H index (since 1981). From top to bottom, panels are (a–c) geomagnetic symmetric ring current index SYM-H, (d–f) auroral electrojet index SME, and (g–i) westward auroral electrojet index SML for the March 1989 storm (left panels), the May 2024 storm (middle panels), and the November 2003 storm (right panels). The x-axis indicates times in units of days after the onset of the SYM-H peaks mentioned in the top panels. Horizontal dashed lines in the bottom panels indicate the SML = −2500 nT level, the threshold for an SSS (Tsurutani et al., 2015).