Abstract

The analysis of intense geomagnetic storms (Dst ≤ −100 nT) from 1996 to 2024 has identified two new storm types, IB and IIB, which start their main phase with a Dst index below 0 nT but show distinct evolutionary patterns. Over this period, 13.43% of the studied storms were type IIB and 6.72% were type IB.
Among 23 intense storms examined, 82.6% were driven by interplanetary coronal mass ejections (ICMEs), including 43.47% magnetic clouds (MC), while 17.4% were associated with corotating interaction regions (CIR). Type IB storms were mainly linked to MC, Non-MC, and CIR structures. In contrast, type IIB storms exhibited a wider range of drivers, dominated by MC but also involving sheath regions, CIR, and complex combinations (e.g., Sheath + Non-MC or MC + Sheath).
Analysis of the solar wind structures for these 23 IB/IIB storms shows that frontal shocks (impact angle θXn < 45◦) are strongly associated with magnetic clouds. Of the 23 interplanetary shocks, 87% were frontal (vs. 13% inclined, θXn ≥ 45◦). Frontal shocks were predominantly fast (60% with Vs > 500 km/s) and linked to MC structures (45% of frontal cases). Inclined shocks displayed greater diversity (MC, Non-MC, CIR). Shocks in type IIB storms were more frequently frontal (93%) than in IB (78%). These findings confirm that fast frontal shocks, especially those associated with MC, are the most common drivers of intense type IIB storms.

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