Space Weather Impacts of Long-Duration Southward IMF Bz Events During Solar Cycle 24

Abstract

Space weather disturbances during Solar Cycle 24 provide an interesting contradiction. The cycle itself was historically weak, marked by low sunspot activity and reduced CME speeds, yet it still produced a handful of intense geomagnetic storms. These storms occurred during long-lasting intervals of southward-pointing interplanetary magnetic field (IMF Bz), which enabled continuous solar wind–magnetosphere coupling. This study examines how such long-duration Bz-negative intervals shaped the space weather environment of Solar Cycle 24. Using OMNI solar wind data and geomagnetic indices including Dst, SYM-H, AE, and Kp, the analysis identifies all events where Bz remained below −5 nT for at least six hours.
The results show that storm intensity depended strongly on the duration of sustained southward Bz rather than on momentary peaks. When Bz stayed steadily negative, even moderate solar wind speeds triggered large geomagnetic responses. The St. Patrick’s Day 2015 storm stands out as the clearest example: a long-duration southward Bz interval drove the Dst index to −223 nT, despite the associated CME not being exceptionally fast. Similar behaviour appeared in several sheath-driven storms, where compressed southward fields persisted long enough to transfer large amounts of energy into the magnetosphere.
These findings highlight that long-duration southward Bz intervals are the most reliable predictors of severe space weather, especially during weak solar cycles. The study reinforces the idea that geomagnetic storms are driven by the integrated effect of sustained energy input rather than by isolated fluctuations in solar wind parameters. This understanding is essential for developing forecasting models that can accurately anticipate storm intensity. By focusing on Solar Cycle 24, the study provides a clear example of how duration, organization, and magnetic structure can outweigh raw solar activity levels in determining space weather outcomes.