Speaker
Description
Predicting the geomagnetic effects of coronal mass ejections (CMEs) remains a major challenge in space weather forecasting. Spacecraft positioned upstream of L1, referred to as sub-L1 monitors, offer a promising observational approach to enhance both the lead time and accuracy of geomagnetic storm predictions.
Between November 2022 and June 2024, STEREO-A passed approximately 0.05 AU ahead of L1 at ±15° heliospheric longitude, which is within the range considered feasible for sub-L1 monitoring. During this period, 14 severe (Dst < -100 nT), one intense (Dst < -200 nT), and one superstorm (Dst < -250 nT) were recorded, the latter being the 2024 Mother’s Day storm. This event marked the first superstorm ever observed simultaneously from both L1 and a sub-L1 vantage point, offering the first concrete demonstration of how upstream monitors could enhance forecasting capabilities for extreme space weather events.
Including the May 2024 event, we identified 39 CME events observed by both STEREO-A and L1 during this period, although not all resulted in geomagnetic activity. We apply solar wind–to–Dst models to both STEREO-A and L1 measurements and conduct a two-pronged statistical analysis. The first analysis adopts a CME-based perspective, evaluating all CMEs observed by both spacecraft - regardless of geomagnetic impact - to investigate how longitudinal separation affects forecast accuracy. To quantify the overall forecast skill, we align STEREO-A measurements with corresponding L1 arrival times and compute standard error metrics and correlation coefficients between modelled and observed geomagnetic indices.
The second analysis takes an impact-based perspective, focusing on the sixteen geomagnetic storms (Dst < –100 nT) recorded during the period. For each event, we compare modelled and observed storm onset, peak intensity, timing, and duration.
Together, these complementary analyses provide a robust assessment of sub-L1 monitoring performance and offer a critical benchmark for the design and implementation of future upstream missions to enhance space weather forecasting capabilities.
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