Journal Article

First Observations of a Geomagnetic Superstorm With a Sub‐L1 Monitor

Bauer, M, Rüdisser, H. T, Möstl, C, Weiler, E, Lugaz, N, Amerstorfer, T, Veronig, A. M, Majumdar, S, Reiss, M, Amerstorfer, U. V, Haberle, V, Le Louëdec, J, Davies, E. E

2025

Overview

Forecasting the geomagnetic effects of solar coronal mass ejections (CMEs) is currently an unsolved problem. CMEs, responsible for the largest values of the north‐south component of the interplanetary magnetic field, are the key driver of intense and extreme geomagnetic activity. Observations of southward interplanetary magnetic fields are currently only accessible directly through in situ measurements by spacecraft in the solar wind. On 10–12 May 2024, the strongest geomagnetic storm since 2003 took place, caused by five interacting CMEs. We clarify the relationship between the CMEs, their solar source regions, and the resulting signatures at the Sun–Earth L1 point observed by the ACE spacecraft at 1.00 AU. The STEREO‐A spacecraft was situated at 0.956 AU and 12.6° ^{\\circ}$ west of Earth during the event, serving as a fortuitous sub‐L1 monitor providing interplanetary magnetic field measurements of the solar wind. We demonstrate an extension of the prediction lead time, as the shock was observed 2.57 hr earlier at STEREO‐A than at L1, consistent with the measured shock speed at L1, 710 kms−1 $\\,{\\mathrm{s}}^{-1}$, and the radial distance of 0.043 AU. By deriving the geomagnetic indices based on the STEREO‐A beacon data, we show that the strength of the geomagnetic storm would have been decently forecasted, with the modeled minimum SYM‐H=−478.5 $\\,=-\\,478.5$ nT, underestimating the observed minimum by only 8%. Our study sets an unprecedented benchmark for future mission design using upstream monitoring for space weather prediction.

• Publisher: John Wiley & Sons, Inc
• Subject: Charged particles / Coronal mass ejection / Geomagnetic activity / Geomagnetic effects / Geomagnetic storms / Geomagnetism / In situ measurement / Interplanetary magnetic field / Lead time / Magnetic effects / Magnetic fields / Magnetic storms / Mission planning / Solar corona / Solar effects / Solar magnetic field / Solar wind / Space weather / Spacecraft / Weather forecasting