2025 THEMIS SCIENCE NUGGETS

Compression of Earth’s magnetopause down to 5 RE during the superstorm on 10 May 2024

Wending Fu and Huishan Fu
School of Space and Earth Sciences, Beihang University, Beijing, China
Key Laboratory of Space Environment Monitoring and Information Processing, Ministry of Industry and Information Technology, Beijing, China

Introduction

On 10 May 2024, a super space storm—characterized by the Dst index plummeting to -412 nT and induced by a strong coronal mass ejection on the Sun—attacked the Earth’s magnetosphere. This geomagnetic storm, according to the human record of Dst index, is the third-strongest one throughout history (only slightly lower than the Quebec event in 1989 and the Halloween storm in 2003). Under such an extreme condition, many satellites were directly exposed to solar ejections, leading to significant degradations in GPS and radio communications, and even caused a sharp orbital decline of the International Space Station. However, how the magnetopause evolves and reforms remains unclear, because only few spacecraft measurements were available in the dayside magnetosphere during previous events. Here, by utilizing THEMIS ground magnetometers array together with in-situ measurements of multiple spacecraft, we for the first time determine the magnetopause standoff distance during such superstorm.

Results

At 17:00 on May 10, 2024, the THEMIS B spacecraft, located upstream of Earth at [48.6, 35.5, 5.1] RE, observed a dramatic change in solar wind conditions (Figures 1a-1d). This unusually strong interplanetary ejecta quickly hit the Earth, triggering a series of severe and enduring geomagnetic storms. In the following hours, the Dst index plummeted to -412 nT (Figure 1e). Simultaneously, the Kp index rapidly reached its maximum of 9 (Figure 1f) and the daily resolution Ap index experienced a sharp increase, peaking at 274 (Figure 1g).

Figure 1. The solar wind conditions and geomagnetic indices during the superstorm.

During this historic superstorm, the THEMIS A, GOES-16 and -18 spacecraft were positioned in the dayside magnetosphere, providing in situ evidence that the magnetopause has been compressed below the geosynchronous orbit and possibly continued to move further inward. When the magnetopause was compressed to a position where the solar wind's dynamic pressure balanced with the magnetospheric magnetic pressure, the subsolar magnetopause terminated inward motion.

Figure 2. Measurements of magnetopause crossing by THEMIS A, GOES-16, and GOES-18 satellites.
(a), The spacecraft positions. (b, d, e), Bz measured by three satellites. (c), x-direction velocity. (f, g), Up-scale Bz and ion density measured by THEMIS A. (h, i), Energetic electron and low-energy electron flux.

At this point, magnetopause reconnections become effective (red cross in Figure 3a), which will open the outmost magnetic field lines of the magnetopause allowing intense disturbances from the solar wind and magnetosheath to enter the magnetosphere and propagate along these reconnected field lines down to ground-based observatories (disturbed black lines in Figure 3a), and conversely, the absence of significant disturbances suggests that its closed magnetic field lines remain within the magnetopause (undisturbed black lines in Figure 3a). During this superstorm, eleven ground-based THEMIS magnetometers array are distributed at different magnetic latitudes across the northern hemisphere and facing the dayside magnetopause (Figure 3c). Among these magnetometers, the latest disturbed one, HOV (N64.0°), corresponding to the L-value in the IGRF model was 5.20, while the outmost undisturbed magnetometer, GAKO (N63.8°), corresponding to the L-value was 5.12. Thus, we can determine that the magnetopause was compressed down to 5.12-5.20 RE during the super geomagnetic storm.

Figure 3. Locating the actual standoff distance of subsolar magnetopause.
(a), A schematic cartoon illustrates the magnetospheric magnetic field lines. The red cross indicates magnetopause reconnections. The magnetic field lines opened by reconnection connect with the interplanetary magnetic field. Strong turbulence in the solar wind and magnetosheath disturbs these magnetic field lines. The last undisturbed magnetic field line marks the position of the magnetopause. (b), Magnetic field lines can map the turbulence in solar wind to geomagnetic observatories on the ground. Eleven THEMIS ground-based magnetometers array on dayside during this superstorm. (c), Magnetic field strength measured by eleven geomagnetic observatories. Observatories that detected significant disturbances are colored in a blue-white gradient, while those without notable disturbances are colored in a gray-white gradient.

Conclusion

Due to the powerful solar eruption, the magnetosphere exhibited significant deformation on 10 May 2024. Space-based observations revealed that the magnetopause was compressed inside the geostationary orbit. The ground-based magnetometer observations indicated the magnetopause located between 5.12 and 5.2 RE. Such standoff distance of the magnetopause is also consistent with the prediction of empirical model, which is 5.15 RE.

Biographical Note

Wending Fu is a PhD student in the School of Space and Earth Sciences, Beihang University. His research focuses on the storm-time magnetospheric dynamics. Huishan Fu is a professor in the School of Space and Earth Sciences, Beihang University. His research focuses on the magnetic reconnection, radiation belts and wave-particle interaction, plasmaspheric dynamics, and solar wind-magnetosphere interaction.

References

Fu, W. D., Fu, H. S., Zhang, W. Z., Yu, Y., and Cao, J. B. (2025). Compression of Earth’s Magnetopause Down to 5 RE During the Superstorm on 10 May 2024. Geophysical Research Letters, 52(5), e2024GL114040. https://doi.org/10.1029/2024GL114040

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Emmanuel Masongsong / emasongsong @ igpp.ucla.edu