Abstract
Geomagnetic substorms transfer solar wind energy into the planetary magnetosphere and ionosphere, producing auroral displays and ground magnetic disturbances, particularly intense during the expansion phase. Despite decades of study, the mechanisms governing the expansion phase remain unresolved. Based on coordinated observations of storm-time intense substorms, we reveal that substorm expansion is temporally embedded within a global cycle of field-aligned currents and auroral electrojets, coupled to large-scale plasma convection. The cycle manifests as a coherent movement of current peaks across magnetic longitude and latitude—first antisunward and equatorward, then sunward and poleward—and coincides with enhanced sunward ionospheric convection. This cycle involves two components of the auroral electrojets: the convection-driven DP-2 current and the expansion-phase DP-1 substorm current. The antisunward-equatorward phase, corresponding to intervals of dominant dayside reconnection, begins with DP-2 and can stepwise transition into DP-1. During the subsequent sunward-poleward phase, reflecting intervals of dominant nightside reconnection, DP-1 either persists from the earlier interval or develops within this phase. These observations show that expansion onset can occur under dominance of either dayside or nightside reconnection, while the full development of DP-1 generally involves nightside reconnection, providing insight into substorm evolution.
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Data availability
Source data are provided with this paper. OMNI and THEMIS data are available at NASA’s Coordinated Data Analysis Web (CDAWeb, https://spdf.gsfc.nasa.gov/pub/data/). The AMPERE field-aligned current is available on https://ampere.jhuapl.edu/browse/. The relevant geomagnetic indices of SuperMAG come from https://supermag.jhuapl.edu/indices/. SuperDARN data can be accessed at https://doi.org/10.20383/102.0447. Source Data are provided with this paper and available at 10.5281/zenodo.18493818.
Code availability
The radar software toolkit (RST) used to produce SuperDARN convection maps is available at https://doi.org/10.5281/zenodo.7467337, and reference therein. IDL SPEDAS used for analyzing data are freely available at https://themis.igpp.ucla.edu/software.shtml. IDL and Python code used for this study are available at 10.5281/zenodo.17776738.
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Acknowledgements
L.D is jointly supported by NSFC grants (42425404) and the National Key R&D Program of China 2025YFF0512100, NSFC grants (42527802,42188101), the Specialized Research Fund for State Key Laboratories of China, and the Strategic Pioneer Program on Space Science II, Chinese Academy of Sciences, grants XDA15350201, XDA15052500. M.H.Z. is supported by NSFC grants 42404178. We acknowledge the use of SuperDARN data. SuperDARN is a network of radars funded by national scientific funding agencies of Australia, Canada, China, France, Italy, Japan, Norway, South Africa, the United Kingdom, and the United States of America. We thank SuperMAG for providing geomagnetic station data and derived geomagnetic indices. We thank the AMPERE team and the AMPERE Science Data Center for providing data products derived from the Iridium Communications constellation. Thanks to the THEMIS mission for providing solar wind data in this event, the Kyoto World Geomagnetic Data Center for SYM-H data.
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L.D. conceptualized the study, analyzed and interpreted data, and wrote the manuscript. T.H.W. analyzed and interpreted data and contributed to manuscript writing. Y.R. collected and processed observational data and contributed to data interpretation. M.H.Z. and J.J.L. processed figure data. W.G., S.W., and C.P.E. contributed to data interpretation. C.W., X.W., and K.L.W. contributed to manuscript revision. All authors reviewed and approved the manuscript.
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Wang, T., Dai, L., Escoubet, C.P. et al. Substorm expansion embedded in a global cycle of field-aligned currents and auroral electrojets. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69753-x
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DOI: https://doi.org/10.1038/s41467-026-69753-x
