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Description
Intense geomagnetic storms are typically initiated by fast coronal mass ejections (CMEs) preceded by a sheath region of compressed solar wind. The arrival of the interplanetary shock at the magnetopause produces a sudden storm commencement (SSC), observed as an abrupt, step-like enhancement of the horizontal geomagnetic field. The rapid magnetic field variations associated with SSCs induce strong geoelectric fields at the Earth’s surface, which can pose significant risks to ground-based technological infrastructure. In this study, we compare geoelectric fields induced during two SSC events: (1) the SSC associated with the idealised perfect interplanetary CME simulated by Welling et al. (2021, doi:10.1029/2020SW002489) using the Space Weather Modeling Framework (SWMF), and (2) the observed SSC of the 29 October 2003 Halloween geomagnetic storm. This investigation supplements a recent study by Viljanen et al. (2026) which compared the simulated Carrington event without an SSC (Blake et al., 2021, doi:10.1029/2020SW002585) to the Halloween storm, and showed that a Carrington-like event can produce 4-10 times larger geoelectric fields.
For the idealized SSC event, the SWMF provides the external 1-s ground magnetic field due to ionospheric, magnetospheric, and field-aligned currents. Using a three-dimensional conductivity model of Fennoscandia, we derive the internal part of the magnetic field produced by telluric currents, thus, obtaining the total field variation, and the geoelectric field at the Earth’s surface. For the 29 October 2003 Halloween storm SSC, ground magnetic field measurements from the stations of the International Monitor for Auroral Geomagnetic Effects (IMAGE) network are employed to calculate geoelectric fields within the same conductivity model. We are especially interested in the peak amplitudes of the time derivative of the ground magnetic field (dB/dt) and of the geoelectric field. From the 1-s recordings of the Halloween storm, we know that the total horizontal dB/dt reached values up to 180 nT/s, whereas the maximum external dB/dt by the SWMF simulation is about 500 nT/s. When the telluric contribution is added to this, the total dB/dt is probably significantly larger, making a perfect SSC many times stronger than the Halloween SSC. A similar feature is expected for the geoelectric field, meaning that an extreme SSC can produce very large geomagnetically induced currents in power grids.