Extreme Geomagnetic Events

Dear Colleagues,

Extreme geomagnetic field events are evident in numerous observations over various time scales. The nucleation of the inner core marked a dramatic change in convection style at Earth’s core, though detecting its signature on the paleomagnetic field is challenging. Most prominent extreme events are reversals and excursions, which are abrupt transitions of the dipole axis observed in paleomagnetic records that last several millennia and punctuate chrons that last several hundred thousand years. Extremely rapid field direction changes were reported during the last reversal. In archeomagnetic records, local short-lived spikes are enigmatic. Since the advent of direct intensity measurements, the geomagnetic dipole has been rapidly decreasing, provoking speculations of an imminent reversal. In parallel, a region of weak field intensity at the Earth’s surface, the South Atlantic Anomaly, has been expanding. Another intriguing phenomenon observed at the Earth’s surface is the recent acceleration of the North Magnetic Pole, which is rapidly moving poleward. The fastest field variability, termed geomagnetic jerks, correspond to local abrupt transitions in the trend of the secular variation (SV). The SV also affects ionosphere-thermosphere systems and the magnetosphere, which in turn affects surface phenomena such as the aurorae areas. The dipole decrease and the South Atlantic Anomaly expansion accentuate the impact of space weather events, increasing geomagnetic storms’ effectiveness and allowing energetic particles to penetrate deeper into the magnetosphere. These observations are challenging to explain in terms of their underlying core dynamics and the impact on magnetospheric conditions.

Recent years have seen remarkable progress in the monitoring of the geomagnetic field and its interpretation in terms of core dynamics on various timescales. Dedicated satellites (e.g., the Swarm constellation) provide unprecedented high-quality data with global coverage. New archeomagnetic and paleomagnetic measurements improve the dataset over millennial to hundreds of millennia timescales. Ensembles of field models reflect the uncertainty of the geomagnetic observations. Recent modelling of magnetic flux expulsion was invoked to explain the entire SV or archeomagnetic spikes. New approaches of core flow modelling account for subgrid and magnetic diffusion effects. In parallel, recent progress has been achieved in the modelling of the geodynamo action in the outer core by thermochemical convection. Current numerical dynamos reach unprecedented resolution and approach Earth-like force balance. These models successfully reproduce key aspects of core dynamics, most notably large-scale field and SV morphologies, Theoretical progress is facilitated in new data assimilation techniques, where core flow models are constrained with statistics from dynamo models. General circulation and magneto-hydrodynamic models have recently been used to assess the effects of SV and extreme geomagnetic field events over the magnetosphere and ionosphere-thermosphere systems.

Extreme events are particularly challenging to explain. Here we invite contributions from all research areas related to geomagnetism to advance the understanding of the underlying core dynamics of extreme geomagnetic events and the consequences of these events on processes outside the solid Earth. These include related geomagnetic observations on various timescales, the analysis of field models, numerical modelling, and theory aiming to explain these enigmatic phenomena. Core dynamics studies may include papers about magnetic induction and diffusion, dynamo action, waves in the core, core–mantle interactions, stratification, double-diffusive convection, etc. These contributions may provide insights into the impact of core dynamics on the extreme geomagnetic events from deep in the core, to the core–mantle boundary, through the Earth’s surface and up to the ionosphere.

Dr. Hagay Amit
Dr. Chris Davies
Dr. Ana Elias
Dr. Ingo Wardinski
Guest Editors

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• Geomagnetic field
• Secular variation
• Core dynamics
• Geodynamo
• Core–mantle boundary
• Inner core nucleation
• Reversals
• Dipole decay
• Jerks
• South Atlantic Anomaly
• Aurorae
• Geomagnetic storms
• Space weather