Recent secular variation and an update to the World Magnetic Model

The secular variation (SV) and secular acceleration (SA) of the Earth’s geomagnetic field, generated in the outer core, poses a difficult challenge for field modellers. We must decipher the scope of spatial and temporal changes that are often poorly resolved and masked by variations of other field sources, and parameterise our models to represent the observed signals accordingly. In retrospect, we can accurately model variations with observations from the global network of ground observatories and satellites, but for practical purposes, field models are often used to predict the future state of the field. Models such as the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM) are produced on a quinquennial basis, and predict the core field for the subsequent 5 years. These models are widely used in academia, industry and by governmental and international organisations for purposes such as navigation. With our current incomplete understanding of the physics of the outer core and the generation of SV and SA, it is difficult to predict the time variations of the core field. Such model predictions are based on simplifying assumptions and can differ from the real field variations. In recent years, various SV and SA trends have been observed, captured in as much details as has ever been possible by the extensive observation network. With respect to the most recent (2015) releases of the IGRF and WMM, we present an analysis of field variations, such as reported jerks and accelerating core flows in the northern hemisphere, and their impact on such model predictions. In light of this, we also present an out-of-cycle 2018 update to the WMM2015, ahead of the next scheduled release in 2020, to better account for the field variations observed since 2015.