A secular variation candidate model for IGRF-13 based on Swarm data and ensemble inverse geodynamo modelling

Earth, Planets and Space

Table 2 The forecast error is expressed as the root-mean-squared difference between the true geomagnetic field, defined by an update of the CHAOS-6 field model (Finlay et al. 2016), and the forecast at the terminal epoch

Forecast error (in nT) over recent periods for different forecasting strategies
	2005.0–2010.0	2009.0–2014.0	2015.0–2019.3
Nocast	400.2	439.4	383.0
Linear extrapolation	72.2	58.2	64.6
Coupled Earth, ensemble, QG-MAC	75.4	77.8	62.9
Steady flow, ensemble, QG-MAC	68.8	60.7	60.7
Steady flow, no ensemble, QG-MAC	71.7	63.3	61.4
Steady flow, ensemble, no QG-MAC	67.3	61.5	65.1
Steady flow, no ensemble, no QG-MAC	72.7	66.5	66.2

Italics values indicate the best strategy for each period
Nocast assumes that the field does not change. Linear extrapolation assumes a linear variation whose slope is specified by the secular variation up to degree 13 at the start of the forecast period, as specified by the initial field model we construct; “Coupled Earth, ensemble, QG-MAC” indicates that the full coupled Earth dynamo model is integrated for the 100 ensemble members, with the median defining the forecast, and an initialization where the QG-MAC constraint is mildly enforced. In the last four rows, “‘Steady flow” implies that the sole three-dimensional induction equation (with magnetic diffusion) is integrated, with or without an ensemble approach, and with or without the mild imposition of the QG-MAC constraint. If an ensemble approach is adopted, it is the median that defines the forecast