Fig. 4

Using Swarm residuals to calculate real-time magnetic field at the altitude of migrating animal. We take the measured field at the satellite height (Z₁) and subtract modelled contributions of the core, crust and magnetosphere fields at this same height (orange/brown), to obtain the real-time solar-wind induced variability, represented as residuals (yellow). This variability varies at a much higher temporal scale than the modelled contributions and can only be measured in situ. We then obtain modelled field values at the elevation Z₂ of the migrating animal (values in blue/teal) and add residuals from height Z₁, which gives us real-time field values at height Z₂. All modelled values are from the CHAOS-7 model [50, 51]. Charts are not to scale: the contribution of the core field typically represents over 98% of the total field