Time series of the original horizontal external magnetic field components H(r, t) (blue) and approximated horizontal external magnetic field components ${\mathbf{H}}^{\mathrm{appr}}(\mathbf{r},t)$ (red) obtained using the SECS method and pre-identified principal components. Time series are demonstrated for the external magnetic field grid points closest to Nurmijärvi (NUR) and Abisko (ABK) geomagnetic observatories. The observatories are marked with red triangles in Figure B2. The original and approximated curves are nearly indistinguishable.