Geomagnetic field Inclination Fig S1. Long-wavelength part of the geomagnetic field at Earth’s surface (International Geomagnetic Reference Model, IGRF 11, see notes on last slide). The arrows indicate the local magnetic North direction and intensity (proportional to the arrow length). The dip angle of the field lines (inclination) is represented by colours. The solid white line delineates the magnetic equator, where inclination is zero. The dashed white lines connects all points on Earth where the local magnetic North direction points to the geographic North pole (zero declination). The position of the south magnetic pole is marked by a white circle. The actual dip-poles (where the locally measured field is exactly vertical) do not exactly coincide with the magnetic poles from the geomagnetic reference model.

Dipolar part of the geomagnetic field Inclination Fig S2. Geomagnetic field with nondipole field mathematically removed. The geomagnetic poles (i.e. the magnetic poles of the dipole field) are anitpodal at (80.5°N, -72.2°W) and (80.5°S, 107.8°E). Since the dipole axis is tilted by 9.5° with respect to the rotational axis, there are only two longitudes (dashed white lines) at which geomagnetic North would correspond to geographic North. The geomagnetic equator (solid white line) is a great circle perpendicular to the dipole axis. Flip back to trace the differences between magnetic and geomagnetic equator.

Nondipolar part of the geomagnetic field Inclination Fig S3: Nondipole field, i.e. total field (Fig. S1) with the dipole part (Fig. S2) mathematically removed. The inclination (colors) does not represent the difference in dip angle between total field and dipole field, but shows the dip angle produced by the nondipole field in the absence of the dipole field. These inclination values help to pinpoint the source (dark violet and diverging arrows) and sink (red and converging arrows) regions of the nondipole field. The length scale for the arrows is the same as in Fig. S1 and S2.

Inclination anomalies due to nondipolar field Fig S4: Actual difference in inclination between total field (Fig. S1) and dipole field (Fig. S2). The pronounced negative inclination anomaly over the Equatorial Atlantic is responsible for the northward bending of the magnetic equator in this region (Compare solid white line in Fig. S1 and Fig. S2).

Geomagnetic field - total intensity F Fig S5: Isolines of total field intensity, contour spacing is 5 T ( 0.05 G in cgs units). Note the large-scale anomalies over Brazil (e.g., 23.0 mT at Sao Paolo) and Siberia (e.g., 61.6 mT at 65°N, 105°E). The total intensity is 57.0 mT at the north magnetic pole (located at 85°N, 133°W) and 66.8 mT at the south magnetic pole (64.5°, 137.5°E).

Intensity anomalies due to nondipolar field Fig S6: Difference between total magnetic field intensity F (Fig. S5) and dipole field intensity, in T. Blue and red contours enclose regions where nondipole field reinforces and diminishes, respectively, the dipole field. Note that major intensity anomalies (South Atlantic, East Asia, Australia) coincide with inclination anomalies (see Fig. S3, S4). Note the absence of an intensity anomaly over the Equatorial Atlantic, where the worldwide largest inclination anomaly occurs.

Geomagnetic field – declination D Fig S7: Lines of constant declination (deviation of local magnetic North from geographic North), contour spacing is 10°. At the magnetic poles (here south magnetic pole), declination is ill defined.

Dipolar part of geomagnetic field – declination D Fig S8: Lines of constant declination for the dipolar part of the geomagnetic field. Contour spacing is 5°. A compass needle would always point to the north geomagnetic pole, but due to the tilt of the dipole axis, the needle would indicate the geographic North direction only at the longitudes defined by the geomagnetic poles. The largest deviations from magnetic North and geographic North would therefore occur at high latitudes at longitudes midway between the geomagnetic pole longitudes. Declination for the dipole field is reflection symmetric about the geomagnetic equator.

Notes and Links Magnetic field elements shown here correspond to the 11th generation of the International Geomagnetic Reference Field, IGRF11 (core field, without permanent contributions from the crust) and were calculated for the year 2010 (Jan 1) from the Gauss coefficients available at http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html . Here, the Fourier series was truncated after order 8, but including higher orders (wavelengths  40° longitudes) does not alter the field distribution in any significant way, because the power in the nondipole field falls off with 1/order. A mesh size of 2.5° in latitude and longitude was used to minimize contouring artefacts and to provide smoothly varying pseudocolors. All graphs shown here were produced with MATLAB 7.6 (Release 2008), with the aid of the Mapping package M_Map downloaded from Rich Pawlowicz‘s webpage, http://www.eos.ubc.ca/~rich/ A worldwide chart of the heterogeneous crustal field with a resolution of 0.5° is available at: http://www.ngdc.noaa.gov/geomag/EMM/emm.shtml Note that the crustal field scale ranges from -200 nT to 200 nT, which is less than 1% of the main field variation at Earth’s surface. See also http://www.ngdc.noaa.gov/geomag/WMM/ for general background information about geomagnetic field models. See http://geomag.org/info/geomag_tutorials.html for info about time-varying contributions to the magnetic field due to ionospheric and magnetospheric currents systems. See http://jupiter.ethz.ch/~cfinlay/gufm1.html for secular variation of the main field and animations showing the temporal evolution of the historical field between 1590 and 1990.