On the geographical distribution of induced time-varying crustal magnetic fields

A long standing question in geomagnetism is whether the time variation of the induced crustal field is a detectable quantity and, if so, at which spatial wavelengths. We tackle this problem with the help of a forward modeling approach using a vertically integrated susceptibility (VIS) grid of the Earth's crust. For spherical harmonic degrees 15-90, we estimate the root mean square of the crustal magnetic field secular variation to amount 0.06-0.12 nT/yr at the terrestrial surface between epochs 1960-2002.5. The geographical distribution of the signal shows absolute values reaching 0.65-1.30 nT/yr over South America. Unfortunately, most of the world magnetic observatories currently lie on quasistationary features where the crustal field signal variations are expected to be very low. However, this long sought signal could be detected over well chosen regions, provided that satellite, observatory, and repeat station measurements are available over several decades. Citation: Thebault, E., K. Hemant, G. Hulot, and N. Olsen (2009), On the geographical distribution of induced time-varying crustal magnetic fields, Geophys. Res. Lett., 36, L01307, doi:10.1029/2008GL036416.