Crustal concealing of small-scale core-field secular variation

International audienceThe Earth’s magnetic field is mainly produced within the Earth’s liquid and electrically conductingcore, as a result of a process known as the geodynamo. Many other sources alsocontribute to the magnetic signal accessible to observation at the Earth’s surface, partly obscuringthe main core magnetic field signal. Thanks to a series of very successful satellites andto advances in magnetic field modelling techniques, considerable progress has, however, beenmade in the recent years toward better identifying the signal of each of these sources. In particular,temporal changes in the field of internal origin happen to be detectable now in sphericalharmonic degrees up to, perhaps, 16. All of these changes are usually attributed to changesin the core field itself, the secular variation, on the ground that the lithospheric magnetizationcannot produce such signals. It has, however, been pointed out, on empirical grounds, thattemporal changes in the field of internal origin produced by the induced part of the lithosphericmagnetization could dominate the core field signal beyond degree 22. This short note revisitsthis issue by taking advantage of our improved knowledge of the small-scale field changes andof the likely sources of the lithospheric field.We rely on a simple extrapolation of the observedspatial spectrum of the field changes beyond degree 16 and use a forward approach based on arecent geological model of lithospheric magnetization. This leads us to confirm that the maincause of the observed changes in the field of internal origin up to some critical degree, NC,is indeed likely to be the secular variation of the core field, but that the signal produced bythe time-varying lithospheric field is bound to dominate and conceal the time-varying coresignal beyond that critical degree, in very much the same way the permanent component of thelithospheric field dominates and conceals the permanent component of the core field beyonddegree 14. All uncertainties taken into account, we estimate NC to lie between 22 and 24.We, however, also note that in practice, the main limitation to the observation of the corefield small-scale secular variation is not so much its concealing by the field of lithosphericorigin but its fast changing nature and small magnitude. This leads us to conclude that whereascumulative small-scale lithospheric field changes might be detected some day, detection ofcore-field secular variation beyond degree 18 is likely to remain a severe challenge for sometim