Centennial variations of the Earth’s magnetic field strength: from archaeomagnetism to core processes

Direct measurements of the geomagnetic field being only available over the historical period (from 1590 to today), global reconstructions beyond that time therefore resort to indirect measurements provided by paleo- and archeomagnetism. In this respect, archeomagnetism can provide particularly well dated data. This thesis aims at analyzing the geomagnetic field intensity variations provided by archeomagnetism over multi-decadal to centennial timescales, from two different but complementary aspects. A first study focuses on the acquisition of archeointensity data in central Asia and their consequences on the knowledge of regional and global geomagnetic field variations. In particular, global geomagnetic field models over the historical period based on direct mea-surements solely need additional constraints to overcome the absence of direct intensitymeasurements before ~1840. Two options have been proposed: either to linearly extrapolate backward the behavior of the axial dipole moment observed since 1840, as in the gufm1 model, or to rely on a global archeointensity dataset. In this study, a regional approach is used, based on new archeointensity data obtained from Bukhara for the historical period. This city is of particular interest owing to its outstanding, well-preserved historical center and the archives just as well preserved providing precise dating constraints on the buildings sampled for this study. The baked clay bricks fragments are analysed using the Triaxe experimental protocol. The obtained intensity variations curve shows a rapid decrease from 1600 to ~1750 followed by an increase until the early 19th. This evolution is in good agreement with other Triaxe data acquired in western Europe and western Russia. These three Triaxe datasets are used to recalibrate the axial dipole moment from the gufm1 model. The resulting evolution is non-linear, with a minimum amplitude during the second half of the 18th century. Although the results presented in this study need to be confirmed by further data acquisition worldwide, it nonetheless illustrates that archeointensity data can provide constraints on the geomagnetic intensity evolution over multi-decadal to centennial timescales at both regional and global scales.The second study focuses on intensity variations inferred from archeomagnetic data, from a theoretical standpoint. Recently, extreme archeointensity events lasting only a few decades, termed geomagnetic spikes, have been proposed in the Near-East during the first millennium BC. They are associated with variations rates up to several μT/yr, while today’s maximum is of order ~0.1 μT/yr. Magnetic flux expulsion at the core’s surface has been proposed to explain such extreme events, but this process has not yet been studied in detail. In this study, a 2D kinematic model of magnetic flux expulsion is implemented, controlled by a single parameter: the magnetic Reynolds number Rm, the ratio of magnetic diffusion to advection times. This model allows for the monitoring of initially horizontal magnetic field lines, advected by a fixed flow pattern constituted by two counter-rotating eddies. As the magnetic field lines are distorted and folded by the flow, the magnetic flux is progressively expelled towards the domain’s boundaries. If the boundary separates the conducting fluid from an insulating medium, the magnetic flux can diffuse through it. To follow the flux expulsion through the insulating boundary, the vertical component of the magnetic field is monitored during the system evolution. The characteristic rise time is found to scale as Rm 0.15, while the maximum instantaneous variation rate scales as Rm 0.45. These scaling laws are then extrapolated at the Earth’s surface. The results show that geomagnetic spikes cannot be generated by flux expulsion. However, other intensity peaks of durations longer than one century and associated with much lower variation rates would be compatible with flux expulsion events.