Florian Lhuillier

More stable yet bimodal geodynamo during the Cretaceous superchron

We report palaeomagnetic and Ar-40/Ar-39 dating results from two sequences of basaltic lava flows deposited at the same locality in western China, yet separated in time by similar to 50 Myr: one set lies within the Cretaceous normal superchron at 112-115 Ma and a second at 59-70 Ma spanning the Cretaceous-Palaeogene boundary. We find that magnetic field directions during the superchron exhibit bimodal populations: one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex, multipolar field. However, the time-dependent variability in field directions was 50% lower during the superchron than after, which implies greater field stability during the superchron. Our results suggest that episodes of less dipolar field behavior occurred within the Cretaceous superchron and raise the question whether a second, more multipolar, field state is more persistent than previously thought.

Intermittence and peculiarities of a statistic characteristic of the geomagnetic field in geodynamo models

The analysis of the statistical characteristics of the geomagnetic field generated in the numerical geodynamo models has shown that the distribution of the spherical harmonic coefficients in some cases is not Gaussian but, instead, has much in common with the Laplace distribution. The shape of the corresponding histograms depends on the time scale, which allows interpreting the obtained data in terms of a mixture of Gaussian distributions. The similar effects associated with the intermittence were observed in the experiments in a turbulent fluid flow. Hence, the behavior of secular variations in the magnetic field of the Earth should perhaps be described in terms of a mixture of several Gaussian stationary processes corresponding to switching between the different regimes of geodynamo generation.