Redefining the boundary between crystalline and sedimentary rock of Eastern Dahomey Basin

This study defines a new boundary between the crystalline and sedimentary rocks of Eastern Dahomey Basin at the southwestern part of Nigeria using a geophysical approach that combines regional aeromagnetic and ground resistivity data. Aeromagnetic data covering the entire Eastern Dahomey Basin were acquired at 500 m line spacing, 80 m mean terrain, and processed into grids of Residual Magnetic Intensity (RMI) map. Filters and corrections such as upward continuation, and reduction to equator were applied to enhance deep magnetic sources and correct for magnetic inclination and declination. Tilt Derivative Angles (TDR) was applied for edge detection. To support the aeromagnetic analysis and interpretation, 104 Vertical Electrical Sounding (VES) surveys and 8 Electrical Resistivity Tomography (ERT) data were also acquired, processed and interpreted along the basement-sedimentary rock boundary. The TDR revealed a significant trend that corresponds to the edge between the basement complex and the sediments of the Eastern Dahomey Basin. A strong match was also noticed between the VES positions and the TDR map. Areas interpreted as basement rocks from the VES stations align with positive values on the TDR maps while the sedimentary terrains have negative TDR values. Our work demonstrates that areas that were previously fixed as sedimentary terrains on geological maps belong to the crystalline basement or transition zone. A new and reliable geological boundary is hereby drawn between the basement and sedimentary rocks. Thus, providing a revised map of the Eastern Dahomey Basin.

Long-distance transequatorial navigation using sequential measurements of magnetic inclination angle

Diverse taxa use Earth’s magnetic field in combination with other sensory modalities to accomplish navigation tasks ranging from local homing to long-distance migration across continents and ocean basins. Several animals have the ability to use the inclination or tilt of magnetic field lines as a component of a magnetic compass sense that can be used to maintain migratory headings. In addition, a few animals are able to distinguish among different inclination angles and, in effect, exploit inclination as a surrogate for latitude. Little is known, however, about the role that magnetic inclination plays in guiding long-distance migrations. In this paper, we use an agent-based modelling approach to investigate whether an artificial agent can successfully execute a series of transequatorial migrations by using sequential measurements of magnetic inclination. The agent was tested with multiple navigation strategies in both present-day and reversed magnetic fields. The findings (i) demonstrate that sequential inclination measurements can enable migrations between the northern and southern hemispheres, and (ii) demonstrate that an inclination-based strategy can tolerate a reversed magnetic field, which could be useful in the development of autonomous engineered systems that must be robust to magnetic field changes. The findings also appear to be consistent with the results of some animal navigation experiments, although whether any animal exploits a strategy of using sequential measurements of inclination remains unknown.

GIUSEPPE FOLGHERAITER: THE ITALIAN PIONEER OF ARCHAEOMAGNETISM

ABSTRACT The history of the science of archaeomagnetism conventionally starts in 1600 with the publication of William Gilbert's monumental work De Magnete, but the theoretical basis of this scientific field has to be positioned at the end of the nineteenth century. In Italy at that time, a number of scientists such as Giambattista Beccaria, Macedonio Melloni and Silvestro Gherardi, were working on magnetic field characteristics and their work variously contributed to the early study of Earth and rock magnetism. A major contribution to the birth of paleomagnetism as a science, and archaeomagnetism as a dating technique, was produced by Giuseppe Folgheraiter (1856–1913) by means of his research on the magnetic properties of volcanic deposits and his attempts to date ancient pottery of different epochs based on the magnetic properties of clay materials. Initially, Folgheraiter studied the rock magnetism of the volcanic rocks of Latium where he replicated the findings of Macedonio Melloni, who had studied Vesuvius lavas, and found that volcanic rocks are affected by a permanent magnetization. In addition, Folgheraiter verified the discovery by Filippo Keller of the Punti distinti. Folgheraiter also made the innovative proposal that lightning strongly influences the magnetic properties of lavas resulting in magnetic disorder. The main analytical effort of Folgheraiter at the end of the nineteenth century was dedicated to the study of the variations of magnetic inclination in different epochs as registered in archaeological pottery. He produced archaeomagnetic sets of analyses on 191 samples grouped into 10 epochs, that resulted in the first reconstruction of a geomagnetic secular variation curve (SVC). Even if nowadays the Folgheraiter analytical results have been replaced by more precise measurements, a great portion of the development of modern archaeomagnetic techniques originated with Folgheraiter’s experiments and intuitions. Many of those advances were improved upon only during the first half of the twentieth century by Emile Thellier (1904–1987). Actually, the well-known work by Thellier, resulting in the birth of the Saint Maur archaeomagnetic laboratory at the Institut de Physique du Globe de Paris, had as a starting point the theories and suggestions developed by Giuseppe Folgheraiter. Based on the studies by Thellier, the well-known secular variation curve for France was derived, later to be perfected by Ileana Bucur in 1994.

Particle acceleration in neutron star ultra-strong electromagnetic fields

In this paper, we discuss the results of a new particle pusher in realistic ultra-strong electromagnetic fields such as those encountered around rotating neutron stars. After presenting the results of this algorithm in simple fields and comparing them to expected exact analytical solutions, we present new simulations for a rotating magnetic dipole in vacuum for a millisecond pulsar by using the Deutsch solution. Particles are injected within the magnetosphere, neglecting radiation reaction, interaction among them and their feedback on the fields. Our simulations are therefore not yet fully self-consistent because the Maxwell equations are not solved according to the current produced by these particles. The code highlights the symmetrical behaviour of particles of opposite charge to mass ratio, $q/m$ , with respect to the north and south hemispheres. The relativistic Lorentz factor $\gamma$ of the accelerated particles is proportional to this ratio $q/m$ : protons reach up to $\gamma _p \simeq 10^{10.7}$ , whereas electrons reach up to $\gamma _e \simeq 10^{14}$ . Our simulations show that particles could either be captured by the neutron star, trapped around it or ejected far from it, well outside the light cylinder. Actually, for a given charge to mass ratio, particles follow similar trajectories. These particle orbits show some depleted directions, especially at high magnetic inclination with respect to the rotation axis for positive charges and at low inclination for negative charges because of symmetry. Other directions are preferred and loaded with a high density of particles, some directions concentrating the highest or lowest acceleration efficiencies.