Search

Absolute past geomagnetic intensity values can mainly be recovered by fired archaeological materials and volcanic rocks. Here, we present 10 new archeointensities from the Mediterranean region that help to better constrain geomagnetic field intensity changes in Europe over the last two millennia. The new archeointensity results were obtained from the Thellier classical method including thermoremanent magnetization (pTRM) checks and both the TRM anisotropy and cooling rate corrections and were derived from at least three specimens. The new data presented, together with a selection of previous archeointensities satisfying a set of quality criteria, confirm the presence of several intensity maxima in Europe over the last 2000 years. In particular, the new archeointensities allow to better define the starting point of the double-oscillation feature that occurred in Europe during the second half of the first millennium CE, and reinforce the existence of a relative maximum at the end of the 14th century - beginning of the 15th century in Western Europe. From selected European archeointensities two new paleosecular variation curves are constructed for Western and Eastern Europe using temporal cubic b-splines in a bootstrap approach. The obtained curves suggest that the occurrence of the intensity maxima is characterized by a period of about 300 ± 50 years. In addition, our results suggest that the maxima do not occur simultaneously in Western and Eastern Europe, pointing out an intensity eastward drift with a mean lag-time of about 100 years. [Display omitted] • New 10 archeointensities for the Mediterranean region for the last two millennia. • Evolution of the double-oscillation in Europe (2nd half of 1st millennium CE). • The intensity maxima depicted seem to be compatible with an eastward drift. [ABSTRACT FROM AUTHOR]

Highlights • TRM anisotropy correction is an essential requirement in paleointensity studies. • Four new high-quality archeointensities are presented for NW Argentina. • The decrease of geomagnetic field intensity started in South America after 1600 CE. Abstract A good characterization of the geomagnetic field strength at centennial to millennial time scales in the Southern Hemisphere is particularly crucial to disentangle the long-term evolution of the South Atlantic Anomaly (SAA), an intriguing geomagnetic feature currently observed at the Earth's surface. Here we present 59 new archeointensities obtained from four well-dated groups of potteries with ages ranging between 1300 and 1500 CE and collected in Northwest Argentina. The new data were obtained in accordance with the Thellier paleointensity method including partial thermoremanent magnetization (pTRM) checks and TRM anisotropy and cooling rate corrections. We have also performed a comparative study of the efficiency of magnetic susceptibility, ARM and TRM anisotropy tensors to correct the TRM anisotropy effect upon intensity estimates. Our results suggest that the magnetic susceptibility tensor systematically underestimates the TRM anisotropy effect by 10–30%. Our new data, together with selected archeointensities already published, confirm that the decrease of the geomagnetic field intensity in South America started around 1600–1650 CE, due to the arrival of the SAA. [ABSTRACT FROM AUTHOR]

Aiming to understand the physical processes underneath the reversals events of geomagnetic field, different numerical models have been conceived. We considered here the so named “domino” model, an Ising–Heisenberg model of interacting magnetic macrospins aligned along a ring. This model was proposed by Mazaud and Laj (1989) and then applied by Mori et al. (2013) to study geomagnetic field reversals. The long series of the axial magnetic moment (dipolar moment or “magnetization”) generated by the “domino” model are empirically studied by varying all model parameters. We present here some results which are slightly different from those given by Mori et al. (2013), and will provide our explanation on the presence of these differences. We also define the set of parameters that supply the longest mean time between reversals. Using this set of parameters, a large number of time series of axial magnetic moment are also generated. After de-noising the fluctuation of these time series and averaging them, we compared the resulting averaged series with the series of axial dipolar magnetic moment values supplied by CALS7k.2, and CALS10k.1b models, finding similar behavior for the all time series. In a similar way, we also compared the averaged 14,000 years long series of dipolar moment with the dipolar magnetic moment obtained by the model SHA.DIF.14k. [ABSTRACT FROM AUTHOR]

The knowledge of the secular variation of the geomagnetic field at different time scales is important to determine the mechanisms that maintain the geomagnetic field and can help to establish constraints in dynamo theories. We have focused our study on the secular variation at millennial and centennial time scale searching for characteristic periods during the last 10 kyr. The frequency study was performed using four recent updated global paleomagnetic field reconstructions (SHA.DIF.14k, CALS10k.2, BIGMUDI4k and SHAWQ2k) by applying three techniques commonly used in signal analysis: the Fourier transform, the Empirical Mode Decomposition, and the wavelet analysis. Short-term variability of the geomagnetic field energy shows recurrent periods of around 2000, 1000–1400, and 600–800 and 250–400 years. The characteristic time around 600–800 years is well determined in all paleomagnetic reconstructions and it is mostly related to the axial dipole and axial octupole terms, but also observable in the equatorial dipole. In addition to this period, longer characteristic times of around 1000–1400 years are found particularly in the equatorial dipole and quadrupole terms in SHA.DIF.14k, CALS10k.2 and BIGMUDI4k while the 2000 year period is only well determined in the total geomagnetic field energy of SHA.DIF.14k and CALS10k.2. The most detailed paleoreconstructions for younger times also detect shortest characteristic times of around 250–400 years. The long-term variation of the geomagnetic energy is only observable in the axial dipole. A characteristic period of around 7000 years in both SHA.DIF.14k and CALS10k.2 has been found. This long period is related to two decays in the dipole field and a period of increasing intensity. The oldest decay took place between 7000 BCE and 4500 BCE and the present decay that started around 100 BCE. We have modeled the 4500 BCE up to present variation as a combination of a continuous decay, representing the diffusion term of the geomagnetic field, and one pulse that reinforces the strength of the field. Results show a characteristic diffusion time of around 11,000–15,000 years, which is compatible with the diffusion times of the dipole field used in geodynamo theories. Unlabelled Image • Frequency analyses on paleomagnetic reconstructions for the last 10 kyr have been performed. • Characteristic period of 7000, 2000, 1400–1000, 600–800 and 250–400 yr are found. • Long trend variation of the geomagnetic field is carried by the axial dipole. • The long trend term is modeled as a combination of diffusion and reinforcement (pulse) processes. [ABSTRACT FROM AUTHOR]