Variations in the geomagnetic dipole moment over the last 12 000 years

An analysis has been made of archaeointensity data for the past 12 000 years. There are 3243 results from different areas of the world covering the past 12 000 years. Of these, 2203 are from the European region and 1040 are from the rest of the world. The archaeointensity data set analysed in the present study is almost three times larger than that used by McElhinny & Senanayake (1982 ). Although there is no major difference between our global data and the earlier data, the data for the non-European region have been improved and we now have a data set for Asia.     

Distribution analysis of errors due to relocation of geomagnetic data using the 'Conversion via Pole' (CVP) method: implications on archaeomagnetic data

Secular variation 'master' curves are built up using geomagnetic historical observations or archaeomagnetic data from a limited area and their use is usually restricted to regions of around 1000 km radius. Relocation of data within this distance is a common practice to enable comparison of data, although the errors due to such process are rarely taken into account. A detailed analysis of the distribution of relocating geomagnetic data has been done using three popular sets of geomagnetic models (IGRF-9, GUFM and CALS7K-2). This study improves the error analysis of relocating geomagnetic directions made up to date and expands it to geomagnetic intensities. Maximum errors correlate with the non-dipole to dipole field ratio. Archaeomagnetists could use this analysis to valuate the error introduced by reducing data.     

Tracking a non-dipole geomagnetic anomaly using new archaeointensity results from north-east China

A collection of ceramics and samples, collected from north-east China with ages ranging from 1000 to 7000 years, have been investigated using a modified version of the Shaw palaeointensity techniques (Shaw 1974; Rolph & Shaw 1985) in which only partial NRMs and TRMs (PNRMs and PTRMs) with blocking temperatures (T_b) above 300 C are used after pre-selection of samples by mineral magnetic analysis. A secular variation curve obtained from this study is quite consistent with previous results from other areas of China (Wei et al. 1987; Tang et al. 1991), as well as with the global model of McElhinny & Senanayake (1982). Comparison of the Chinese results with contemporaneous results from Greece (Aitken et al. 1989) has allowed us to track the movement of a large non-dipole anomaly as it drifts westwards.     

Centennial to millennial geomagnetic secular variation

A time-varying spherical harmonic model of the palaeomagnetic field for 0–7 ka is used to investigate large-scale global geomagnetic secular variation on centennial to millennial scales. We study dipole moment evolution over the past 7 kyr, and estimate its rate of change using the Gauss coefficients of degree 1 (dipole coefficients) from the CALS7K.2 field model and by two alternative methods that confirm the robustness of the predicted variations. All methods show substantial dipole moment variation on timescales ranging from centennial to millennial. The dipole moment from CALS7K.2 has the best resolution and is able to resolve the general decrease in dipole moment seen in historical observations since about 1830. The currently observed rate of dipole decay is underestimated by CALS7K.2, but is still not extraordinarily strong in comparison to the rates of change shown by the model over the whole 7 kyr interval. Truly continuous phases of dipole decrease or increase are decadal to centennial in length rather than longer-term features. The general large-scale secular variation shows substantial changes in power in higher spherical harmonic degrees on similar timescales to the dipole. Comparisons are made between statistical variations calculated directly from CALS7K.2 and longer-term palaeosecular variation models: CALS7K.2 has lower overall variance in the dipole and quadrupole terms, but exhibits an imbalance between dispersion in   g ¹₂  and   h ¹₂  , suggestive of long-term non-zonal structure in the secular variations.