The direction of the geomagnetic field in Britain is now moderately well established for the last 2, 000 year based on analyses of the directions of magnetic remanence isolated at some 466 archaeological sites in Britain and parts of N.E. France. The vast majority of these observations are of fired, in situ archaeological materials, with only 21 site observations being based on sediments. Most of these findings are only available in virtually inaccessible field reports and theses, or similar such publications, so the summary mean site British values have been placed into a database; this has then been extended to include may directional observations on a global basis. These data are now available as Dbase and Access databases, as well as in ASCII form. It is hoped that such data will enable more detailed secular variation studies to be undertaken and encourage other workers to add their observations to this database, particularly those that may otherwise be observed scatter with that expected from archaeomagnetic, geomagnetic and archaeological site sources suggest that the precision of the mean direction is only a partial estimate of the total error, evaluated as being of the order of ±5° for any given site that may have taken place after the last heating. There is also clear evidence for invalid age assignments in some of the published data but increasing archaeomagnetic data are now enabling such errors to be re-evaluated and the technique is thus improving as more data accumulate.

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Archaeomagnetic field intensity in Finland (Lat. 61°N, Long. 22°E) has been determined for the past 6400 years with the Thellier technique of bricks, potsherds and baked clays. The normalized archaeointensity curve for Finland, based on 23 mean values, shows an increase from ∼4360 BC to the maximum at AD 500, after which it decreases to the present value. The high intensity at AD 500 as found in Finland, but not in Bulgaria and Japan, is not a consequence of the applied Thellier technique since we are able to reproduce the known field intensities in the laboratory, and some of the dated bricks yield values which are in broad agreement with Nurmijärvi Magnetic Observatory data. We also show that variations in grain size of the magnetic carriers, cooling rate or magnetic refraction are unlikely to cause systematic errors in intensity larger than a few percent. Based on results of self-made bricks we show that the archaeointensity, however, depends on the angle between the laboratory and ancient field directions. If these are not aligned errors up to ±10% in archaeointensity can occur and are probably caused by anisotropy of remanence or by other factors not yet understood. The high intensity at AD 500 in Finland can be modelled by a radial nondipole field source at the core-mantle boundary producing enhanced field at higher latitudes (Finland) and relatively weaker fields at lower latitudes (e.g. Bulgaria and Japan). Extrapolation of the present field (IGRF 1990) back in time shows similar field behaviour.

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A total of 170 ceramic specimens from 16 archaeological sites representing 16 well determined ages from 4000 BC to 150 AD were investigated using the Thellier method. These paleointensities were compared with data from countries near Egypt after reduction to a common latitude to determine a secular variation curve of the geomagnetic field for the time period from 4000 BC to 150 AD. The field intensity increases from about 30 μT around 3500 BC to a maximum of 70 μT at about 400 BC when it starts to decrease until 150 AD. The Thellier double heating method was used together with the following additional test measurements: a) during the Thellier experiments the laboratory field F_Lab. pointed in the direction of stable NRM, b) magnetic susceptibility was measured after each double heating step to check for chemical alteration, c) a TRM check was made, by repeating the pTRM induction at a lower temperature, d) recent samples which were fired in a known field were used to test the reliability of our experimental setup and method. Based on rock magnetic measurements magnetite was identified as the predominant carrier of magnetization in the ceramics and bricks.

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Archaeointensity data have been obtained for the past 7, 500 years by analyzing ancient Chinese ceramics using a new version of the Shaw techniques (Shaw, 1974) and the Coe (1967) version of the Thellier techniques (Thellier and Thellier, 1959). Several rock magnetic parameters and measurements, such as Curie temperature, low temperature susceptibility, hysteresis and magnetic anisotropy, have been made on sub-samples. These measurements have enabled us to preselect the most suitable samples for archaeomagnitude analysis. There is acceptable agreement between the results obtained from separated areas of China. Major features of the geomagnetic field in China determined from this study are generally consistent with those of the global average field model of McElhinny and Senanayake (1982) but with detailed differences which we may be due either to local non-dipole field effects or lack of resolution in the global model. Comparison of the present results with contemporaneous results from Greece (Aitken et al., 1989) shows significant differences that may be due to the movement of a large non-dipole anomaly. Samples younger than 4, 000 years ago were usually dated by archaeological context and/or artefact type; radiocarbon dating was used to date the older samples.

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A record of palaeosecular variation has been obtained from a stalagmite from Vancouver Island, Canada, spanning the period 16.9 ± 6.0 to 13.3 ± 2.3 ka. Dating control was provided using the uranium-thorium disequilibrium method. The main magnetic remanence carrier in the speleothem is a fine-grained magnetite which is thought to be of detrital, rather than chemical, origin. The record is apparently free from any depositional inclination errors and compares well with a contemporaneous speleothem record obtained from the same cave (Latham et al., 1987). Good correlation also exists with several North American lacustrine records. The record provides evidence of clockwise looping of the position of the virtual geomagnetic pole over western Canada from about 14.9 to 13.6 ka, and in the interval from 16.9 to 15.2 ka the data can be modelled by a stationary, oscillating, inward-pointing source to the north-west of the sampling site.

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Two lava sections in southwestern Iceland were revisited; sectoin IG of Kristjánsson et al. (1988) of Brunhes chron age and section K of Wilson et al. (1972) of early Matuyama chron age. Paleodirections obtained from 22 laves at the former section do not seem to have averaged out the secular variation completely because of their small angular standard deviation, and this suggests rapid accumulation of the lavas. The latter section consists of 29 lavas and contains the Gauss to Matuyarna transition near its base. The time interval during which the laves at this section were erupted also seems to be short, judging from the small directional dispersion. Thelliers' paleointensity method was also applied to the samples from this section. A characteristic low paleointensity was obtained from the transitional lava flow. However, low paleointensities were also obtained from the lavas which erupted some time after the transition, and which record directions close to those of an axial geomagnetic dipole.

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Paleointensity determinations following the principle of the Konigsberger Thellier Thellier (KTT) method, but with measurement at high temperatures have been carried out. The high temperature measurements were made with a continuous thermal demagnetization system, which operates as a modification of a 2G horizontal access cryogenic magnetometer. The high temperature measurements permit comparison of the NRM lost and TRM acquired within temperature stages incrementally, rather than in the cumulative manner of the KTT method. When this approach was applied to a Hawaii 1960 lava erupted in a known geomagnetic field intensity, the NRM/TRM ratio for one specific temperature range of 400-450°C gave anomalously high values, while the expected NRM/TRM ratios were obtained from all other temperature ranges. This anomalous behavior is related to the destruction of a phase carrying NRM in this temperature range. The advavantage of making measurements at high temperature is that it permits independent determinations of the incremental ratio of NRM lost/TRM gained. This ability to isolate anomalous temperature ranges may permit reliable intensity determinations in materials, such as lunar rocks and meteorites, whose complicated behaviour on heating has precluded intensity determinations in the past.

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The present magnetic field is usually described by models such as the IGRF which are described by Gauss coefficients obtained by spherical harmonic analysis of the geomagnetic field. The use of Gauss coefficients in paleosecular variation (PSV) studies will therefore make it possible to compare field structures obtained through paleomagnetism with those observed directly. For making such comparisons, it is important to understand how the magnetic field maps into geomagnetic poles and what are their statistical properties, because geomagnetic poles are frequently used in PSV studies. We show how Gauss coefficients map into pole positions, and we develop statistical tools to treat the angular scatter of magnetic poles calculated from models based on Gauss coefficients. We use these methods to evaluate the models of Constable and Parker (1988) and McFadden et al. (1988) which are typical of PSV models using Gauss coefficients. Our results suggests the importance of the (l = 2, m = 1) harmonic in the dynamo process of the earth.

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There have been many models of the secular variation (SV) of the Earth's magnetic field proposed over the past few decades. Most if not all of them are models of Earth's magnetic field during so-called normal times, in other words, times during which the field is not reversing or undergoing an excursion. However, in studying data from Icelandic lava flows and from other areas, it has become obvious that there is no real distinction between so-called normal SV and the larger deviations of the Earth's field which are classified as excursions and/or reversals, except by the magnitude of the deviation, in which there is a continuous variation. Recently, reversal patterns have been studied in more detail, and it has been found that the exact start or end of a reversal is frequently only fuzzily defined, there being precursors to reversals, or events following reversals, which cannot be described either as belonging to the reversal per se or being a part of the normal SV pattern. In this paper, information from spherical harmonic models of the field during the past three centuries will be used to show that the field today has an abnormally large latitudinal variation of virtual geomagnetic pole (VGP) angular standard deviation, and should not be used to constrain SV models. Data for lava flows and other rock types from a number of different areas will be used to determine the scatter of the field at various observation latitudes, using VGPs from all latitudes, so that eventually a SV model may be produced which does not reject low latitude VGP information. Other factors will be described which any SV model should be able to produce.

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