intensity of the geomagnetic field in western europe over the past 2000 years new data from ancient french pottery 1 we studied 14 groups of french pottery fragments dated between the 4th and 16th centuries the potsherds were analyzed using the thellier and thellier 1959 method revised by coe 1967 intensity values were corrected for thermoremanent magnetization trm anisotropy and cooling rate dependence of trm acquisition we first analyzed modern ceramics produced following ancient techniques and fired in a wood burning kiln inside of which field intensity was measured the recovered mean intensity is within 3 of the expected value which proves the reliability of our experimental procedure thermal experiments carried out at rapid and slow cooling rates clearly indicate that the cooling rate correction is critical in archeointensity studies our data indicate that large variations in intensity occurred in france over the last 2000 years two relative maxima in intensity are observed one between the 8th and 10th centuries and the second between the 14th and 15th centuries similarities are observed between the archeointensity data from france and ukraine yielding some evidence for eastward drift of geomagnetic sources between western and eastern europe from a d 800 to a d 1700 we also show that the dipole moment evolution proposed by mcelhinny and senanayake 1982 and yang et al 2000 for the last two millennia is likely biased toward higher values mainly because of the absence of correction for the cooling rate dependence of trm acquisition in most published archeointensity studies we finally underline a possible relationship valid at least in western europe between changes in direction and intensity of the geomagnetic field

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

validity of archaeomagnetic field recording an experimental pottery kiln at coppengrave germany palaeomagnetic data obtained from archaeological materials are used for reconstructions of the earth s magnetic field of the past millennia while many studies tested the reliability of this recorder for palaeointensity only a few studies did this for direction the study presents an archaeomagnetic and rock magnetic investigation applied to an experimental pottery kiln which was operated in 2003 to produce stone ware this kind of high quality pottery needs a temperature of at least 1160 c shortly before heating of the kiln direct absolute measurements of the absolute geomagnetic field vector have been carried out close to it after cooling of the kiln 24 oriented palaeomagnetic samples have been taken although curie temperatures are about 580 c that is the typical temperature for magnetite thermal as well as alternating field demagnetisations reveal also a considerable amount of hematite as magnetic carrier this mixture of magnetite and hematite is dominated by pseudo single domain grains demagnetisation removed in some cases weak secondary components but in most cases the specimens carried a single component thermoremanent magnetisation the mean characteristic remanent magnetisation direction agrees on 95 per cent confidence level with the directly measured field direction archaeointensity was obtained from five specimens with the thellier coe method and with the multiple specimen palaeointensity domain state corrected method six of these specimens also provided a result of the dekkers bohnel method which overestimated the archaeointensity by about 9 per cent compared to the direct value while after correction for fraction the value agrees very well for the multiple specimen palaeointensity domain state corrected method only fractions between 25 and 75 per cent have been used and specimens showing alteration have been excluded above 450 c many specimens showed alteration of the magnetic grains because median destructive temperatures were often above this value in most cases the fraction was less than 50 per cent nevertheless the obtained intensity 48 48 0 24 is on 95 per cent confidence level in agreement with the direct observation behaviour of the specimens during the thellier experiments was not ideal because of narrow unblocking temperature spectra and alteration nevertheless the obtained mean archaeointensity is also in agreement with the direct field observation here the relative palaeointensity error is about 6 per cent and very high compared the multiple specimen palaeointensity domain state corrected method the investigation demonstrates that a pottery kiln can provide a very precise estimate of the ancient geomagnetic field vector

paleointensity on volcanic glass of varying hydration states abstract we have characterised the magnetic properties of variably hydrated volcanic glasses that were collected from rhyolitic deposits at blahnukur torfajokull iceland the glasses span the range from fresh obsidians to highly fractured perlites that contain 2 wt water lava hydration plays a key role in the formation of perlite and hence these rocks are ideal to study hydration effects on remanence carriers and reliability of the paleomagnetic record the total volatile content of the different samples was determined as a proxy for the degree of perlitisation hydration it was found that coercivity of remanence saturation magnetisation and saturation of remanence decrease with increasing hydration i e that magnetic remanence carriers get lost and that magnetic stability is reduced additionally thermal demagnetisation of a three component isothermal remanence revealed that mainly the high coercive material is destroyed within the more strongly hydrated samples while lower coercive material seems to be less affected grain sizes of all but one samples are in the pseudo single domain range the one exception shows multi domain characteristics it was impossible to unambiguously identify the remanence carriers but titanomagnetites are most likely responsible for the lower coercivity component while hemoilmenites possibly represent the higher one a modified thellier method was used to determine paleointensity values as most of the samples are hydrated it is not astonishing that the overall paleointensity data is not of very high quality however it is important to note that there are hydrated samples with well defined arai diagrams although seemingly of high quality these paleointensity values are incorrect as there is a trend towards lower paleointensity values observed with increasing perlitisation we attempted to test for magnetic anisotropy and cooling rate dependency but this was hampered by alteration during the experiments however we argue that both anisotropy and cooling rate dependency are unlikely to be responsible for the observed trend in paleointensity thus even well defined paleointensity values can be erroneous when obtained from hydrated glass this emphasises the need for unaltered samples and additional attention during paleointensity determinations

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

paleointensity of the geomagnetic field recovered on archaeomagnetic sites from france abstract twenty new paleointensity determinations have been obtained from archaeomagnetic sites from france time intervals investigated range from 65 to 380 ad and from 1370 to 1700 ad two hundred forty two samples of tiles and bricks were treated with the classical thellier method for paleointensity determination rock magnetic studies identify magnetite substituted maghemite and hematite as the main magnetic minerals as is usual with archaeomagnetic samples especially tiles magnetic anisotropy was observed in all samples the effect of thermoremanent magnetization trm anisotropy upon the paleointensity values was corrected after the determination of the tensor of trm anisotropy in each sample these determinations allow corrections of the field strength obtained which are usually lower than 20 the effect of the cooling rate on the trm intensity acquired was also determined and seems relatively low on our samples most frequently eight to 10 samples per site were used to calculate the mean intensity standard deviations are typically lower than 7 of the average value during the roman period small variations of the field intensity in paris are observed between 62 and 70 t while during the more recent period the intensity of the field decreases from 62 to 52 t an analysis of the paleointensity data available for the last 2000 years obtained from sites in western europe was performed using a weighing factor which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination this weighing factor clearly shows that most of the existing data might not be reliable the analysis performed indicates that more data is needed especially from the time interval 500 to 1400 ad before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in western europe

influence of cooling rate on thermoremanence of magnetite grains identifying the role of different magnetic domain states it is widely accepted that cooling rate can strongly influence the intensity of the thermal remanent magnetization trm acquired by rocks during cooling to ambient temperatures if ignored this effect might lead to underestimates or overestimates of the ancient magnetic field intensity to date however the cooling rate dependence of trm acquired by particles with different domain states has never been systematically analyzed from the theoretical or experimental point of view in this study we present measurements of the trm of synthetic magnetites with well defined grain sizes that were quenched with constant cooling rates of 0 05 0 1 1 3 10 and 15 k min while single domain sd and small pseudo single domain psd samples are found to show larger trms after slow cooling the trms of larger psd and multidomain md magnetites are not affected by an increase or decrease of the cooling rate overall our results suggest that only smallest magnetite grains acquire a cooling rate dependent trm therefore cooling rate corrections of paleointensity determinations are only necessary for samples dominated by sd remanence carriers while rocks dominated by psd and md carriers such as basalts which are most commonly used for paleointensity studies do not require such corrections

the use of the anisotropy of magnetic remanence in the resolution of the anisotropy of magnetic susceptibility into its ferromagnetic and paramagnetic components abstract the anisotropy of magnetic susceptibility ams is often controlled by both ferromagnetic sensu lato and paramagnetic minerals the anisotropy of magnetic remanence amr is solely controlled by ferromagnetic minerals jelinek trav geophys 37 1993 introduced a tensor derived from the isothermal amr whose normalized form equals the normalized susceptibility tensor provided that the ferromagnetic fraction is represented by multi domain magnetite the present paper shows the close correlation between these tensors for a collection of strongly magnetic specimens containing multi domain magnetite in addition acceptable correlation between the tensors was also found for a collection of specimens containing single domain magnetite a new method is developed for the ams resolution into ferromagnetic and paramagnetic components using the amr some examples are presented of this resolution in mafic microgranular enclaves in granodiorite and in gneisses of the ktb borehole

thermal diffusivity of olivine group minerals at high temperature thermal diffusivity d data from 12 oriented single crystals and seven polycrystalline samples of olivine group minerals were acquired with the laser flash method at temperatures t of up to 1500 c samples included forsterite fe mg binary olivines sinhalite and chrysoberyl specimens were characterized using infrared spectroscopy and electron microprobe analysis crystal orientation and chemistry both affect d for our single crystals d 100 d 001 d 010 at all temperatures thermal diffusivity decreases with increasing t and becomes virtually constant at high temperatures at room temperature d 001 of pure forsterite has the highest observed values but substitution of a small amount of co in forsterite 0 3 wt coo lowers d by 20 substitution of 10 fe for mg in forsterite as in typical mantle olivine lowers d by 50 at room temperature mantle olivine has d 3 25 1 66 and 2 59 mm2 s for the 100 010 and 001 orientations respectively the values decrease to 0 93 0 87 mm2 s at 790 985 c for 100 0 54 51 mm2 s at 590 740 c for 010 and 0 83 0 79 mm2 s at 740 890 c for 001 two dunite samples have d of 0 55 0 56 mm2 s at 890 1080 c showing the effect of preferred orientation of grains dominated by 010 thermal diffusivity of polycrystalline samples is controlled by the large amounts of olivine present minor phases offset the curves for d t from the value of the olivine mineral our laser flash measurements isolate the phonon component of heat transfer from radiative transfer and show that the phonon contribution becomes nearly constant for the high temperatures expected in the mantle the other microscopic component diffusive radiative transfer depends strongly on temperature and this temperature dependence likely exerts greater control on mantle convection

multiple specimen absolute paleointensity determination an optimal protocol including ptrm normalization domain state correction and alteration test abstract a recent proposal of a multiple specimen technique promises to be a viable alternative to the classical thellier thellier method of absolute paleointensity determination however to exploit the full potential of the multiple specimen approach a thorough understanding of its theoretical foundation and a detailed experimental verification of its implicit assumptions is required here the validity of the multiple specimen technique is studied on a collection of synthetic samples covering grain sizes ranging from single domain sd over intermediate pseudo single domain psd to multidomain md the experimental data indicate that the multiple specimen method in its present form systematically overestimates paleointensity for intermediate psd to md particle sizes this finding is investigated theoretically by a statistical theory of weak field thermoremanence and quantified by a phenomenological thermoremanence model based on this theoretical framework and on the new experimental evidence an extended version of the multiple specimen technique is designed which is more reliable in the critical domain state range the new measurement scheme improves normalization and quantifies the psd and md overestimate which then can even be corrected for furthermore the proposed measurement scheme includes a thermal repeat measurement to assess the effect of alteration upon the accuracy of the final paleointensity result the new technique is verified experimentally for the synthetic samples investigated

paleointensity estimates from historic and modern hawaiian lava flows using glassy basalt as a primary source material abstract published paleointensity estimates derived from lavas extruded in known fields are highly variable and rarely recover the expected field strength within an accuracy of better than 10 inconsistent results on modern volcanic rocks lend even greater uncertainty to intensity experiments performed on lava flows emplaced during periods of unknown geomagnetic field strength the majority of published paleointensity data are collected from the slowly cooled massive centers of lava flows where the magnetic grains are more likely to be multi domain and produce non ideal experimental results glassy volcanic material found on subaerial lava flow tops and in sub aqueous and subglacial environments however is rapidly cooled and therefore most likely of all volcanic materials to behave as single domain particles demanded by neel theory we present a new paleointensity study of historic and modern hawaiian lavas and test the viability of subaerially emplaced glassy basaltic material as an accurate recorder of magnetic field intensity six of eight lava flows sampled on the big island of hawaii 1843 1859 1935 1950 1960 1990 c e produce well behaved arai plots and recover an average intensity to within 2 7 t of the expected field strength or better than 8 accuracy we apply very strict selection criteria including a minimum of three specimens per site to prevent extraneous field estimates from affecting the final results individual volcanic glass results from the 1960 c e lava flow have a much lower variance than published data from the same volcanic unit glassy materials should therefore be collected wherever possible as they allow recovery of geomagnetic field strength with unprecedented accuracy

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

an evaluation of the influence of the experimental cooling rate along with other thermomagnetic effects to explain anomalously low palaeointensities obtained for historic lavas of mt etna italy methodological aspects in obtaining reliable absolute palaeointensity estimates have attracted renewed attention in recent years obtaining a reliable palaeointensity from lavas however still is notoriously difficult in many cases lavas have been shown to be a non ideal recorder of the palaeofield here we assess the influence of various thermomagnetic effects on the outcome of palaeointensity experiments on a suite of recent lavas from mt etna several palaeointensity studies have targeted recent flows of mt etna before often palaeointensity underestimates were found we apply both thellier and multispecimen style palaeointensity experiments and obtain estimates generally 20 35 per cent below the expected reference value in line with previous studies we explored subtle chemical alteration viscous magnetic behaviour and differences between the natural and laboratory cooling rates used during the palaeointensity experiments as possible causes for the observed underestimates of the palaeofield for the latter a furnace with controlled programmable cooling rates was constructed it can cool samples very slowly with rates that are in the same order of magnitude as natural cooling rates typical of mt etna lava flows 10 k day both viscous magnetic behaviour and differences in cooling rate appear to influence the outcome of the palaeointensity experiments however the magnitude of the changes observed is not sufficient to explain the observed underestimates we therefore hypothesize that the observed underestimates may be primarily explained by transdomain processes occurring during the heating steps of the palaeointensity experiments

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

importance of cooling rate dependence of thermoremanence in paleointensity determination 1 the practical effect of cooling rate on the magnitude of thermal remanent magnetization trm is experimentally tested using both natural and synthetic magnetites for synthetic and natural single domain sd magnetites trm intensity increases as the cooling rate decreases because a longer exposure time for slower cooling offers more chances to achieve an equilibrium magnetization multidomain md magnetites have the opposite response trm decreases as the cooling rate increases because slower cooling allows more time to achieve lower magnetization because of self demagnetization for pseudo single domain psd magnetite the effect of cooling rate on the remanence intensity appears to be insignificant intermediate in behavior between sd and md for a sd trm differences are restricted to lower temperatures but diminish near the curie point strongly indicating a serious problem in a conventional treatment of cooling rate correction on the practical side merely applying trm differences as a correction can overcorrect the true paleointensity another important significance of the present study is that a cooling rate difference alone can cause a slight nonlinearity in an arai plot for a sd

testing determinations of absolute paleointensity from the 1955 and 1960 hawaiian flows abstract a new exhaustive study of the flows erupted in 1955 and 1960 from the kilauea volcano big island of hawaii has been conducted in order to constrain the dispersion inherent to the data of absolute paleointensity reported so far and ultimately to determine what are the most critical parameters to obtain suitable determinations the mean directions derived from both flows agree with the expected field within less than 2 paleointensity experiments have been performed using a double heating thellier procedure completed by systematic ptrm checks almost all samples display a linear nrm trm straight line but only 20 of the samples have provided suitable results less than 10 away from the expected field in the other cases the deviation from the expected paleointensity is related to the quality of the ptrm checks which thus provides a threshold value for acceptance of ptrm checks all successful determinations less than 10 away from the expected value have a maximum drat selkin and tauxe 2000 that never exceeds 5 while the mean value of all deviations is lower than 3 5 another interesting feature is the existence of a link between the unblocking temperature spectra and the accuracy of the paleofield which points out the origin of difficulties in the presence of a wide distribution of grain sizes and or the existence of several magnetic phases the results also indicate that magnetite with a narrow range of high unblocking temperatures and little variability in grain size is a favorable situation to obtain suitable determinations with a high success rate these observations may well explain the dispersion inherent to the previous studies of the hawaiian flows

high coercivity remanence in baked clay materials used in archeomagnetism a study of the high coercivity remanence in archeological baked clays has been carried out more than 150 specimens from 46 sites across europe have been analyzed selected on the basis of the presence of a fraction of their natural remanence that was resistant to alternating field demagnetization to 100 mt the study was based on the stability of isothermal remanence to alternating field and thermal demagnetization and its variation on cooling to liquid nitrogen temperature results indicate that the high coercivity remanence may be carried by magnetite hematite and in isolated cases partially oxidized magnetite and goethite in addition a high coercivity thermally stable low unblocking temperature phase has been identified the unblocking temperatures of both the isothermal remanence and the alternating field resistant natural remanence exhibit similar unblocking temperatures suggesting that the same phases carry both signals the high coercivity low unblocking temperature phase contributes to the natural remanence sometimes carrying a stable direction and behaving ideally during palaeointensity experiments and sometimes not an unambiguous mineralogical identification of this phase is lacking although likely candidates include hemoilmentite related to clay source lithology and substituted hematite or magnetic ferri cristabolite both possible products of thermal transformation of iron bearing clays

evaluation of the multispecimen parallel differential ptrm method a test on historical lavas from iceland and mexico summary obtaining reliable geomagnetic palaeointensity pi estimates is still a difficult objective most common techniques in pi studies are based on various modifications of the thellier method but high failure rates and large uncertainties are common furthermore only rocks which obey stringent criteria are assumed to yield faithful pi estimates in general magnetic particles must be of single domain size the magnetization must be a thermoremanent magnetization and chemical alteration of magnetic carriers must be avoided during the laboratory experiment recently a new method for obtaining pis was proposed which on the grounds of results from two historic lavas and three simulated experiments is assumed to be independent of magnetic domain state this method uses multiple samples subjected to only one heating step at moderate temperatures here we present a test of the multispecimen parallel differential ptrm method on 11 historical lavas from iceland and mexico from which the actual field intensity is known either from magnetic observatory data or from magnetic field models our results show that the majority of pi estimates after the multispecimen parallel differential ptrm method yielded results that are very close or indistinguishable within the range of error from the expected intensity and thus largely confirm the findings from the initial study however there is in general a small overestimate which we show is associated with multidomain material when compared to a thellier type pi experiment carried out on sister samples from five icelandic lava flows the average multispecimen pi is closer to the actual field intensity samples from one lava characterized by low curie temperatures failed the thellier approach due to laboratory induced alteration but here the multispecimen method returned a reasonable pi even though there is evidence that the method is not entirely independent of magnetic domain state as previously proposed we regard this method as a valuable alternative in future pi studies

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined