NEW ARCHAEOMAGNETIC STUDIES NEAR XI'AN,CHINA*

Archaeomagnetic and mineral magnetic studies were made of known-age samples from fired structures and sediments from archaeological sites near Xi'an, China. The fired materials retained a stable record of the direction of the past geomagnetic field, which compared well with previous investigations and could contribute to an archaeomagnetic calibration curve. However, discrepancies with documented observations of the field were noted.     

Specialized Salt Production During the Ancient Maya Classic Period at Two Paynes Creek Salt Works,Belize: Chan b’i and Atz’aam Na

Underwater excavations at the Classic period (a.d. 300–900) ancient Maya salt works of Chan b’i and Atz’aam Na in Paynes Creek National Park, Belize, reveal activity areas associated with a substantial salt industry for distribution to nearby southern Maya inland sites where this biological requirement was scarce. Wooden architecture and salt making artifacts are abundantly preserved in a peat bog composed of red mangrove. Excavations, screening, and analysis of recovered material at the submerged underwater sites reveal that the artifacts are overwhelmingly briquetage: pottery vessels used to evaporate salty water by heating over fires to make salt. The spatial distribution of briquetage in relation to the interior and exterior of buildings reveals that salt production was occurring indoors. The plethora of briquetage and the scarcity of domestic artifacts indicate that the sites were specialized salt works and not physically attached to households.     

Magnetic field of the magnetospheric ring current and its dynamics during magnetic storms

This review examines models existing in the literature which describe the magnetic field produced by the ring current (DR) at the Earth's surface based on the energy balance equation. The parameters of this equation, the injection function F and decay parameter τ are considered to depend on parameters of the interplanetary medium and the DR intensity. The existing models are shown to be able to describe the DR variations with sufficient accuracy (r.m.s. deviation δ between the experimental and modelled values of DR for 170 magnetic storms is 5 < δ < 15 nT and the correlation coefficient between the two is 0.85 <r<1). The models describe that part of the geomagnetic field variation at low latitudes during a magnetic storm that is controlled by the geoeffective characteristics of the interplanetary medium and which thus responds immediately to its variations (the driven part).The values of τ are significantly less during the main phase of a magnetic storm than during the recovery phase. This reflects the difference in the main mechanisms of ion loss from the ring current during the two phases of the storm. These are the interaction of ions with hydromagnetic waves during the main phase of the storm with its intervals of intense plasma injection into the inner magnetosphere and charge exchange with the cold hydrogen geocorona during the recovery phase.     

Combined archaeomagnetic and thermoluminescence study of a brick kiln excavated at Fontanetto Po (Vercelli,Northern Italy)

A combined archaeomagnetic and thermoluminescence study was carried out as part of a rescue archaeological excavation on a kiln discovered during the installation of methane gas pipelines beneath a rice field, along the southern border of Fontanetto Po village (Vercelli province, Italy). A total of 23 independent brick samples have been collected, oriented in situ with an inclinometer; the use of magnetic and sun compass was not possible due to the existence of metallic tubes beneath the kiln and a plastic cover above it. Standard archaeomagnetic procedures have been used for the determination of the archaeomagnetic inclination and absolute geomagnetic intensity. Stepwise thermal demagnetization shows a very stable characteristic remanent magnetization and the calculated mean inclination of the 23 samples is I = 65.3° with α₉₅ = 2.4° and k = 156. Archaeointensity experiments have been performed using the classical Thellier method as modified by Coe, with regular partial thermoremanent magnetization (pTRM) checks. The cooling rate and remanence anisotropy effects upon thermoremanent magnetization (TRM) have been investigated in all the specimens. A total of 19 archaeointensity determinations (at specimen level) that correspond to linear NRM–TRM plots were used for the calculation of the site mean archaeointensity that is 46.4 ± 2.9 μT. Archaeomagnetic dating results show two possible dating intervals for the last 1000 years, calculated at 95% confidence interval: a first one from 1511 to 1614 AD, and a second one from 1768 to 1872 AD. Thermoluminescence (TL) study has been also performed on two brick samples from the kiln's internal wall, using conventional laboratory procedures. According to the thermoluminescence results the kiln's last usage lies between 1796 and 1914 AD. This age is in good agreement with the second dating interval obtained by the archaeomagnetic analysis. The combined archaeomagnetic and thermoluminescence results suggest that the last firing of the kiln could have occurred between the end of the 18th century and the beginning of 20th century.     

RESEARCH REPORT: MAGNETIC INVESTIGATIONS AND THE AGE OF A MEDIEVAL KILN AT KUNGAHÄLLA (SOUTH‐WEST SWEDEN)*

The results from a magnetic survey and archaeomagnetic investigation of a medieval brick kiln at Kungahälla in Bohuslän (south‐west Sweden) are reported. Detailed magnetic total field and magnetic gradient measurements over known traces of the kiln showed marked local magnetic anomalies of up to 200 nT, revealing the rectangular shape of the kiln remains. Palaeomagnetic investigations of the remanent magnetization of 12 bricks from the kiln floor gave precise definitions of both the direction of the archaeomagnetic field [(D_m, I_m) = (66.7°, 8.8°), k = 655, α₉₅= 1.7°] and the palaeointensity [B_m= 69.0 ± 3.6 µT]. An archaeomagnetic date is obtained by comparing the direction of the archaeomagnetic field found in the kiln with a geocentral dipole‐transformation of the British master‐curve for secular variation. Of two possible archaeomagnetic dates (ad 1280 ± 50 or ad 1480 ± 50), the ad 1280 ± 50 age is in good accordance with the C‐14 ages and archaeological dates as well as with direct historical evidence, whereas a c. 100 years younger TL date appears to be too young.     

Geomagnetic field variations as a dating tool: Application to Sicilian lavas

Over 40 historically dated Mt Etna lava flows have been carefully sampled for the purpose of palaeomagnetic analysis. The natural remanent magnetization (NRM) of each lava flow contains a record of both the magnitude and direction of the Earth's magnetic field at the time of its eruption. Our analysis has thus enabled us to determine the change of the magnetic field in Sicily through historic times and to construct curves which show this secular variation of the field in both direction and magnitude. In this paper, the curves are used to reconsider the historical dates attributed to a number of Etna lavas, dates which had since been questioned by recent palaeomagnetic and geological studies. In a similar manner, these secular variation curves can be used as a powerful archaeological dating tool by comparison with the NRM of fired artifacts.     

ARCHAEOMAGNETIC TILT CORRECTION ON BRICKS

Fired bricks are used in archaeomagnetic work to determine the Inclination I of the earth's magnetic field at the time of firing. However, in addition to the usual mean error of between ± 1/2° and ± 2° due mainly to inexact levelling of the bricks during the firing, there is a systematic error in the measured mean value of the Inclination that may amount to 2° or 3° at high magnetic latitudes (I? 80°), while at lower magnetic latitudes (I< 60°) it is usually less than 1/2°. A method of correction based on the measured statistical spread of the bricks is suggested.     

Combining an archaeomagnetic and radiocarbon study: dating of medieval fireplaces at the Mühlegasse,Zürich

During the restoration of the Radium cinema located in the old Town of Zürich, a stratigraphic sequence of medieval fireplaces came to light. Based on ceramic finds, a preliminary age of about 1000 AD was assigned. In this paper we show the results from archaeomagnetic and radiocarbon dating, which combined with the stratigraphic information yield a better constrain of the period of use of these fireplaces. Whilst the radiocarbon technique continues to be the most widespread methodology for dating young archeological artifacts, archaeomagnetic dating is becoming increasingly more established in Europe thanks to the increased number of available measurements. A Bayesian model on the period of use of these fireplaces was produced with OxCal, by a combination of magnetic and radiocarbon ages, together with the stratigraphic information. The results show that the fireplaces were probably used during 1 to 2 generations (e.g. 20–40 years each), and are in good agreement with the archaeological context of this site. To make use of the magnetic measurements for future reconstructions of the field, we carried out an additional age model using only the radiocarbon and the stratigraphic information. Although some of the structures show large directional scatter due to poor baking in antiquity, the results confirm the trend of the geomagnetic field around 1000 AD.     

VLF radiation generated by a loop antenna in F2 layer of the ionosphere—1. Theory

In 1987 an experiment on the transmission and reception of VLF electromagnetic waves was carried out in the Earth's ionosphere. The interpretation of the experiment required an elaboration of the theory of a loop antenna in a plane-stratified plasma with parameters changing along the external magnetic field. Formulae for the wave field at arbitrary distances from the antenna have been obtained. Asymptotic expansions for the wave field permit us to construct an effective numeral algorithm for the calculation of the signal when the far zone approximation does not hold and in the presence of the fine structure.     

Full‐Vector Archaeomagnetic Dating of a Medieval Limekiln at Pinilla Del Valle Site (Madrid,Spain)

Archaeomagnetic dating based on the full geomagnetic field vector was carried out on a limekiln excavated at Pinilla del Valle archaeological site (Madrid, Spain). The limekiln tradition in this area is largely documented by historical sources for recent centuries but the date of the studied kiln's last use was unknown. The combination of mean archaeomagnetic directional and absolute archaeointensity results shows that the kiln was last used between ad 1296 and 1413, in good agreement with two independent radiocarbon dates. This study provides the first archaeomagnetic evidence that the local limekiln tradition dates back to at least late medieval times. Furthermore, the inclusion of these data in the Iberian secular variation curve and geomagnetic field models will help to improve the archaeomagnetic dating technique.     

Inversion of oblique ionograms including the Earth's magnetic field

A technique for determining ionospheric electron distribution from oblique ionograms is presented, based on the inversion method of Reilly and Kolesar (Radio Sci. 24, 575, 1989). It makes use of an equivalent operating frequency and an additional term to account for magnetoionic effects associated with the Earth's magnetic field. The technique is demonstrated by application to synthetic oblique ionograms, and to an experimentally obtained ionogram.     

ARCHAEOMAGNETIC FIELD INTENSITY DURING THE ROMAN PERIOD AT SIWA AND BAHRYN OASIS,EGYPT: IMPLICATIONS FOR THE FIDELITY OF EGYPTIAN ARCHAEOMAGNETIC DATA

A preliminary survey was conducted in Siwa and Bahryn Oasis to test the possible influence of various rock magnetic and experimental constraints on the fidelity of the Egyptian archaeomagnetic field record. Five potsherds from the Roman era, which lasted from 1981 bp to 1555 bp , have been investigated. Archaeologists dated the localities to ～1620 bp . Ten ceramic specimens, two of each potsherd, were subjected to archaeointensity determination, including tests for domain state effects, magnetic anisotropy and magnetic cooling rate dependency. Six successful archaeointensity determinations are obtained from three individual cooling units, revealing an average field value of 37.7 μT for the late Roman period in Siwa and Bahryn oasis, which is comparable to the present‐day field strength. The error propagation of the individual uncertainties related to all applied experimental techniques results in a maximum uncertainty estimate of 4.4 μT. The obtained field value is significantly smaller than early results and slightly smaller than some more recent determinations of the field intensity in Egypt. The difference is attributed to a combined effect of alteration, magnetic anisotropy and magnetic cooling rate dependencies. Along with other high‐quality data from the south‐east Mediterranean, our data suggest a field intensity minimum during the Egyptian Roman era.     

Archaeomagnetic dating of archaeological sites from Switzerland and Bulgaria

Baked materials from three archaeological sites in Switzerland and one in Bulgaria have been examined and dated archaeomagnetically. The detailed rock-magnetic properties of the burnt clay collected have already been studied therefore and have been published elsewhere (Geophysical Journal International 153 (2003) 146). The main aim of this paper is to determine the terminus ante quem for the last firing of the clay collected from pottery kilns or burnt layers. The directional and intensity results of the ancient geomagnetic field corresponding to the time of the structure's last firing are given for each site. These geomagnetic parameters are then compared with reference curves to provide archaeomagnetic dates for the last use of the kilns. Application of archaeomagnetism as a dating tool is only possible for those regions that already have reference curves. Bulgaria is fortunate in having established reference curves covering almost the last 8000 years. Two Swiss sites were dated using only directional results on the basis of existing palaeodirectional results for Switzerland as well as the French directional reference curves. The archaeomagnetic dates of three of the sites were then compared to archaeological evidence and available radiocarbon dates. The palaeointensity data obtained from this study will enrich the existing archaeomagnetic database for western Europe.     

Automatic fitting of quasi-parabolic segments to ionospheric profiles with application to ground range estimation for single-station location

A method of automatically fitting a quasi-parabolic segment (QPS) model to measured profiles of electron concentration is described. The model consists of many smoothly joined quasi-parabolic segments. The base of the ionosphere is represented by a quasi-linear segment. This model has the advantage of possessing analytic solutions for ray quantities in a spherical ionosphere (neglecting the effect of the Earth's magnetic field). It is also able to represent measured vertical ionospheric profiles to within a few percent in electron concentration.The usefulness of the model is illustrated by an application to the single-station location (SSL) of a distant radio transmitter from angle-of-elevation measurements. The no magnetic field results can represent the ordinary mode and approximate results for the extraordinary mode can be obtained using a frequency scaling technique.     

D-region electron density diurnal changes observed in association with the PCA event of July 1974

Well defined diurnal phase and amplitude deviations of very low frequency radio signals propagation were detected during the solar proton event that occurred from 3 to 8 July 1974. Phase advances observed on signals transmitted from GBR (U.K.), received at Atibaia (Brazil) and at Inubo Radio Observatory (Japan), were compared with simultaneous NWC (Australia) transmissions received at Atibaia, and at Syowa Station (Antarctica). Emphasis was given to the propagation paths NWC-Atibaia and NWC-Syowa, because both propagate completely in the southern hemisphere, crossing regions where the Earth's magnetic field behaves anomalously. The comparison allowed the determination of parameters typical of the D-region at a given height in the lower ionosphere, for geomagnetic regions defined through the McLlwain parameter(L). An exponential model was adopted to fit the vertical distribution of the diurnal electron density. The experimental results showed a delayed contribution to the total ionization observed which was attributed to a slow precipitation of energetic particles in the South Atlantic Geomagnetic Anomaly region.     

Synthesis of oblique ionograms from vertical ionograms using quasi-parabolic segment models of the ionosphere

This paper describes a method of synthesizing oblique ionograms from vertical ionograms based on representing the ionosphere by multiple quasi-parabolic segments (QPS). The advantage of this approach is that it allows analytical solutions to be obtained for several ray parameters when the Earth's magnetic field is neglected and the ionosphere is spherically stratified. The no-field results are representative of the ordinary mode. In addition, results for the extraordinary mode can be obtained by introducing a perturbation to the effective frequency and modifying the no-field results. The method is illustrated by comparing synthesized oblique ionograms with observed ionograms and with results obtained for the ordinary mode using the classical method.     

Dayside high latitude magnetic impulsive events: their characteristics and relationship to sudden impulses

On 17 December 1990 a series magnetic impulsive events (MIEs) were observed at high latitudes near local noon. EISCAT, situated some 5 hours of MLT away from the noon sector, detected simultaneous impulsive electron density enhancements at heights between 90 and 120 km. The MIEs at noon were also associated with riometer absorption spikes. The correlated EISCAT and riometer observations indicate that there was an elongated electron precipitation region some 3000 km wide stretching from local noon to morning. In close association with the impulsive electron precipitation, VLF emissions were observed by groundbased stations in the morning side. We interpret the large scale electron precipitation and VLF emissions as signatures of a global compression of the Earth's magnetosphere. This is confirmed by the specific type of magnetic variations simultaneously recorded at the worldwide network of magnetometers. We conclude that the small scale MIEs with their drifting ionospheric current vortex structures can (but do not necessarily have to) occur in conjunction with large scale SIs. Moreover, MIEs and SIs have a common origin: the interaction of solar wind inhomogeneities with the Earth's magnetosphere. They do, however, represent different effects of the same primary agent.     

The effect of mid-latitude electron precipitation on the geoelectric field

A simple model is outlined to describe electron precipitation from the population of charged particles trapped in the Earth's magnetic field; almost all of the precipitation is shown to occur in the region of the South Atlantic Anomaly (SAA). When the effect of a dawn-to-dusk electric field across the magnetosphere is included in the model, a diurnal modulation of the precipitated electron flux is predicted. Experimental evidence which supports the diurnal modulation model is described; the measurements which are discussed are principally those from University of Houston rocket payloads flown in the region dominated by the SAA. The resulting diurnal variation in atmospheric conductivity in the SAA is shown to be such that it could account for part of the daily UT variation in the geoelectric field. The observed seasonal variation in the pattern of geoelectric field modulation is also shown to be consistent with the proposed source of the daily variation. The difficulty of altering global conductivity by intense ionization of the mesosphere is discussed and the point is made that further in situ investigation of the Antarctic mesosphere is needed.