SUMMARY

The first archaeomagnetic secular variation (SV) curves for the whole Southern Balkan Peninsula are presented. These are based on all data within a 700 km circle centred at Thessaloniki (40.60oN, 23.00oE). This data set consists of 325 directional and 625 intensity data mainly from Greece, Bulgaria, Serbia and southern Hungary. Some data from southern Italy are also included. The sliding moving window technique, was used to calculate a continuous SV curve for intensity while the directional SV curves were calculated using the bivariate extension of the Fisher statistics. These curves are well constrained and clearly show the main features of the geomagnetic field variation in this region during the last eight millennia. Comparisons with the predictions of the SCHA.DIF.3K and SCHA.DIF.8K regional and the CALS7K.2 and ARCH3K.1 global geomagnetic field models show a good agreement for the last 3000 years but differences for older times. The Balkan SV curves identify several rapid changes of the geomagnetic field in eastern Europe and can be used as reference curves for archaeomagnetic dating in the Balkan Peninsula.

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Secular variation of the Earth's magnetic field in the Balkan region during the last eight millennia based on archaeomagnetic data

In this study six new intensity determinations are presented, obtained from five well dated archaeological sites, located in northern Greece and in Paros, Cyclades Islands. The fired structures consisted of ceramic and pottery kilns belonging to the Hellenistic, Roman and Byzantine periods. Between 8 and 21 samples of highly fired baked clays, tiles and bricks were taken, homogeneously distributed over the structures. The samples were analysed using the classical Thellier method, providing the past intensities and directions of the geomagnetic field recorded at each site. The intensity values have been corrected for anisotropy of thermal remanent magnetisation and cooling rate effects. Differences in the mean archaeointensities per site ranging from 1% to 11%, before and after TRM anisotropy and cooling rate corrections, were obtained. The new results indicate a decrease of 20% of the geomagnetic field strength in Greece, during the last four centuries BC. In order to compare our results with previously published data, a catalogue of archaeo- and palaeointensity results for the Aegean area has been established, covering the last 7 millennia. It consists of 336 data from Greece, western Turkey and Former Yugoslavia, collected from various authors. Weighting factors have been applied to these data, that then have been treated with a hierarchical Bayesian modelling, and a geomagnetic field intensity variation curve for Greece was constructed. A good agreement is observed when comparing the curve for Greece with the Bulgarian secular variation curve (SVC) for intensity. Satisfactory coincidence is also found with the archaeointensity data from Mesopotamia. Despite the presence of some time gaps, a more precise secular variation intensity curve has been constructed for Greece which, combined with a forthcoming directional SVC, will help for dating purposes.

New archaeointensity results from archaeological sites and variation of the geomagnetic field intensity for the last 7 millennia in Greece

The long-term spatial and temporal variation of aurora borealis events from 1600 to the present were studied using catalogues and other records of these phenomena. Geographic and geomagnetic coordinates were assigned to approximately 45 000 auroral events with more than 160 000 observations. They were analysed separately for three large-scale areas: i) Europe and North Africa, ii) North America, and iii) Asia. Variations in the cumulative numbers of auroral events with latitude (in both geographic and geomagnetic coordinates) were used to distinguish between the two main solar sources: coronal mass ejections and high-speed streams from coronal holes. We find significant long-term variations in the space-time distribution of auroras. We mainly identify these with four Gleissberg solar activity cycles whose overall characteristics we examine. The Asian observations are crucial in this context, and therefore merit further studies and verifications.

Long-Term Trends and Gleissberg Cycles in Aurora Borealis Records (1600 – 2015)

Archeointensities in Greece during the Neolithic period: New insights into material selection and secular variation curve

Geomagnetic intensity measurements using bricks from greek churches of the first and second millennia a.d.

The auroral frequency of occurrences (A) for the 20th solar cycle and for the geomagnetic latitudes 54‡–63‡ N has been investigated in relation to sunspot numbers (Rz), number of flares (F), the solar wind streams derived from the coronal holes (H) and the geomagnetic index (Ap). The relationship between A and the other indices were found to be strongly latitude dependent. At around 57‡–58‡ N, a drastic change in this relationship occurs, and an attempt is made qualitatively to evaluate this latitudinal variation.

Latitude dependence of auroral frequency in relation to solar - terrestrial and interplanetary parameters

Statistical analyses on available archaeomagnetic intensity data mainly from the Aegean area and supplemented by some from Bulgaria and Czechoslovakia have revealed the existence of periodicities.

Maximum entropy and power spectrum analyse of geomagnetic intensity variations from archaeomagnetic data: emphasis on the 200-year period

Archaeointensity results from Greece for the last 2000 yr have been obtained using thermal and alternating field (AF) demagnetisation techniques. Most of the samples consisted of tiles and bricks derived from Byzantine churches which were epigraphically dated. The accuracy of the dating of the samples (to within a few years) and the comparison of the two techniques for measuring geomagnetic intensity combine to provide a good basis from which to derive meaningful conclusions — i.e. (i) About 70% of the archaeointensity measurements, made by the two methods, were highly compatible, differing less than ±7% from their mean. In 60% of the cases, the AF technique yielded higher palaeointensity results than that for thermal demagnetization. The results for the thermal and AF techniques are discussed in accordance with reliability criteria and attention is drawn to the reliability of archaeointensity data in general. (ii) An overall similarity of the intensity trend was observed for the last 400 years between the AF, the thermal demagnetization and a modelled intensity curve derived from the spherical harmonic coefficients since AD 1600. (iii) A decrease of intensity amounting to 40% is noted in the new data for the period since AD 400, and (iv) a non-dipole disturbance was noted at around AD 1300 in southern Greece, where the intensity changed by about 30% within a period of 30 years or so.

Greek archaeomagnetic intensities: Some aspects of reliability and geophysical implications

Geomagnetic virtual pole positions (VGP's) calculated from archaeomagnetic directional data are compared with three ancient accounts of low latitude observations of the Aurorae boreales, viz by Aristotle (384–322 BC), Seneca (55 BC-40 AD) and simultaneously by Chinese astronomers in Hangzhou, China and by European observers in Prague in 1138 AD, October 13th

Aurorae Boreales and Geomagnetic Inclinations as Aids to Archaeomagnetic Dating

The analytical representation and the application of maximum entropy spectral analysis to inclination data of the last 2000 years revealed the possible existence of periodic terms of approximately 1000, 500 and 260 years. The best fitting of the inclination data was achieved by a network of cosinusoidals as well as cubic splines.

Y. Liritzis

Papers

Latitude dependence of auroral frequency in relation to solar - terrestrial and interplanetary parameters

Maximum entropy and power spectrum analyse of geomagnetic intensity variations from archaeomagnetic data: emphasis on the 200-year period

Greek archaeomagnetic intensities: Some aspects of reliability and geophysical implications

Aurorae Boreales and Geomagnetic Inclinations as Aids to Archaeomagnetic Dating

Spectral analysis of archaeomagnetic inclinations for the last 2000 years