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A reappraisal of instrumental magnetic measurements made in Western Europe before AD 1750: confronting historical geomagnetism and archeomagnetism
© The Author(s) 2017
- Received: 3 December 2016
- Accepted: 6 February 2017
- Published: 17 February 2017
- Historical magnetism
- Secular variation
- Geomagnetic directions
- Western Europe
Since the dawn of archeomagnetic research, data obtained in Europe from archeological artifacts and volcanic deposits were used to build regional-scale geomagnetic directional secular variation curves (e.g., Chevalier 1925; Aitken and Hawley 1966; Tanguy 1969; Thellier 1966, 1981). An issue rapidly arose regarding the continuity between archeo-/paleomagnetic results and the most ancient instrumental or “direct” measurements of declinations (D) and inclinations (I). In Western Europe, magnetic declinations have been directly measured since the middle of the sixteenth century, whereas only a very small number of inclination data became available more than a century later (e.g., Jackson et al. 2000; Jonkers et al. 2003). These series of pioneering D and I measurements from the sixteenth and seventeenth centuries, especially those obtained in London and in Paris, were the subject of several compilations (e.g., Malin and Bullard 1981; Alexandrescu et al. 1996, 1997). Comprehensive datasets further incorporating measurements reported in ship logbooks by mariners during their navigation around the world have been used to construct global geomagnetic field models encompassing the past four centuries, in the form of Gauss coefficient time series. At present, the gufm1 model developed by Jackson et al. (2000) is used most frequently. It enabled, in particular, studies to decipher the evolution of the main core flow patterns at the core surface (e.g., Hulot et al. 2002; Holme 2007; Finlay et al. 2016). Note that the first examples of global reconstruction of the ancient field date back to the nineteenth century; one can, for instance, mention the work of the Swedish scholar Carlheim-Gillensköld (1896), whose computations went back to the sixteenth century. More recently, a regional field model was also constructed for Europe using a spherical cap harmonic (SCHA) analysis technique incorporating both archeomagnetic and historical geomagnetic data (Pavón-Carrasco et al. 2009).
The objective of this study is to better constrain the continuity and/or compatibility between the earliest Western European instrumental directional data and the available French archeomagnetic results. For this purpose, we present a new analysis of the historical geomagnetic data spanning the AD 1500–1750 time interval. This analysis therefore provides a reliability test of the directions expected in Europe from gufm1 and, as a consequence, for that of the older segment of the gufm1 model.
The catalogs of historical instrumental magnetic measurements are subject to sporadic reviews. For the Western European region, practically all references can be found in the recent article of D’Ajello Caracciolo et al. (2011), whereas all records obtained worldwide contained in the global database used for the construction of the gufm1 model (Jackson et al. 2000) are available on the Web (ftp://ftp.nmh.ac.uk/geomag/Shipborne/).
Instrumental inclination data are scarce before AD 1750, and perhaps approximate given the rudimentary design of instruments used at that time (see illustration, for instance, in Cabeo 1628 or Kircher 1641). Note also that the ancient writings often lacked precision on dates and/or locations of measurements (see below). However, it seems certain that the observers tried to make measurements as accurate as possible. Here we took into account these results without any unfavorable consideration. Whenever possible, we returned to the original documents written by the scholars of that time.
Group 1 data from London compiled by Malin and Bullard (1981) are the most numerous and arguably the best documented (yellow circles in Fig. 2a with corresponding dates preceded by “L” for London). Most of the older London data were previously given very low or no weight for the construction of an average curve, mainly due to an ignorance of the context in which values were recorded (see Table 3 in Malin and Bullard 1981). However, one can see that apart from L1661 and L1684 their consistency is very satisfactory with a clear directional evolution toward the West and with slightly increasing inclinations. The French data compiled by Alexandrescu et al. (1996) are much less numerous (pink circles in Fig. 2a with dates preceded by “P” for Paris). P1660 and P1668 were disregarded by Alexandrescu et al. (1996) because they did not appear in agreement with the general directional trend defined by data from London (and also following a similar comment made by J. Raulin; see §6 in Alexandrescu et al. 1996), whereas P1671 was retained. However, from Fig. 2a, one could easily argue for excluding P1671 while retaining P1660 and P1668. The unique and well-known measurement made in Rome by Athanase Kircher and consensually dated at 1640 (red circle labeled R1640 in Fig. 2a) is also in particularly good agreement with the directional trend seen from the London data. We note that in Kircher (1641), the inclination value is 65°50′ on page 401, but 65°40′ on page 410. Nevertheless, regarding the distribution of the data, we do not see any reason to reject this particular datum, in contrast with doubt raised by Lanza et al. (2005) and D’Ajello Caracciolo et al. (2011). The Italian dataset does not contain any other inclination value until the measurements made by Alexander von Humboldt in 1805–1806 (e.g., Cafarella et al. 1992).
Group 2 contains 16 other inclination data from Western Europe found in the global database and reported in Fig. 2b. Geographically, they are located in an area bounded by Malta to the south, Sweden (from the observatory Uraniborg created by Tycho Brahe) to the north, Ireland to the west and Bavaria, in Germany, to the East. In Fig. 2b, all declinations associated with the inclination data were estimated using the gufm1 model. It is worth mentioning that two inclinations dated at 1584 and 1607 obtained by astronomers Tycho Brahe and Johannes Kepler were deduced from the continuation to the Earth’s surface of rays of aurora curtains aligned following the local direction of the geomagnetic field (Mairan 1733; see page 29 in Carlheim-Gyllensköld 1896). It is remarkable to see how these two results lie close to the overall trend defined by most available data (Fig. 2a, b). In contrast, four data from Italy appear significantly distant from this trend. One of these records was reported from Ferrare by Cabeo (1628). It is curiously dated 1629, thus 1 year after Cabeo’s publication. The three others are part of a group of four strictly identical inclination values (59.25°) all dated 1639 and reported by Kircher (1641) from Malta, Palermo, Messina and Napoli. None of them were listed in the Italian catalog of Cafarella et al. (1992), but were considered by Carlheim-Gyllensköld (1896). Their distinct positions in Fig. 2b originate from the effect of reduction to Paris because of the different latitudes of these cities. The original paragraph in which these values have been mentioned is ambiguous: Kircher (1641) wrote on page 401 that he went to Malta to perform a measurement, thus finding an inclination of 59.25°, but he did not say whether he went to the three other Italian cities to achieve other measurements. He only wrote that the three values were almost the same as the one measured in Malta, and from all the observations he knew at that time, he concluded that the inclinations were varying as a function of latitude, thus in contradiction with the data in question. Owing to this clear ambiguity, the present compilation retains only the inclination from Malta.
The geomagnetic directional measurements made in Western Europe before AD ~1750 show an overall good consistency and define a coherent directional evolution in spite of their relatively large geographical coverage. Only a few discrepant values are identified.
The new historical geomagnetic curve computed for Western Europe shows good agreement with the most recent segment of the French archeomagnetic directional variation curve (Bucur 1994). We therefore confirm the good consistency previously underlined by Thellier (1981) between the oldest historical magnetic measurements and the French archeomagnetic data.
The directions expected from the gufm1 model do not agree with the historical geomagnetic field measurements available in Western Europe for the sixteenth and most of the seventeenth century, nor with the French archeomagnetic data. This likely indicates that the older segment of gufm1 lacks reliability and that this model should be revised. A similar inference was previously made from the comparison of archeomagnetic data obtained in La Réunion Island and gufm1-derived directions (Tanguy et al. 2011). Moreover, Genevey et al. (2009) and Hartmann et al. (2011) reached the very same conclusion from archeointensity analyses carried out on ceramics from France and Brazil.
In their current form, neither the older part of the gufm1 model nor the recent sixteenth–eighteenth-century segment of the regional magnetic field model constructed by Pavón-Carrasco et al. (2009) can be used for archeomagnetic dating purposes or for studies of the geomagnetic field in the past millennium.
MLG designed the study and performed the data analysis. MLG and YG discussed the data and interpretation and wrote the paper together.
We thank Jean-Claude Tanguy for stimulating recent and past discussions, Eva Fareau for her help in finding ancient documents, Suzette Roux and Michel Rival for their assistance in Latin translation. We are grateful to France Lagroix and Vincent Courtillot for a careful reading of the manuscript. We also thank F. D’Ajello Caracciolo and an anonymous reviewer for their constructive comments. This is IPGP contribution no. 3817.
The authors declare that they have no competing interests.
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