Geomagnetic field archaeointensities from Britain and the application of the microwave palaeointensity method to materials of differing dielectric properties

摘要

The strength of the geomagnetic field is a subject of both scientific and public interest, with the decay over the past 160 years leading to speculation as to whether we are entering a geomagnetic reversal. Prior to 1840, there was no capability for direct measurements of geomagnetic field strength; to investigate the field strength at this time, palaeomagnetic and archaeomagnetic determinations must be made. Here we investigate the strength of the field in Britain over the past 1500 years as recorded by archaeological artefacts including ceramics, brick and burnt sandstone. Results are derived using both microwave and thermal demagnetisation. The theory of microwave demagnetisation is fully explained and equations governing the absorption of energy by an archaeomagnetic sample in a microwave cavity are derived. As a result, the possibility of demagnetising a palaeomagnetic sample using microwaves without significant heating is demonstrated for the first time. Geomagnetic field archaeointensities from seven British sites are reported. A meta-analysis of global archaeointensity and palaeointensity data from 1590 to 1990 reveals that significant bias has been introduced to field models through inconsistent error estimation. It is shown that the principle source of uncertainty in archaeointensity should be considered as systematic, rather than experimental and that data of arbitrarily high precision can only marginally increase our knowledge of the field. Correspondingly, it is argued that while large data sets are informative enough to constrain the evolution of the geomagnetic field, archaeointensity can only have a limited application as an archaeomagnetic dating tool. It is demonstrated that when the uncertainties are properly quantified, the global data implies that the recent decay of the dipole, evident in magnetic observatory data from 1840 until the present day, is part of a longer term trend, starting as early as 1600.