The secular variation of the geomagnetic field in Egypt in the last 5000 years

Abstract

The palaeo-intensities (F _a) of the geomagnetic field in Egypt at some ages are determined by archaeomagnetic measurements and found to be:F _a=36.2 μT at 3100 B.C., F_a=46.8 μT at 3000 B.C.,F _a=36.5 μT at 2780 B.C., 49.0 μT at 2500 B.C., 36.4 μT at 2200 B.C., 57.5 μT at 1990 B.C., 62.1 μT atca 1400 B.C., 61.5 μT at 1400 B.C., 69.9 μT at 600 B.C., 59.3 μT at 550 B.C., 79.9 μT at 460 B.C., 73.7 μT at 450 B.C., 69.7 μT at 320 B.C., 56.2 μT at A.D. 50, 64.9 μT, at A.D. 400, 54.4 μT at A.D. 300, 57.5 μT at A.D. 700 and 43.0 μT at A.D. 1975.

The palaeo-inclinations (I _a) at some ages are found to be:I _a=24.2° at 420 B.C., 44° at A.D. 50, 60.7° at A.D. 703 and 42° at A.D. 1795.

The measured values ofF _a are affected by the anisotropy of magnetic susceptibility of the samples by 13% to 20% of the expected correct value. The suitable correction of this effect is by multiplyingF _⊥ by 1/((1+0.2(θ/90)) andF _∥ by 1/((1–0.13 (θ/90)), whereF _⊥ andF _∥ are the resultant values ofF _a if the laboratory field is perpendicular or parallel to the wall of the sample during the Thelliers' experiments, respectively, and θ is the angle between the direction of natural remnant magnetization of the sample and the direction of the laboratory field.

The results of this paper, together with the previous results for Egypt and the neighbourhoods, lead to the production of the secular variation curve of the geomagnetic field in Egypt for the last 5000 years. The intensity of the field shows a periodicity of about 400 years with multiples.