Secular variation of the geomagnetic virtual dipole field during the last 2000 years as inferred from the spherical harmonic analysis of the available archeomagnetic data

Abstract

Recently excellent archeomagnetic data sequences have been bublished from several parts of the world. Using these sequences, an attempt is made to trace the secular variation of the virtual geomagnetic dipole field characterized by the three first order spherical harmonic coefficients\(\bar g_1^0 ,\bar g_1^1 and\mathop h\limits^-- _1^1 \). The archeomagnetic data (declination, inclination and total intensity) are transformed into the first order coefficients mentioned by a simple mathematical method. The secular variations of these coefficients, however, contain both dipole and non-dipole components. The separation of these is also attempted.