Sunspot cycle influence on the geomagnetic field

Abstract

Summary The variation of the geomagnetic field associated with the sunspot cycle is examined using a much more extensive data set than hitherto. A method which takes account of the irregular nature of the solar cycle is used to represent the magnetic variation, particular attention being paid to the confidence limits of the results. We obtain measures in both vertical and horizontal magnetic intensity at 81 observatories distributed throughout the world. These results are used to derive a spherical harmonic model, from which the contribution that results from the sunspot cycle modulation of the daily geomagnetic variation is subsequently removed. The potential is then separated into parts of External (inducing) and Internal (induced) origin, with amplitude ratio I/E= 0.12 ± 0.07 and phase difference = - 8 ± 30 deg. Examination of the residuals indicates the presence of local variations, on a timescale comparable with the solar cycle, that are uncorrelated with solar activity. These variations probably result from dynamo action within the core.