SOLAR-CYCLE VARIATIONS OF THE 1ST-DEGREE SPHERICAL HARMONIC COMPONENTS OF THE GEOMAGNETIC-FIELD

Spherical harmonic analyses were made of the annual mean values of the geomagnetic components at 34 observatories for the period from 1940 to 1973. Annual values of the Gauss coefficients were separated into parts of external and internal origin. The external part of the first-degree component is parallel to the internal dipole and changes with solar activity. The maximum change from peak to trough amounts to 47 nanoteslas during this period. During one solar cycle, two peaks appear in the variation of this component. The first peak corresponds to the period of the maximum solar activity, the second at the declining stage of the cycle. Examination of the time variations of the first-degree components was extended to an analysis for 1900 to 1973 based on annual means from 21 observatories. Although its amplitude is small, the internal part changes with a three-year time lag behind the external field change. After Fourier analyses of the time series of both the internal and external components, their ratio was computed for the 10-year-period band. The modulus of the internal/external ratio (i/e) was obtained to be 0.32 and the phase angle (internal-external) to be 105�°. © 1979, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.