Solar cycle variations of the first-degree spherical harmonic components of the geomagnetic field.

Abstract

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 1050.

Toward a better representation of the secular variation. Case study: The European network of geomagnetic observatories

“…We confirmed the well known presence in the annual means of geomagnetic elements of a 11-year solar-cyclerelated (SC) signal due to incomplete averaging out external effects and their induced counterparts, modulated by solar activity (Chapman and Bartels, 1940;Yukutake, 1965;Bhargava and Yacob, 1969;Alldredge, 1975Alldredge, , 1976Courtillot and Le Mouël, 1976;Alldredge et al, 1979;Yukutake and Cain, 1979;Demetrescu et al, 1988;Verbanac et al, 2007;Wardinski and Holme, 2011) and quantitatively showed that the time change of the signal is of the same order of magnitude as that of the internal ingredients of the measured field. As a consequence, the SC signal should be filtered out before any discussion on SV.…”

“…Alexandrescu et al, 1995), but, we think, with increased possibilities for external effects to leak into the models. The presence in annual mean data of a solar-cycle-related (SC) signal due to incomplete averaging out of external effects, modulated by the solar activity, has long been recognized (Chapman and Bartels, 1940;Yukutake, 1965;Bhargava and Yacob, 1969;Alldredge, 1975Alldredge, , 1976Courtillot and Le Mouël, 1976;Alldredge et al, 1979;Yukutake and Cain, 1979;Demetrescu et al, 1988;Verbanac et al, 2007;Wardinski and Holme, 2011). Because at the observing point these effects comprise both the direct and the induced fields, and having in view the frequency difference of the SC variation in comparison to the secular variation, a suitable filter applied to data would eliminate both the external fields and their induced effects.…”

Solar cycle variation in geomagnetic external spherical harmonic coefficients.

“…This topic has been treated in several of the references cited earlier. It is also being treated by YUKUTAKE and CAIN (1979) in a forthcoming study which was done partly in collabration with this study.…”

Solar cycle variations in the annual mean values of the geomagnetic components of observatory data.

“…However, it is not easy to extract the solar cycle variations out of large amplitude secular variations, major parts of which originate in the earth's core. Various attempts have been made to extract the solar cycle variations and to separate them into external and internal parts by spherical harmonic analyses (YUKUTAKE, 1965;COURTILLOT and LEMOUEL, 1976;ALLDREDGE,1976;HARWOOD and MALIN, 1977;ISIKARA,1977;YUKUTAKE and CAIN, 1979). Most of them succeeded in extracting the solar cycle variation in the external component of the first degree spherical harmonic terms (gO1, g11 h11) of the magnetic potential.…”