Effects of solar activity on annual means of geomagnetic components

Abstract

Fourier analyses were made of the geomagnetic horizontal annual means from 20 observatories and the vertical component annual means from 17 observatories from 1900 to 1972 for most of the observatories. Curves synthesized from the Fourier coefficients for periods from approximately 4 to 13 yr have patterns easily explained in terms of a westward-flowing ring current that generally increases as the sunspot numbers increase but lags behind the sunspot numbers by a year or two. External spherical harmonic coefficients should be included in models of the earth's magnetic field. By using the horizontal component curves, synthesized as described above, for seven selected observatories and the vertical component curves for six selected observatories, values of both internal and external first-order zonal spherical harmonic coefficients were determined for a 22-yr span. The amplitude and phase of the ratio of these coefficients are consistent with an earth having a conductivity of 33 mhos/m from the center of the earth to a radius of 0.9 RE surrounded by a nonconducting shell to the surface. [1952] indicated that geomagnetic secular change impulses were evident in most observatory records. An impulse was defined as the apparent sudden change in the slope of a curve of annual means of a geomagnetic component versus time. The impulses for all observatories tended to occur simultaneously (impulse epochs). They found the impulse concept useful in the development of isoporic charts. Weber and Roberts [1951] and Walker and O'DeaAlldredge [1975] has shown that the impulse concept can be adequately explained in terms of solar activity and the resulting buildup and decay of the ring current and related effects.In this paper, magnetic data are processed in a direct way to show effects which are clearly related to the solar cycle. Because only the long-term average effects of the solar cycle activity on the geomagnetic field are being investigated, the annual means of the magnetic components for all days are used. [1974] have made similar studies, using different analysis techniques. Although the results of this paper are in general agreement with theirs, there are, as we will see, important differences. Table 1. When two observatories are shown in the first column, the period of operation of each is indicated on the next line. In each case these were nearby stations (only a few tens of kilometers apart). In most cases, in the analysis for these pairs of stations the base line of the first was shifted to match that of the second, and then the data for both stations were treated as though they were one station. The two observatories are close enough that the geomagnetic latitude given for the second observatory can adequately represent the entire data set. This study was limited to H and Z because they should reveal the effects of external sources more directly than would other components, such as Yukutake [1965], Bhargava and Yacob [1969], and Rivin D^T^ PROCESSING TO SHOW SOLAR EFFECTS This study utilizes the annual means of t...

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.

“…On the other hand, YUKUTAKE (1965), ALLDREDGE (1976), RIVEN (1976), COURTILLOT andLE MOUEL (1976), andHAR WOOD andMALIN (1977) have found that values of the first degree external coefficients vary consistently with the solar cycle. In all of these analyses, when the solar cycle effect was being investigated, the data were first filtered to retain only those variations with periods near the solar cycle.…”

Geomagnetic variations with periods from 5 to 30 years at Hong Kong.

“…The data from several dozen observatories had been used in analyses of the secular variations by many authors, e.g. YUKUTAKE (1965), BHARGAVA and YACOB (1969), CURRIE (1973), RIVIN (1974), ALLDREDGE (1976ALLDREDGE ( ,1977, COURILLOT and MODEL (1976), SRIVASTAVA and AB-BAS (1977). However they did not use the data of Hong Kong, except RIVIN's article (1974).…”

“…The absence of a solar cycle (10-11 years) is important for the analysis, which will be discussed in a later paragraph. In order to compare the phase relation of these signals with the sunspot numbers, this paper used the method similar to ALLDREDGE (1976). The residual values were fit by a sum of trigonometric sine terms, the data being treated as an odd function with a fundamental period twice that of the observed interval.…”