A discussion of sources and description of the earth's magnetic field and its secular variations.

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The first degree external spherical harmonic coefficients are obtained for each year of a 12-year period centered at 1958.5, using annual mean values of X, Y, and Z components from 54 magnetic observatories. Values of the coefficient g0e of the zonal first degree harmonic clearly show a solar cycle variation. The peak-to-peak amplitude of the variation is approximately 29 nT, which is consistent with values obtained earlier by first filtering data to retain only those variations having periods near the solar cycle and then subjecting the filtered data to a spherical harmonic analysis. The variation in g0e is found to correlate extremely well with the annual mean Ds, index and with the annual number of days having Ap>60. Based on statistics of the mean square successive difference, an explanation is presented why the obtained solar cycle variation in g0e, which is very much smaller in magnitude than the standard deviations calculated by the conventional method, is statistically meaningful. The determined absolute (not relative) values of gie are in agreement, within several nanotesla, with the expectation from a theoretical model of solar wind compression of the magnetosphere and an analysis of the Dst index.

Section: Introductionmentioning

confidence: 99%

Solar cycle variation in geomagnetic external spherical harmonic coefficients.

Alldredge

Stearns

Sugiura

1979

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“…However, the geomagnetic field does undergo seasonal and shorter fluctuations, and some form of correction for these effects should be made in using a field model as a reference for a spatial magnetic survey (ALLDREDGE and STEARNS, 1974). In Table 4, it is seen that POGO6/71 gives a near zero mean residual associated with a low r.m.s, deviation for the 1970.9 airborne survey, which is approximately midway between the epochs of the other two surveys.…”

Section: Some Comparisons Among Geomagneticmentioning

confidence: 97%

“…The model POGO6/71 may be useful for such studies in this region. HAINES and HANNAFORD (1972, 1974, 1976 and by HANNAFORD andHAINES (1969, 1974). No corrections for diurnal and other disturbances are normally made to the data.…”

Section: Some Comparisons Among Geomagneticmentioning

confidence: 99%

“…Since the typical survey procedures mean that individual flights are distributed across the survey area and through local time in a fairly non-systematic manner, it is unlikely that systematic biasses due to diurnal and similar effects are present in the data set viewed as a whole. Three extensive airborne vector magnetometer surveys were conducted by the Earth Physics Branch (in 1969.1, 1970.9, and 1972.9) over western and Arctic Canada (HAINES and HANNAFORD, 1972, 1974, 1976 have been made by using analyses of flight intersection errors (HANNAFORD andHAINES, 1969, 1974), with standard errors of about 40 nT in the vertical component being obtained. This figure reflects the combined effect of magnetometer and platform inaccuracy, uncertainties in the aircraft field, navigation errors and temporal geomagnetic variations.…”

Section: Some Comparisons Among Geomagneticmentioning

confidence: 99%

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Some comparisons among geomagnetic field models, observatory data and airborne magnetometer data: Implications for broad scale anomaly studies over Canada.

Coles¹

1979

The secular change characteristics of several geomagnetic field models have been compared with the observed secular changes at sixteen magnetic observatories in Canada and nearby regions, for the period 1960 to the present. The significance of inadequacies in the secular terms of the field models when used as reference fields for the compilation of large scale magnetic surveys is discussed. Errors of the order of 50 nT/year are encountered with some models. A model derived primarily from satellite data shows the best description of the secular change in the vertical component in western Canada during the interval 1969 to 1973, which includes the epochs of several airborne surveys. The revised IGRF75 describes the secular changes more closely than does the original IGRF (International Geomagnetic Reference Field), but cannot account for recent secular accelerations. Comparisons between field models and an extensive airborne data set show good agreement in the vertical component between the airborne survey data and two satellite-based models. Correction surfaces are developed to reduce the survey data to a common epoch, by considering differences between observatory monthly means and the corresponding values predicted by the field model.

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Using only internal coefficients they were surprised to find that using sin 0 weighting they reproduced the IGRF coefficients almost exactly, but that without this weighting there were significant changes in the zonal (m=0) coefficients of odd degree.

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mentioning

confidence: 99%

The Effect of a Field of External Origin on Spherical Harmonic Analysis Using Only Internal Coefficients

Lowes¹

1976

ALLDREDGE and STEARNS (1974)discussed the effect on spherical harmonic analysis of the geomagnetic field of omitting the n=1 terms of external origin (i. e. those associated with the ring current). In a numerical test they added a 100nT uniform field to a field synthesised from a set of (internal) IGRF coefficients, and then reanalysed by least squares fitting to (X, Y, Z) component data at points on a 10X10grid.Using only internal coefficients they were surprised to find that using sin 0 weighting they reproduced the IGRF coefficients almost exactly, but that without this weighting there were significant changes in the zonal (m=0) coefficients of odd degree. Their result is in fact to be expected. With the sin 0 weighting, the least squares' process is to a very good approximation equivalent to determining the coefficients by integrating over the sphere. Now, over the sphere, any harmonic field of external origin is orthogonal to any harmonic field of internal origin (see e. g. JAMES, 1973); similarly the field of any one harmonic is orthogonal to the field of any other harmonic, and each harmonic contributes independently to the mean square vector field (LowES, 1966). So the best the least squares can do is to reproduce the internal coefficients and leave the 100 nT external field unfitted.However without the sin 0 weighting the polar regions are overweighted and there is no longer orthogonality. The least squares will do its best to reduce the (polar weighted) residual field by changing the internal coefficients, and it does in fact manage to reduce the total (polar weighted) residual field to 96nT (vector) r. m. s. by introducing a spurious internal field of 24nT r. m. s. (Properly veraged over the surface the r. m. s. residual field will in fact be (1002+242)1/2 103nT.) That the errors are in the zonal harmonics of odd degree follows from the geometry of the uniform field. It is interesting that when they analysed the total intensity F they obtained the much larger spurious internal fields of 92 and 94nT respectively with and 515