Unusual behaviour of the IGRF during the 1945–1955 period

Abstract

The International Geomagnetic Reference Field (IGRF) for 1900-2000 is tested in this paper. A peculiar behaviour is found in the time variations of higher-degree coefficients of the IGRF models. During the 1945-1955 period the spherical harmonic coefficients g m n and h m n with n > 7 show unusual jumps, while their variations before and after this period are quite smooth. These irregular variations have little effect on the main features of the surface magnetic pattern. However, when we extrapolate the field pattern downward through the insulating mantle to the core-mantle boundary (CMB), the contributions of the higher-degree coefficients become more important and are likely to affect the shape of the geomagnetic energy spectrum, distribution of magnetic flux, and magnetic determination of the conducting core radius. It seems necessary to re-examine and revise these coefficients in the IGRF models.

Revision of the high-degree Gauss coefficients in the IGRF 1945–1955 models by using natural orthogonal component analysis

“…As for IGRF 1945IGRF , 1950IGRF and 1955, the great deviations are attributed to unusual jumps of the highdegree Gauss coefficients of these models. In fact, ragged time-variations occur not only in the highdegree coefficients (n ≥ 7), but also in some lower-degree coefficients, for instance, g Barton (1997), and figure 1 in Xu (2000)). A complete comparison for all Gauss coefficients is made and shown partly in Fig.…”

“…In order to examine the NOC models, the magnetic energy beyond the core-mantle boundary is calculated by using the revised Gauss coefficients, and compared with those by the original IGRF models (Xu, 2000), as shown in Fig. 9.…”

“…However, Barton (1997) noted some unusual variations in the high-degree Gauss coefficients (mainly for n = 7, 8) in the IGRF 1945the IGRF -1955. Xu (2000) tested all Gauss coefficients up to n = 10, and indicated that these unusual behaviors would lead to unreliable jumps in the magnetic energy and magnetic flux for 1945 attributed these unusual variations to that each of these models is based on an independent data set. These coefficients need revision to avoid misinterpretation of the secular variations.…”

Westward drift in secular variation of the main geomagnetic field inferred from IGRF

“…However, magnetic components usually have more complicated contour maps, since magnetic components are the gradient of the potential. Unusual behaviors in IGRF models (for instance, IGRF1945-1955, (Xu, 2000, 2002) will seriously distort the distribution of the component fields, but they have less significant effects on the magnetic potential. Figures 7(a), (b), (c) and (d) show respectively the SV of the potential W and the components X , Y and Z for [1945][1946][1947][1948][1949][1950].…”

“…It should be pointed out that the unusual jumps in highdegree Gauss coefficient of IGRF 1945-1955(Barton, 1997Xu, 2000Xu, , 2002 seriously distort the contour maps of X , Y and Z components, and make it difficult to accurately estimate drift rate (see Fig. 7 in the next section).…”