The Geomagnetic Field 1900-1995, Including the Large-Scale Field from Magnetospheric Sources, and the NASA Candidate Models for the 1995 Revision of the IGRF

Abstract

This paper reports on a continuous representation of the main geomagnetic field of degree 13 for the 1900-1995 time period, including a degree 1 representation of the field of external origin, designated GSFC(S95). The model employs a cubic B-spline basis with equi-spaced knots for the temporal variation in the secular variation of the internal field. Hence, the temporal variation of the spherical harmonic coefficients is represented by integrals of cubic B-splines. In the derivation, a suite of different forms is utilized for representation of the external field: (1) GSFC(S95-a), in which the external terms are proportional to the annual averages of the as index, (2) GSFC(S95-s), in which the external terms are represented by unconstrained cubic B-splines, (3) GSFC(S95-sc), in which the cubic B-spline representation of the external field is constrained to be near the GSFC(S95-a) model for years prior to about 1940, and, (4) GSFC(S95-nx), in which there is no external field representation. The NASA candidate models for the 1995 revision of the IGRF are extracted from GSFC(S95-sc). Data sources include the Magsat and POGO satellites, observatory annual means, decimated land survey, marine total-field, aeromagnetic, and repeat data. Random data uncertainties are assigned by statistical binning procedures, while systematic error is accounted for via the correlated weight matrix procedure of Langel et al. (1989). The data are not sufficient to resolve all model parameters, and thus, regularization via quadratic penalty functions is employed. For the internal field this included minimizing the average of the square of the radial field secular variation and acceleration over the core-mantle boundary and through time. Comparison of the GSFC(S95-sc) model with the ufml model of Bloxham and Jackson (1992) for their common time span shows good general agreement, especially with respect to secular variation coefficient signatures and overall data statistics. The major differences are manifested in a better fitting of Magsat and POGO data by GSFC(S95-sc), but better fitting of early survey data by ufml. This is expected and reflects the relative influence of the data and penalty function in the particular model. The external field of GSFC(S95-sc) exhibits a very prominent solar-cycle variation in the q° coefficient, though with about a 2-year time lag. The internal field exhibits a well behaved R,,, spectrum throughout the time span indicating sufficient constraints being applied to the poorly observed, high-degree secular variation. Finally, the westward drift synthesized for degrees 2 to 5 shows a 0.76 linear correlation with the length-of-day (lod) variations of Jordi et al. (1994) when the latter leads the former by 11 years.

International Geomagnetic Reference Field: The Seventh Generation

“…This is consistent with the results of animations of longer spherical harmonic time sequences at the Earth's surface (Thompson, 1984) and at the core-mantle boundary (Bloxham et al, 1991). Sabaka et al (1997) consider the behaviour of the continuous GSFC (S95-sc) model from 1900 to 1995. They compare their model with continuous model ufm I (Bloxham and Jackson, 1992), examine the evidence for geomagnetic j erks, and discuss the correlation between westward drift and decade variations in the length of day.…”

“…This is unsatisfactory for applications that require a fixed reference standard (e.g., reduction of aeromagnetic survey data to obtain anomaly values). A continuous model is physically plausible, would appeal to persons studying the temporal variation (see Sabaka et al, 1997), and would be convenient to use. WG V-8 resolved not to pursue this approach for the 1995 revision of the IGRF, but to re-consider it in 1997.…”

Evaluation of the dynamic cutoff rigidity model using dosimetry data from the STS-28 flight

“…with the International Geomagnetic Reference Field for epoch 1995.0 (Sabaka et al, 1997) in the manner / described by Fliickiger and Kobel (1990). The Boberg et al (1995) extension was used to describe the magnetospheric fields for magnetic activity levels exceeding Kp values of 5.…”

Comparison of Candidate Models for DGRF 1990 and IGRF 1995

“…The GSP and GS models utilized a careful selection of data from all available sources: magnetic observatories, repeat stations, satellite surveys, aeromagnetic surveys, marine magnetic surveys, and land magnetic surveys (Sabaka et al, 1997).…”