THE GEOMAGNETIC-FIELD 1900-1995, INCLUDING THE LARGE-SCALE FIELD FROMMAGNETOSPHERIC SOURCES, AND THE NASA CANDIDATE MODELS FOR THE 1995 REVISION OF THE IGRF

This paper reports on a continuous representation of the main geomagne tic field of degree 13 for the 1990-1995 time period, including a degr ee 1 representation of the field of external origin, designated GSFC(S 95). The model employs a cubic B-spline basis with equi-spaced knots f or the temporal variation in the secular variation of the internal fie ld. Hence, the temporal variation of the spherical harmonic coefficien ts is represented by integrals of cubic B-splines. In the derivation, a suite of different forms is utilized for representation of the exter nal field: (1) GSFC(S95-a), in which the external terms are proportion al to the annual averages of the aa index, (2) GSFC(S95-s), in which t he external terms are represented by unconstrained cubic B-splines, (3 ) GSFC(S95-sc), in which the cubic B-spline representation of the exte rnal field is constrained to be near the GSFC(S95-a) model for years p rior to about 1940, and, (4) GSFC(S95-nx), in which there is no extern al field representation. The NASA candidate models for the 1995 revisi on of the IGRF are extracted from GSFC(S95-sc). Data sources include t he Magsat and POGO satellites, observatory annual means, decimated lan d survey, marine total-field, aeromagnetic, and repeat data. Random da ta uncertainties are assigned by statistical binning procedures, while systematic error is accounted for via the correlated weight matrix pr ocedure of Langel et al. (1989). The data are not sufficient to resolv e all model parameters, and thus, regularization via quadratic penalty functions is employed. For the internal field this included minimizin g the average of the square of the radial field secular variation and acceleration over the core-mantle boundary and through time. Compariso n of the GSFC(S95-sc) model with the ufm1 model of Bloxham and Jackson (1992) for their common time span shows good general agreement, espec ially with respect to secular variation coefficient signatures and ove rall data statistics. The major differences are manifested in a better fitting of Magsat and POGO data by GSFC(S95-sc), but better fitting o f early survey data by ufm1 This is expected and reflects the relative influence of the data and penalty function in the particular model. T he external field of GSFC(S95-sc) exhibits a very prominent solar-cycl e variation in the q(1)(0) coefficient, though with about a 2-year tim e lag. The internal field exhibits a well behaved R-n spectrum through out the time span indicating sufficient constraints being applied to t he poorly observed, high-degree secular variation. Finally, the westwa rd drift synthesized for degrees 2 to 5 shows a 0.76 linear correlatio n with the length-of-day (led) variations of Jordi et al. (1994) when the latter leads the former by 11 years.