Brief Description:
This software package contains the FORTRAN subroutine FELDG to calculate
magnetic field components and the subroutines SHELLG and INTELG to calculate
McIlwain's L parameter. By using a polynomial representation of the geomagnetic
potential with inverse Cartesian coordinates, the algorithm for the field
computation is considerably simplified and can easily be extended to obtain
spatial derivatives of higher order. FELDG can be used with Gauss- normalized
as well as Schmidt-normalized magnetic field coefficients for any of the
internal source field models. SHELLG determines McIlwain's L-value by
integrating along the magnetic field line to the conjugate point. The advantage
of using the inverse coordinates system is a significant reduction in the
number of steps needed to evaluate the integral invariant. The INTELG makes use
of a precalculated (with SHELLG) tabulation of L-values for a specific magnetic
field model. L is determinate by interpolation and Fourier expansion from this
table. Such a procedure reduces the computation time for L considerably. Kluge
(1970) reports a root- mean-square error of about 0.05% for L-values up to ten.
Pre-calculated L-tables are, however, available only for the IGRF 1965, GSFC (12/66), and POGO (10/68) magnetic field models. Hilberg (1971)
compared the L-values calculated with INVAR, SHELLG, and INTELG for a wide
range of latitudes, longitudes, and altitudes. He found differences up to 5%,
with the INVAR L- values always exceeding the other two calculations. The study
also notes artificial discontinuities in the longitudinal variation of L-values
obtained with INVAR. The average execution time for B
and L calculation of INVAR, SHELLG, and INTELG is 70, 46, and 12 msec.
Availability: FELDG and SHELLG are included in the BILCAL/IGRF package.
References:
G. Kluge, A Generalized Method for the Calculation of the Geomagnetic Field
from Multipole Expansions, European Space Operations Centre, ESOC Internal
Note No. 61, Darmstadt, 1970.
G. Kluge, Computer Program SHELL for the Calculation of B and L from Models of the Geomagnetic Field, ESOC Internal Note No. 67, Darmstadt, 1970. (B20244)
G. Kluge, Computation of the Magnetic Shell Parameter L from a Compressed Data Table, ESOC Internal Note No. 72, Darmstadt, 1970.
G. Kluge, Direct Computation of the Magnetic Shell Parameter, Comp. Phys. Comm. 3, 31, 1972.
G. Kluge and K. G. Lenhart, Numerical Fits for the Geomagnetic Shell Parameter, Comp. Phys. Comm. 3, 36, 1972.
R. H. Hilberg, Magnetic Shell Parameter Calculations, KMS Industries, Report KMS 71-02, 1971
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