EarthRef.org Reference Database (ERR) -- Cain et al. 2003

A study was done to determine the difficulty of obtaining a field model for 1995 using magnetic observations taken since that epoch. ¿rsted vector data taken from 1999 to 2001 in conjunction with scalar data from all surface ship surveys and secular variation data from magnetic observatories taken since 1995 were utilized. Spherical harmonic models were constructed for the interval from 1995 through early 2001 using selected, magnetically quiet, nighttime intervals. Corrections were made for Dst variations assuming a constant, i.e., ratio (0.28). The data were corrected for magnetic anomalies using Magsat coefficients (m102389) above n = 21. In order to compute secular change data, some observatory hourly values were reduced to remove the annual variations, which ranged up to 40 nT at a few stations. Coefficients were adjusted up to n = 21 in spatial terms, n = 13 in linear secular variation terms, and n = 12 in parabolic secular variation terms. There was also a 30 nT, n = 1 external term. Component maps of this model (f052101) truncated to n = 10 were compared with the IGRF2000 and generally found to be within 20 nT at the surface except for a 110 nT difference in the northern polar region. The model agreed with the OIFM at satellite altitude to within a few nanoteslas except for a few tens of nanoteslas at the poles. Model differences at 1995 from the IGRF1995 peaked over 800 nT in the region west of South America. However, statistical estimates in this region and epoch show model errors up to several hundred nanoteslas. Use of a linear model or one truncated to lower degree is seen to reduce these differences and maximum errors at 1995, but at the expense of less accuracy elsewhere. Besides the large differences from the 1995 IGRF it was also noted that there are a number of areas where the field changes require parabolic coefficients, even over this short a time span. A study was done to determine the difficulty of obtaining a field model for 1995 using magnetic observations taken since that epoch. ¿rsted vector data taken from 1999 to 2001 in conjunction with scalar data from all surface ship surveys and secular variation data from magnetic observatories taken since 1995 were utilized. Spherical harmonic models were constructed for the interval from 1995 through early 2001 using selected, magnetically quiet, nighttime intervals. Corrections were made for Dst variations assuming a constant, i.e., ratio (0.28). The data were corrected for magnetic anomalies using Magsat coefficients (m102389) above n = 21. In order to compute secular change data, some observatory hourly values were reduced to remove the annual variations, which ranged up to 40 nT at a few stations. Coefficients were adjusted up to n = 21 in spatial terms, n = 13 in linear secular variation terms, and n = 12 in parabolic secular variation terms. There was also a 30 nT, n = 1 ext