Reply [to “Comment on ‘Error made in reports of main field decay’”]



I thank Maus et al. for providing this opportunity to explain spherical harmonic analysis (SHA) methods to our Eos readers. Gauss devised the SHA as a means for separating the external and internal geomagnetic field contributions at an analysis spherical surface. The SHA starts with the selection of an analysis center and axis through that center defining the study coordinates of longitude Φ, latitude θ, and radius r of the analysis sphere. Using Maxwell's equations, the three orthogonal geomagnetic field components at the analysis spherical surface are converted to equivalent potential function values to which two special series of terms are then fitted.