Remarks on the mean-square values of the geomagnetic field and its components

When considering functions on the Earth's (spherical) surface, mean-square values are often used to indicate their (relative) magnitude. If a function is separated into it, (essentially) spherical harmonic components then, provided these individual harmonic components are orthogonal over the surface, the concept of spatial power spectrum can be introduced, with each harmonic contributing separately to the total mean square value; this is true for the geomagnetic field vector B, its horizontal component vector H, and its vertical component z. However, because of the lack of orthogonality this concept is not applicable to the horizontal X and y components individually; problems which arise from this are discussed