Cautions for the estimation of equivalent current systems from geomagnetic data

Spherical harmonic geomagnetic data analysis is a convenient technique to estimate equivalent current systems and to separate the external from the internal fields. However, there are certain cautions which need to be taken in the procedure, as demonstrated by some examples of the analyses of geomagnetic daily variations during solar active and quiet periods. Questions concerning the proper level of series truncation, problems with sharp gradients of the northward amplitude in narrow latitudinal zones, and dependence of results on which horizontal component is selected (northward, eastward or both) are discussed for the northern hemispheric equivalent current systems.