Abstract: The relationship of geomagnetic field at the earth's surface with S_q^e current systems is analysed. It is pointed out that there exsist some differences between the true focus of the S_q^e current system and the "apparent focus" determined by using S_q^e variations in the geomagnetic field at the earth's surface. Practical approaches to estimating S_q^e current focus by means of geomagnetic data are discussed and examined by using geomagnetic data during the period of IGY/C.