The magnetic fields caused by the magnetospheric and ionospheric current systems are calculated. These current systems are Chapman-Ferraro current system (C-F), magnetotail current system (MTL), symmetric ring current system (SRC), region 1 field-aligned current system (FAC1), partial ring current-region 2 field-aligned current-ionospheric current system (PRFI), and the ionospheric dynamo current system (IDC). The variable magnetic fields recorded at the ground surface are the superposition of these fields and other disturbance fields. The spatial-temporal characteristics of these fields are comparatively studied, and a method for separating dynamo-field from the total field is developed. In order to describe conveniently dynamo-field intensity, a new magnetic index, “S_q index”, is proposed.