Abstract
PC index was originally introduced as a characteristic of the polar cap magnetic activity generated by geoeffective solar wind coupling with the magnetosphere. Subsequent researches showed that the PC index follows changes of the solar wind electric field EKL through the field-aligned current system (R1 FAC) responding to variations of the solar wind parameters. Appearance of magnetospheric disturbances is specified by the PC index value (with a typical threshold level ~ 1.5 ± 0.5 mV/m) and by the PC index growth rate. The disturbance progression strongly follows the PC index variations, the intensity of substorms (AL) and magnetic storms (Dst) being linearly related to the PC magnitude. In view of these statistically justified relationships, the PC index is regarded at present as a proxy of the solar wind energy input into the magnetosphere. A great advantage of the PC index application over other methods, based on the satellite measurements, is a permanent on-line availability of information on the magnetic activity in both northern (PCN) and southern (PCS) polar caps, providing a means for monitoring the magnetosphere state and for nowcasting the magnetic disturbances development.