**9. Conclusions**

The *PC* index uniquely responds to variations of the interplanetary electric field *E*KL coupling with the magnetosphere, the *E*KL effect being transported in the polar caps by means of the field-aligned currents generated in the equatorial magnetosphere due to the plasma pressure gradients. On the other hand, the *PC* index growth predetermines the development of magnetospheric disturbances (substorms and magnetic storms). These experimental results convincingly testify that *PC* index serves as an indicator of the solar wind energy that enters into the magnetosphere during the solar wind-magnetosphere coupling. Advantage of the *PC* index application over other methods, based on the ground-based or satellite data, is permanent on-line availability of information on the magnetic activity in both northern and southern polar caps and, correspondingly, accessibility of information on the solar wind energy input into the magnetosphere. The *PC* index in this charge might be useful for monitoring the space weather, nowcasting the actual state of the magnetosphere, fitting the solar wind-magnetosphere coupling functions, and validating the utility of the solar wind data presented on OMNI website.

A special procedure agreed in 2011 by the Arctic and Antarctic Research Institute (responsible for the production of *PCS* index) and Space Institute of the Danish Technical University (responsible for the production of *PCN* index), ensures the calculation of the 1-min *PC* indices in quasi-real-time based on data of magnetic observations at the polar cap stations Vostok and Qaanaak (Thule). The *PCN/PCS* indices are freely available at the website http://pcindex.org.
