**6.** *PC* **index as a verifier of the solar wind geoefficiency**

In spite of statistically justified agreement in response of *E*KL field and *PC* index to action of the solar wind, the correspondence between *E*KL and *PC* can be essentially distorted in the concrete events. The typical examples of consistency and inconsistency between *E*KL field and *PC* index are presented in **Figure 10**, where the upper panel shows the courses of *E*KL (green) and *PCmean* = (*PCN + PCS*)/2 (violet), the middle panel is for *PCN* and *PCS* indices (blue and red lines), the lower panel shows the *AL*/*AU* indices of magnetic activity (which indicate intensity of negative and positive disturbances in the auroral zone), the substorm onsets being marked by vertical dotted line.

#### *The Polar Cap Magnetic Activity (*PC *Index) as a Tool of Monitoring and Nowcasting... DOI: http://dx.doi.org/10.5772/intechopen.103165*

**Figure 10a** demonstrates concerted changes of *E*KL, *PC,* and *AL* in course of an isolated magnetic substorm on October 2, 2000, when the disturbance started against the background of quiet magnetic field in response to the *PC* growth related to the *E*KL field increase. **Figure 10b** demonstrates a specific event on August 17, 2001, when the substorm started in response to the *PC* index jump, but this jump in the ground-based *PC* index was registered ~10 min ahead of the appropriate increase of the estimated *E*KL field. **Figure 10c** gives example (February 20, 1998), when the electric field *E*KL was unchanged and quiet (*E*KL ~ 1 mV/m), whereas the *PCN* and *PCS* indices demonstrated jump above 2 mV/m, which was accompanied, as usual, by the development of substorm with intensity of AL ~ -400nT. In contrast, on 21 October 1999 (**Figure 10d**) the *E*KL field demonstrated a sharp increase above 2 mV/m for long, but this increase was not followed by the *PC* index growth. It is worthy to note that validity of the *PC* index behavior in all cases was certified by reaction of *AL* index, as a substorm indicator.

It should be reminded that *E*KL field is estimated by data on the solar wind parameters, such as the solar wind speed Vsw and interplanetary magnetic field (IMF) components, available at the OMNI database (https://omniweb.gsfc.nasa. gov/). These parameters are fixed onboard the spacecraft located far upstream of the magnetosphere, usually at the Lagrange point L1, far upstream of the magnetosphere (at the distance of ~1.5 million km from the Earth). Thereupon they are reduced to the Earth's magnetopause, under the silent presumptions that the solar wind observed in the Lagrange point always encounters the magnetosphere, the Vsw and IMF characteristics is not altered on the way from the L1 point to the magnetopause. That is why the inconsistency between the "estimated" *E*KL field and *PC* index should be considered as evidence that the solar wind measured by distant monitors did not contact with magnetosphere at all (case of **Figure 10c**), either touched sideways to magnetosphere (**Figure 10d**) or traveled in space with acceleration, as in case of August 17, 2001 (**Figure 10b**), when the real contact of solar wind with magnetosphere (and jump of *PC* index) occurred ahead of the "estimated" contact. As results [58, 59] showed, the solar wind, fixed by distant monitors, did not contact with the Earth's magnetosphere in about 20% of time history and extended in space with acceleration in ~1.5% of examined substorm events. Under these circumstances, the *PC* index takes on great significance as a filter of the OMNI data applicability for analyses of the solar-terrestrial relationship.
