**8. Discussion**

The *PC* index has been affirmed as a proxy of the solar wind energy input into the magnetosphere (Resolution No. 3, IAGA 2013; Resolution No. 2, IAGA 2021). What are the physical implications of this certification? There are three well-known concepts explaining the solar wind's influence on the magnetosphere.

According to the first concept, put forward by [49], the IMF carried by the solar wind contacts with the terrestrial magnetic field at the dayside magnetopause, where the geomagnetic field is northward. When the IMF is southward, the terrestrial field lines will interconnect with the interplanetary field lines, and the electric potential *ΔV = l*eff*E*(where *E* is the interplanetary electric field and *l*eff is the effective extent of reconnection zone at the dayside magnetopause) will be mapped along infinitely conducting magnetic field lines into the polar ionosphere ensuring the cross-polar cap potential. As a result, the antisunward plasma convection is generated in polar caps, where the ionospheric plasma attached to interplanetary field lines moves together with the solar wind. The merged field lines will reconnect again in the tail neutral sheet, giving rise to the return magnetic flow, which ensures the dayside magnetosphere balance. The Dungey's hypothesis provided the big impulse for the researches of the IMF influence on processes in the magnetospheric. Nevertheless, the hypothesis was criticized from the outset with reference to such theoretical problems as validity of the frozen-in condition in the real magnetosphere, ignoring the turbulence in the real magnetosheath and plasma sheet, necessity to make distinction between the physical laws in passive and active plasma regions and so on.

It should be noted that the original Dungey hypothesis does not even mention the field-aligned currents owing to absence of any information about their existence in those times. At present, the FAC systems registered in the satellite experiments are commonly regarded as favoring the Dungey concept. Indeed, the NBZ FAC system fixed in the near-pole area and BY FAC system fixed in the day-time cusp area [30, 32, 34, 35] can be regarded as a result of interconnection of the interplanetary and terrestrial fields under influence of the IMF northward BZ and azimuthal BY components. However, it is well to bear in mind that these FAC systems are always observed against the background of the permanent R1 FAC system, which continues to exist even under condition of the northward IMF. Moreover, the R1 FACs are positioned far inside the magnetosphere, within the plasma sheet boundaries [39, 62]. The permanent availability of the R1 FAC system (affected by *EKL* field) in presence of independent NBZ (or BY) FAC systems, responding to influence of the northward

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

(or azimuthal) IMF influence, seems to be inconsistent with the interconnection as a reason of the permanent availability of the R1 FAC system. It is possible, that mechanism of interconnection generates, under condition of southward IMF, a specific "SBZ" FAC system (in a similar manner to NBZ and BY systems), but product of this "SBZ" system is added to the effect of permanent R1 FAC system.

The second concept, known as a "viscous-like interaction", was put forward by *Axford and Hines* [63], who suggested that the antisunward plasma convection on closed field lines along the boundary layer of magnetosphere can be ensured via the transfer of the solar wind momentum to the magnetospheric plasma across the magnetopause. At present the viscous-like interaction is regarded as a little effective mechanism, it may be responsible for not more than 15% of the polar cap voltage under the normal solar wind conditions. In addition, it was shown [64] that the particles that originated in the magnetospheric low-latitude boundary layer (LLBL) are positioned in the 09–15 MLT time sector equatorward of day-time cusp, whereas the mantle particles are positioned in the same sector poleward of cusp region. The regularity has been confirmed later by Wing et al. [40]. It implies that the whole boundary layer between the solar wind and magnetosphere plasmas is mapped into the narrow daytime sector, not into the dawn and dusk sectors of the auroral oval, where the R1 FAC system is positioned.

The third concept, formulated ten years later by [65], was elaborated in [66, 67]. According to this concept, the solar wind impact on magnetosphere violates the magnetostatic equilibrium in the outer magnetosphere resulting in the formation of the plasma pressure gradients within the magnetosphere. Redistribution of the plasma pressure leads to generation of large-scale dawn-dusk electric field and initiates the magnetospheric field-aligned currents responsible for cross-polar cap electric potential. The concept of the field-aligned currents generated in the equatorial magnetosphere due to formation of the plasma pressure gradients was supported later by statistically justified data on the plasma gradients distribution in the plasma sheet [40, 41]. Thus, *Tverskoy's* concept predicted, in fact, the existence of the magnetospheric field-aligned currents discovered by [24–26]. Concept of [65] declared that plasma gradients in the magnetosphere are determined by the solar wind impact on the magnetosphere, however, the mechanisms ensuring the link between the solar wind parameters (Vsw and IMF BY, BZ components) and the magnetospheric plasma redistribution have not been defined. As for solar wind dynamic pressure (Psw) influence, it should be reminded that the Psw impulses compressing the magnetosphere lead to magnetic disturbances (polar cap magnetic activity and substorms) only if they are accompanied by the corresponding changes in the *E*KL field [68].

Thus, the experimental results unambiguously testify that the geoeffective solar wind generates, through the field-aligned currents, magnetic activity in the polar caps. The *PC* index, characterizing the polar cap activity, demonstrates the best relation to the electric field *E*KL *= Vsw\*(By*<sup>2</sup>  *+ Bz*<sup>2</sup> *)*1/2*sin*<sup>2</sup> *θ/2,* which is termed as the solar wind electric field. It should be kept in mind that *E*KL field is displayed only in the Earth's coordinate system (i.e. in the magnetosphere) which is stationary relative to the moved solar wind. Nevertheless, neither of the three above concepts does explain the link between the *PC* index and *EKL* field. What is the *PC* index significance in such a case?

The *PC* index serves as an indicator of capacity of the solar wind influence on the magnetosphere resulting in generation of the electric fields and field-aligned currents responsible for the magnetospheric convection and polar cap magnetic activity and for development of magnetic disturbances. It is very significant that the value and

behavior of the *PC* index are related to the solar wind parameters being independent of the intensity and duration of the magnetospheric disturbances. Indeed, features of the *PC* index growth define the onset and intensity of the disturbance, the *PC* index behavior being independent on the disturbance development; to the contrary, the *PC* index decline below the threshold level (~ 1.5 mV/m) is followed by prompt decay of disturbances. This specific feature of the index makes it possible to use the *PC* index not only for monitoring the magnetosphere state but also for nowcasting the disturbance progression.
