**2. Mechanism of the solar wind influence on magnetic activity in the polar caps**

Magnetic alterations typical of the polar caps in periods free of magnetic disturbances in the auroral zone (substorms) were found by *Nagata and Kokubun* [12]. They were named DP2 magnetic disturbances [13] to distinguish them from magnetic substorms (DP1). Current systems of DP2 disturbances consist of two vortices with currents flowing sunward in the near-pole region without any peculiarities in the auroral zone, the current intensity is correlated with the southward IMF [14, 15]. As it was shown later [16, 17], the two-vortex DP2 current system is terminated by the geomagnetic latitudes Ф = 50–60° with the focuses located at the morning and evening poleward boundaries of the auroral oval, and the DP2 disturbances can be observed in the polar cap in absence of the southward IMF.

Other types of magnetic disturbances typical of the sunlight summer polar cap are the "near-pole DP variation", named as DP3 disturbances, which are observed under conditions of northward IMF [16, 18], and magnetic disturbances related to *The Polar Cap Magnetic Activity (*PC *Index) as a Tool of Monitoring and Nowcasting... DOI: http://dx.doi.org/10.5772/intechopen.103165*

azimuthal IMF [19–21], named as DP4 disturbance [16]. The DP3 system consists of two vortices with opposite anti-sunward directed currents in the very limited nearpole area. The DP4 system includes currents flowing along geomagnetic latitudes with maximal intensity in the daytime cusp region (Ф ~ 80°), the current direction being determined by sign of the IMF azimuthal component.

**Figure 1** shows current systems of DP2, DP3, and DP4 magnetic disturbances, generated under action of the southward BZS (a) and (b), northward BZN (c), and azimuthal BY (d) IMF components [16]. The multi-functional analysis of relationships between the IMF and geomagnetic variations has been fulfilled by *Troshichev and Tsyganenko* [17] to separate effects of the IMF Bx, By, Bz components in case of their combined influence. Results of the analysis have also demonstrated availability of the DP2, DP3, and DP4 current systems associated with action of the southward, northward, and azimuthal IMF components, respectively. The electric field structure

**Figure 1.**

*Current systems of DP2, DP3, and DP4 disturbances related to variations of IMF components: (a) southward BZS = -1nT, (b) southward BZS = -0.25nT, (c) northward BZN, (d) azimuthal BY [17].*

and intensity derived from magnetic DP2 and DP3 disturbances [16] turned out to be in total agreement with results of direct measurements of electric fields at satellite OGO-6 [22] and in balloon experiments [23].

Mechanism of generation of the polar cap magnetic disturbances became clear when the field-aligned magnetospheric currents were detected onboard the OGO 4 spacecraft [24] and Triad spacecraft [25, 26]. These experiments have fixed a layer of the field-aligned currents on the poleward boundary of the auroral oval (Region 1 FAC system), with currents flowing into the magnetosphere in the morning sector and flowing out of the ionosphere in the evening sector, and layer of the field-aligned currents on the equatorward boundary of the auroral oval (Region 2 FAC system), with opposite directed field-aligned currents. The currents in Region 1 are observed permanently, even during the quiet conditions, whereas Region 2 currents are detected only in periods of magnetic disturbances (*Iijima and* [27]). The intensity of the field-aligned currents demonstrates the strong dependence on the IMF BY and BZ components and the solar wind electric field [28, 29]. During substorm events, the average latitude width of Regions 1 and 2 increases by 20–30% and complicated small-scale structures are superimposed upon the large-scale field-aligned currents, especially in the nighttime sector (**Figure 2**).

The field-aligned currents of reverse polarity were found [30] in the near-pole area, at latitudes of Ф > 75°, under conditions of the IMF northward component (not shown in **Figure 2**). Later these currents were named as NBZ FAC system [31, 32]. The specific BY FAC system, controlled by the azimuthal BY IMF component, was separated in the daytime cusp region [33–35]. This FAC system consists of two current sheets located on the equatorward and poleward boundaries of the cusp, the current directions and intensity being determined by the IMF BY sign [34, 36]. Influence of the BY FAC system strongly distorts the effects of the regular R1 and NBZ FAC patterns.

It should be noted that R1 and R2 FAC systems presented in [24–26] were outlined by the poleward and equatorward auroral oval boundaries. The same result was

**Figure 2.** *Pattern of field-aligned currents derived from Triad data [25].*

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

obtained by [37] by measurements onboard the Viking and DMSP-F7 satellites and by [38] by measurements onboard the ISEE 1 and 2 satellites. It implies that generators of R1/R2 FAC systems are positioned within the closed magnetosphere, not on the dayside magnetopause. Results of the R1/R2 FAC mapping to the equatorial plane [27, 39] have also demonstrated that R1 and R2 field-aligned current systems are located within the closed magnetosphere. Availability of the appropriate plasma pressure gradients in the closed equatorial magnetosphere has been displayed in [40, 41].

The numerical simulations of ionospheric electric field and currents generated by field-aligned currents were fulfilled in [42, 43] with use of satellite data [25, 26] on the FAC intensity and structure and data on ionospheric conductivity in the polar caps. The results of numerical simulations have clearly demonstrated that DP2, DP3, and DP4 magnetic disturbances in the polar caps are generated by the corresponding R1, NBZ, and BY FAC systems, the R1 FAC system being presented constantly irrespective of the IMF BZ polarity. As this takes place, magnetic effect of the ionospheric Pedersen currents in the summer polar cap with high-conductive ionosphere is roughly compensated by the distant magnetic effect of the field-aligned currents, as a result, the magnetic disturbances distribution is determined by ionospheric Hall currents, in full agreement with the theorem of *Fukushima* [44]. On contrary, in the winter polar cap with the low-conductive ionosphere, effect of the ionospheric Hall and Pedersen currents is insignificant, and the polar cap magnetic disturbances are determined by the distant effect of the field-aligned currents. The conclusion was made [45] that the field-aligned currents are responsible for generation of magnetic activity in the polar cap.

Relationship between the *PC* index and really observed field-aligned currents under concrete conditions was examined in [46] based on measurements onboard the SWARM satellites. The analysis, carried out for growth phase of isolated substorms started against the background of magnetic quiescence, showed that increase of the R1 FAC intensity in dawn and dusk sectors of the auroral oval was always accompanied by the *PC* index growth. On contrary, correlation between the *PC* index and field-aligned currents in the noon and the midnight sectors of the oval during the substorm growth phase was absent. In paper [47] relationship between the R1 field-aligned currents and *PC* index was examined for different types of magnetospheric disturbances. The high correlation between the *PC* index and FAC was found, with zero time lag, for all examined events. Thus, the experimental results are indicative of the magnetospheric field-aligned currents as a driver of the polar cap magnetic activity.
