**7. Relationships between** *EKL* **field and** *PC, AL, Dst* **indices in 23/24th cycles of solar activity**

Invariability of relationships between the *E*KL, *PC* and *AL, Dst* parameters were checked [60] with the use of the daily quantities of these parameters in course of 23/24 solar cycles. **Figure 11** shows, as an example, courses of the *E*KL*,* and *PCN, PCS* daily values in years of solar maximum (2000, 2015) and solar minimum (2008, 2019). One can see that daily values of *E*KL and *PC* on average were higher the threshold level 1.5 mV/m in years of solar maximum and lower the level in years of solar minimum, the *PCN, PCS* (and *PCmean*) indices being strongly responded to alterations of the *E*KL field irrespective of the solar activity phase.

To display relationships between the *PC* and *AL*, *Dst* indices, which are quite different in scales of values and changes, the relative values of *PC*, *AL* and *Dst* (i.e. the running quantities related to their average value for period under examination) were taken in [60]. **Figure 12** shows the courses of the relative daily quantities *PCmean,* 

#### **Figure 11.**

*Coordinated variations of the daily values of* EKL *field and* PCN, PCS *indices in epochs of solar maximum (2000, 2015) and solar minimum (2008, 2019) [60].*

#### **Figure 12.**

*Coordinated variations of daily values of the* PCmean *and* AL, Dst *indices in epochs of solar maximum (2000, 2015) and solar minimum (2008, 2019) [60].*

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

*AL, Dst* in 2000, 2015 and 2008, 2019 years, the *PC mean, AL* and *Dst*traces being marked, correspondingly, by black, red, and green colors. It should be reminded in this connection that correlation between the two quantities remains the same irrespective of the kind, absolute or relative, of quantity. One can see that red traces of *AL* index in **Figure 12** practically hide the traces of *PCmean* index. It means that the daily values of *PC* and *AL* indices alter in almost one-to-one correspondence, irrespective of solar activity, which was maximal in 2000, 2015 and minimal in 2008, 2019. The perfect correspondence in courses of the daily *PCmean* and *AL* quantities is consistent with excellent correspondence in variations of the 1-min *PCN, PCS,* and *AL* indices in course of substorm events, noticed in [8, 9].

The daily *Dst* index demonstrates much worse correspondance with *PCmean* than with *AL* index in agreement with reduction of correlation between the corresponding 1-min values while passing from *AL* index to *SymH* index [9]. Indeed, the *AL* index reacts to *PC* index changes in a few minutes, whereas the storm progression responses to the maximal *PC* value with longer and different delay times (from 30 minutes to some hours), as a result, the storm return phase can last for some hours without evident counterparts in the *PC* index (see for example green traces in **Figure 11**, which extended beyond black and red traces). The reason is that the *PC* index dynamics (i.e. by changes in value, duration and rate of the *PC* jumps and drops), determines the different disposition of the DR currents within the magnetosphere and, therefore, the different times of their growth and decay.

As **Table 1** shows, the correlation between *E*KL field and *PCN, PCS* indices is higher in years of solar minimum (2008, 2019) and worse in years of solar activity maximum (2008, 2015). According to [60] reduction of correlation is related to solar protons (SPE), which intrusion in polar caps extremely increased ionospheric conductivity in the polar caps and violated the regular relationship between the *E*KL and *PC* values. The daily *AL* index demonstrates the perfect correlation with *PC* irrespective of the solar activity phase.


#### **Table 1.**

*Relationships between the daily values of* EKL *field and* PCN, PCS *indices and between the* PCmean *and* AL, Dst *indices described by linear functions* (Y = a + b\*X) *and corresponding coefficients of correlation (R) for years of solar maximum (2000, 2015) and solar minimum (2008, 2019) and for entire period (1997–2020).*

Correlation between the yearly values of *E*KL, *PC* and *AE*, *Dst* indices in course of 23/24 solar activity cycles was examined [61]. The analysis has demonstrated the remarkable consistency in their variations in 1998–2019, the parameters being perfectly correlated with the yearly values of solar wind velocity *Vsw* and interplanetary magnetic field, as well as with their product electric field *E*KL. As this takes place, the best correlation of the yearly values *E*KL, *PC, AE*, *Dst* was observed with the total IMF field |*B*| (R = 0.96, 0.84, 0.88, 0.86 correspondingly), not with the IMF Bz component (R = 0.81, 0.72, 0.69, 0.74), in evident contradiction with the Dungey's concept.

Results of analyses [60, 61] indicate that calibration coefficients determining relationship between the *E*KL field and *PC, AL, Dst* indices remain unchanged during 23 and 24 cycles of solar activity. It means that mechanisms ensuring the solar wind influence on magnetosphere are valid irrespective of solar activity.
