**5.** *PC* **index as a precursor of the magnetic storms progression**

Term geomagnetic storm is designated for the geomagnetic field depression produced by ring currents flowing in the inner magnetosphere [1]. Intensity of magnetic storms is estimated by 1-hour Dst index [56] or its 1-min analog - *SymH*

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

index. Relationship between the *PC* index and the magnetic storms progression was examined in [57] with the use of the *PC mean* and *SymH* values. The running window of 30-min width was used in the analysis for smoothing the 1-min *PC* and *SymH* indices and *E*KL field to reveal a link between the processes occurring with quite different time scales (the time scale typical of the *E*KL field variations and polar cap magnetic activity is *~*1 minute, whereas the ring currents are formed in the inner magnetosphere with a time scale of tens of minutes). Results [9] have demonstrated that exceeding the threshold level *PC* ~ 1.5 mV/m is a necessary condition for beginning the magnetic storms like to substorms, but duration of the threshold exceeding should be longer than 1 hour to ensure the storm development. Progression of geomagnetic storms generally follows the time evolution of the 30-min smoothed *PC* index, irrespective of the type and intensity of magnetic storms. Correspondingly, the magnetic storm beginning can be identified as a moment when the PC index steadily exceeds the threshold level. Such identification of the storm beginning turns out to be very fruitful in case of storms with positive DCF effect at the initial storm phase, which is provided by the magnetopause currents responded to the solar wind dynamic pressure.

Three types of magnetic storms were separated in [9, 57] based on peculiarities of the *PC* index behavior, as follows: "**classic storms**", related to ICME impact, with clearly expressed maximum of depression, "**pulsed storms**", related to SIR impact, with periodically repeating oscillations in *PC* and *SymH* indices, and "**combined storms**", which are regarded as the effect of simultaneous ICME and SIR action. **Figure 7** shows, as an example, the relationship between the *PC* time evolution (upper panel) and the storm progression of *SymH* (lower panel) for different lengths of the storm growth phase in case of classic storms of different intensity (the scale diminution in **Figure 7c** and **d** should be taken into account). Thin red lines show the run of the *PC* and *SymH* indices in course of individual events, thick black solid lines show the behavior of the *PC* and *SymH* values averaged for each storm category and group, the threshold level of *PC* = 1.5 mV/m being marked by the solid horizontal line on upper panels of each figure. The vertical solid lines indicate the moment of the key date T0when the *PC* value steadily (in lapse >1 hour) exceeded the threshold level of *PC* = 1.5 mV/m.

**Figure 8** shows relationships between the *PC* and *SymH* values for pulsed storms in category of Dst = −(30–60) nT with roughly constant (a) and decayed (b) amplitudes of *PC* fluctuations, in category of Dst = −(60–90) nT with roughly constant amplitude of *PC* fluctuations (c), and in category of Dst = −(90–120)nT with varying amplitude of *PC* fluctuations (d). The beginning of the pulsed storms is determined, like classic storms, by exceeding the *PC* index above the threshold level *PC* = 1.5 mV/m. However, the further development of storms turns out to be quite different: instead of steady *PC* growth, the pulsed magnetic storms demonstrate the repeated irregular *PC* fluctuations with different periods and different magnitudes (roughly constant, either decayed or alternating) extended over ten hours. The appropriate response in the geomagnetic field depression presents the *SymH* fluctuations of modified periods and smoothed amplitudes, which implies the different processes' actions in the magnetosphere. In order to derive the generalized relationship between the *PC* and *SymH* indices for pulsed storms, the *PC* and *SymH* characteristics, averaged over the main phase duration, were used.

The mean values of *PCmax* and *SymHmin* derived for different categories of storm intensity and growth phase duration (see black solid lines in **Figures 7** and **8**) were used in [57] to derive a relationship between the appropriate *SymH*MIN and *PC*MAX quantities for classic and composite storms. Results of the analysis are presented

**Figure 7.**

*Relationships between the* PC *evolution (upper panel) and the* SymH *progression (lower panel) for 5 categories of classic storms: (a) Dst = −(30-60nT), (b) Dst = −(60-90nT), (c) Dst = −(90-120nT), (d) Dst = −(120-200nT) and (e) Dst = −(200-400nT) with different* PC *growth durations [57].*

#### **Figure 8.**

*Relationships between the* PC *and* SymH *indices in case of pulsed storms for category* SymH *= −(30–60) nT with roughly permanent (a) and decayed (b) amplitudes of* PC *fluctuations, for category* SymH *= −(60–90) nT with permanent amplitude of* PC *fluctuations (c), and for category* SymH *= −(90–120) nT with alternating PC amplitude (d) [57].*

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

in **Figure 9** for 26 categories of classic storms (a) and 22 categories of composite storms (b), and for their total (c) with inclusion of the mean data for 4 categories of pulsed storms (olive circles), the standard deviation for each category being marked by vertical bars. The relationships between the storm intensity (*SymH*MIN) and the foregoing *PC*MAXvalue are described best by the linear dependences shown in Figure with corresponding correlation coefficients R.

Delay times ΔT in response of *SymH*MIN to the *PC*MAX occurrence lie in the range from half-hour to some hours, being dependent on dynamics of the *PC* index alterations. The shortest delay times (ΔT ~ 30–45 min) are observed in case of strong but

#### **Figure 9.**

*Relationship between the associated mean values of* SymHMIN *and* PCMAX *for classic (a) and composite (b) storms. Panel (c) presents integrated dependence of* SymHMIN *on* PCMAX*, with inclusion of data for pulsed storms (large squares) [57].*

short PC increases when the ring current is quickly formed at a large distance from the Earth and then quickly deceases. The longer delay times are observed in case of the prolonged and irregular PC index dynamics, which initiates the ring currents formation (and subsequent decay) at various distances from the Earth, with the correspondingly different DR current lifetimes. As a result, the longer and unsteady the *PC* growth period is, the larger is delay time.

Thus, the intensity of magnetic storms (*SymH*MIN) is predetermined by value of the *PC*MAXindex, like magnetic substorms. However, it is well to bear in mind that this correspondence was obtained with the use of mean, averaged for 30 minutes, indices for different categories of storms (see **Figure 7**). The actual value of *SymH*MIN (and delay time ∆T) in each concrete storm event will be depended on the *PC* index dynamics, i.e. on the PC index growth (and decay) rates and duration. It implies that formulas (5)–(7) provide reliable estimations of the storm intensity (*SymH*MIN) for low (< 8 mV/m) *PC* values, the discrepancy between the estimated and actual results being increased while raising the *PC* index value and duration of action.
