**Abstract**

*PC* index was originally introduced as a characteristic of the polar cap magnetic activity generated by geoeffective solar wind coupling with the magnetosphere. Subsequent researches showed that the *PC* index follows changes of the solar wind electric field *E*KL through the field-aligned current system (R1 FAC) responding to variations of the solar wind parameters. Appearance of magnetospheric disturbances is specified by the *PC* index value (with a typical threshold level ~ 1.5 ± 0.5 mV/m) and by the *PC* index growth rate. The disturbance progression strongly follows the *PC* index variations, the intensity of substorms (AL) and magnetic storms (Dst) being linearly related to the *PC* magnitude. In view of these statistically justified relationships, the *PC* index is regarded at present as a proxy of the solar wind energy input into the magnetosphere. A great advantage of the *PC* index application over other methods, based on the satellite measurements, is a permanent on-line availability of information on the magnetic activity in both northern (*PCN*) and southern (*PCS*) polar caps, providing a means for monitoring the magnetosphere state and for nowcasting the magnetic disturbances development.

**Keywords:** are Solar wind—magnetosphere coupling, magnetospheric field-aligned currents, magnetic activity in polar caps, PC index, magnetopheric substorms, magnetic storms, monitoring, and nowcasting

### **1. Introduction**

The term "solar wind" is referred to flows of low-energy solar plasma including the magnetic field, which is ejected continuously by the Sun's surface. The Earth's magnetosphere is a result of the solar wind impact on the dipole-like geomagnetic field, the form and size of the magnetosphere being determined by the solar wind parameters such as the solar wind velocity Vsw and the solar magnetic field |B| named usually as an interplanetary magnetic field (IMF). It is totally accepted that the solar wind energy incomes into the magnetosphere, the accumulated energy being realized in form of magnetospheric substorms and magnetic storms. Geomagnetic storms are

associated with formation of powerful currents flowing around the Earth at the distance of ~3–7 RE and displayed as a planetary depression of the geomagnetic field (*Dst* variation) revealing the most power at low- and mid-latitudes [1, 2]. Magnetospheric (or magnetic) substorms [3, 4] are characterized by even stronger magnetic disturbances up to 2000 nT, but they are typical of limited auroral zone, where they are displayed as aurora and are accompanied by a variety of phenomena and processes in the auroral ionosphere.

The polar cap magnetic activity is one of the specific manifestations of the solar wind influence on the magnetosphere, which is displayed in the high-latitude region disposed of poleward of the auroral zone. As it was shown in [5], the polar cap magnetic disturbances correlate the best with the solar wind electric field determined by formula of [6] *EKL = Vsw\*(By2 + Bz2)1/2sin2θ/2,* where By and Bz are the azimuthal and vertical IMF components and *θ* is the clock angle between geomagnetic dipole and IMF tangentional component *BT = (By2 + Bz2)1/***2**. The corresponding *PC* index was put forward [7] as a measure of the solar wind electric field *E*KL coupling with the magnetosphere. The numerous subsequent studies (see [8, 9]) showed that the *PC* index growth determines the development of magnetospheric disturbances, the intensity of the magnetic storm and substorms being related to the *PC* index value. Thus, the *PC* index proved to be strongly responding to the solar wind *E*KL field changes, and regulating, afterwards, the development and intensity of the magnetospheric disturbance. Based on these experimental results, the International Association of Geomagnetism and Aeronomy (IAGA) approved the *PC* index as "a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling" [10]. The 1-min *PC* index is calculated on-line by magnetic data from near-pole stations Qaanaaq (Thule) in the Northern hemisphere (*PCN*) and Vostok in the Southern hemisphere (*PCS*) beginning in 1997. The unified method for derivation of the *PCN* and *PCS* indices was put forward in [11].

In this chapter the following topics, revealing the *PC* index significance, are examined: mechanisms of the solar wind influence on the polar cap magnetic activity, relationship between the *PC* index and the solar wind electric field *E*KL, relationship between the *PC* index and magnetospheric disturbances, the *PC* index use for checking the actual state of magnetosphere.
