**1. Introduction**

The magnetosphere comprises a number of current systems contained within the geomagnetic bubble carved out in the solar wind flow of tenuous ionized gasses flowing from the solar surface carrying solar magnetic fields out into space.

Dungey [1] formulated the concept of magnetic merging processes taking place at the front of the magnetosphere between the interplanetary magnetic field (IMF), when southward oriented, and the geomagnetic field, followed by the draping of the combined solar and geomagnetic fields and associated ionized plasma over the poles creating an elongated magnetospheric structure. In the extended magnetospheric tail region, the geomagnetic field from the northern and southern hemisphere would reconnect releasing the solar magnetic fields. The restored geomagnetic field would then be convected sunward at lower latitudes to resume merging with the solar wind field at the front of the magnetosphere.

The high-latitude antisunward ionospheric and magnetospheric plasma drift across the polar cap (PC) and the return flow in the sunward motion along dawn and dusk auroral latitudes generate the two-cell "forward convection" patterns, later termed DP2 (Polar Disturbance type 2) by Obayashi [2], Nishida [3], and Nishida and Maezawa [4]. Subsequently, Dungey [5] extended his model to include cases where IMF is northward (NBZ conditions), which in stronger cases would reverse the convection patterns in the central polar cap and generate sunward transpolar "reverse convection" plasma flow later termed DP3 (Polar Disturbance type 3) possibly inside a residual two-cell forward convection system. Although many details have been added later [6], these solar wind-magnetosphere interaction models still prevail now, 60 years later. The strictly southward or northward IMF directions in the idealized models have been extended to all IMF directions while retaining the basic features of northward versus southward IMF orientation.

In addition to the magnetopause currents (MPC) marking the interface between the solar wind and geomagnetic space, the magnetosphere comprises the polar cap transpolar currents and the auroral current systems both intimately connected to the plasma convection, the tail current sheet connecting the magnetopause flanks, and the ring currents of ions encircling the Earth at middle, low, and equatorial latitudes at distances of 4–6 earth radii. It is the primary objective of the present contribution to demonstrate that these magnetospheric current systems are closely interrelated in terms of the polar cap (PC) indices, notably the dual polar cap PCC indices.

The polar cap indices, PCN (North) and PCS (South), based on magnetic data recorded at the central polar cap observatories in Qaanaaq (Thule) in Greenland and Vostok in Antarctica, respectively, were developed from the initial concept by Fairfield [7] through the pioneering works of Kuznetsov and Troshichev [8], Troshichev and Andrezen [9], and Troshichev et al. [10]. Further PC index developments were made by Vennerstrøm [11], Troshichev et al. [12, 13], Stauning et al. [14], and Stauning [15–20].

To derive PC index values, magnetic variations related to the transpolar convection of plasma and magnetic fields are calibrated against the values of the merging electric field (coupling function), *E*<sup>M</sup> (=*E*KL, [21]), derived from parameters in the impinging solar wind. The calibration parameters are based on the statistical processing of solar wind and geomagnetic data throughout an epoch of accumulated values. Through their association with *E*M, the PC indices represent the merging processes between the solar wind magnetic fields extending from the Sun and the terrestrial magnetic fields at the magnetospheric boundaries and could be considered representative of the energy transfer from the solar wind to the magnetosphere. This energy may be temporarily stored in the magnetospheric tail configuration to be dissipated in processes such as auroral substorms, upper atmosphere heating, and ring current enhancements. In further developments, interactions between the solar wind and polar cap convection processes include the effects of the related fieldaligned current systems and also the consideration of reconnection processes at the nightside [22].

Janzhura et al. [23] have used the PC indices in substorm studies to predict the duration of the growth phase at substorm developments. For isolated events, they estimated that substorm onset would occur as the PC index level reached 2 mV/m. From the investigations of a large number of substorms, Troshichev et al. [24] concluded that substorm onset was likely to happen when the PC index starting from a low level exceeded 1.5 0.5 mV/m.

#### *Magnetospheric Current Systems and the Polar Cap Index DOI: http://dx.doi.org/10.5772/intechopen.104573*

Troshichev et al. [25] and Troshichev and Sormakov [26] have used PC indices to predict the maximum intensities (SYM-H minima) during geomagnetic storms. In the studies of geomagnetic storms by Stauning et al. [27] and Stauning [28, 29], the PC indices have been implemented in gradient source functions used to predict the development of ring current intensities characterized by Dst index values.

Among important applications of real-time PC indices are forecasts of strong substorms that may threaten power grids through their geomagnetically induced current (GIC) effects [30, 31]. An investigation of GIC-related high-voltage power line disturbances in Scandinavia [32] has demonstrated that the PC index values most often would remain at a high level for more than 2–3 h up to reported major power line cuts. The lengthy pre-event intervals, which are also reflected in the ring current indices [29], are most likely needed for enabling the merging processes at the front of the magnetosphere and subsequent transpolar convection characterized by the PC indices to load the tail configuration with enough energy to generate violent substorm events. The enhanced merging processes during extended pre-event intervals make the polar cap expand to enable substorm activity reaching subauroral latitudes, where important power grids reside. According to these investigations, PC index levels above 10 mV/m maintained throughout more than 1 h should cause alert for subauroral power grids [32, 33].

Strong auroral currents in the polar ionosphere characterized by large PC index values may cause heating of the upper atmosphere, which would then expand to cause anomalies in satellite orbits. The ring current intensities characterized by the Dst indices, which are related to PC index values, have been associated with further space weather effects such as spacecraft charging. The resulting electrostatic discharges in spacecraft structures may cause harmful anomalies in satellite electronic systems [34].

The report ISO/TR23989:2020 [35] issued by the authoritative Technical Committee of the International Organization for Standardization (ISO) for the natural and artificial space environment discusses the operational estimation of the solar wind energy input into the Earth's magnetosphere. The report aims at providing guidelines for the use of operative ground-based information on the polar cap magnetic activity defined by the PC indices. The report notes: "*The solar wind energy incoming into the magnetosphere predetermines development of the magnetospheric disturbances: magnetic storms and substorms. Magnetospheric disturbances include a wide range of phenomena and processes directly affecting human activity, such as satellite damage, radiation hazards for astronauts and airline passengers, telecommunication problems, outages of power and electronic systems, effects in the atmospheric processes, and impact on human health.*"
