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The Polar Cap (PC) index has been approved by the International Association of Geomagnetism and Aeronomy (IAGA XXII Assembly, Merida, Меxico, 2013) as a new index of magnetic activity. The PCindex can be considered to be a proxy of the solar wind energy that enters the magnetosphere. This distinguishes PCfrom ALand Dstindices that are more related to the dissipation of energy through auroral currents or storage of energy in the ring current during magnetic substorms or storms. The association of the PCindex with the direct coupling of the solar wind energy into the magnetosphere is based upon analysis of the relationship of PCwith parameters in the solar wind, on the one hand, and correlation between the time series of PCand the ALindex (substorm development), on the other hand. This paper (the first of a series) provides the results of statistical investigations that demonstrate a strong correlation between the behavior of PCand the development of magnetic substorms. Substorms are classified as isolated and expanded. We found that (1) substorms are preceded by growth in the РСindex, (2) sudden substorm expansion onsets are related to “leap” or “reverse” signatures in the PCindex which are indicative of a sharp increase in the PCgrowth rate, (3) substorms start to develop when PCexceeds a threshold level 1.5 ± 0.5 mV/m irrespective of the length of the substorm growth phase, and (4) there is a linear relation between the intensity of substorms and PCfor all substorm events. Magnetic substorms start when PC index exceeds threshold level PC ~1 mV/mSubstorm sudden onsets are related to sharp increase of PC growth rateHigh correlation between PC and AL is typical of periods before and after SO

The polar cap magnetic activity PCindex is regarded as indicator of the solar wind energy that enters into the magnetosphere during the solar wind‐magnetosphere coupling (Resolutions of XXII IAGA Assembly, 2013). This study presents the results of statistical analysis of relationships between the yearly values of PCindex and such indicators as the magnetic activity indices (AEand Dst), the solar wind parameters (velocity Vsw, density Nsw, dynamic pressure Pdyn, vertical interplanetary magnetic field (IMF) component Bz, total IMF field |B|, electric field EKL), and the solar activity parameters (sunspot number, coronal mass ejections, stream interaction regions) in course of 23–24 solar cycles. It is shown that the yearly values of the quiet daily curve magnitude (which serves as a level of reference in estimation of the PCindex value) changed during 1998–2019 in remarkable agreement with variation of the Solar UV irradiation (150–200 nm) absorbed in the Earth's ionosphere. The yearly values of PCindex perfectly correlate with the yearly values of Vswand |B|and their product electric field EKL, as well as with the yearly values of AEand Dstindices testifying that PCindex serves as the ground‐based indicator of the solar wind energy input into the magnetosphere. The yearly values of EKL, PC, AE, Dstcorrelate with the total IMF field |B|much better that with the IMF Bzcomponent, testifying that efficiency of the solar wind impact on the magnetosphere is defined by the total IMF value, in contrast with Dungey's concept of reconnection. The PCindex was introduced as a characteristic of magnetic activity in the polar caps. At present, the PCindex is regarded as indicator of the solar wind energy that entered into the magnetosphere in course of the solar wind‐magnetosphere coupling. The study examines variations of the PCindex during 23 and 24 cycles of solar activity in relation to solar wind parameters such as solar wind velocity Vsw, solar wind density Nsw, solar wind dynamic pressure PCdyn, the interplanetary magnetic field vertical component BZand total value |B|, as well as in relation to magnetic storms (Dst‐index) and magnetospheric substorms (AEindex), and in relation to the solar activity characteristics: sunspot number, coronal mass ejections, and high‐speed streams. It is shown that the annual PCvalues perfectly correlate with the solar wind velocity Vswand total interplanetary magnetic field (IMF) field |B|, as well as with annual values AEand Dstindices. Efficiency of the solar wind impact on the magnetosphere is determined first of all by value of |B|, not by southward IMF component that is not in conformity with Dungey's concept of reconnection as a reason of magnetic activity. The annual PCvalues perfectly correlate with the solar wind velocity Vswand total interplanetary magnetic field (IMF) field |B|, and with the magnetospheric disturbancesEfficiency of the solar wind impact on the magnetosphere is determined first of all by value of |B|, not by southward IMF componentThe annual values of EKLand PC, AE, Dstdemonstrate the double peak structure during the solar cycle The annual PCvalues perfectly correlate with the solar wind velocity Vswand total interplanetary magnetic field (IMF) field |B|, and with the magnetospheric disturbances Efficiency of the solar wind impact on the magnetosphere is determined first of all by value of |B|, not by southward IMF component The annual values of EKLand PC, AE, Dstdemonstrate the double peak structure during the solar cycle

We studied O+drift‐bounce resonance using Magnetospheric Multiscale (MMS) data. A case study of an event on 17 February 2016 shows that O+flux oscillations at ~10–30 keV occurred at MLT ~ 5 hr and L~ 8–9 during a storm recovery phase. These flux oscillations were accompanied by a toroidal Pc5 wave and a high‐speed solar wind (~550 km/s). The azimuthal wave number (m‐number) of this Pc5 wave was found to be approximately −2. The O+/H+flux ratio was enhanced at ~10–30 keV corresponding to the O+flux oscillations without any clear variations of H+fluxes, indicating the selective acceleration of O+ions by the drift‐bounce resonance. A search for similar events in the time period from September 2015 to March 2017 yielded 12 events. These events were mainly observed in the dawn to the afternoon region at L~ 7–12 when the solar wind speed is high, and all of them were simultaneously identified on the ground, indicating low m‐number. Correlation analysis revealed that the O+/H+energy density ratio has the highest correlation coefficient with peak power of the electric field in the azimuthal component (Ea). This statistical result supports the selective acceleration of O+due to the N= 2 drift‐bounce resonance. MMS observed the enhancement of O+/H+flux ratio in the magnetosphere during the recovery phase of a small stormDrift‐bounce resonance with low‐mPc5 waves is responsible for the selective acceleration of energetic O+The transverse Pc5 waves that selectively accelerated O+through the drift‐bounce resonance are driven by high‐speed solar wind