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The polar cap magnetic activity index (PC) was approved by IAGA as an index characterizing the solar wind energy input into the magnetosphere. Relation of the PC index to solar wind parameters and solar activity during 23/24 solar cycles has been studied in paper (Troshichev et al., 2021, https://doi.org/10.1029/2020JA028491). The paper was commented by Dr. Stauning, who states that results obtained in the study are based on erroneous data presented on web site http://pcindex.org. According to Dr. Stauning, the following invalidities served as a reason for this assertion: incorrect determination of quiet daily curve (QDC), taken as reference level for counting the PC index value, and incorrect QDC amplitudes, incorrect yearly average values of PC index (and EKL field), incorrect methods of the analysis. In our answer we argue the correctness of our analysis and resulted conclusions. Plain Language Summary: The quality of the IAGA endorsed PC index is supplied by choice of proper quiet daily variation (QDC), which makes it possible to account the effect of irregular solar UV irradiation related to solar flares. This special features of the PC index derivation made it possible to reveal the correlation between the QDC amplitude and the solar flares intensity in course of solar 23/24 cycle. The value of the yearly PC index may be estimated with use of median or mean PC quantities, resulting in essential discrepancy of the deduced yearly PC index values. The relative values of median quantities were used in the analysis. The preliminary PCS indices turned out to be incorrectly evaluated in case of 4 months in 2011, but this circumstance did not affect the general regularity of PCN/PCS changes over period 1997–2019. Divergence between values of the corresponding PCN and PCS indices, typical of solstice seasons in periods of high solar activity, is conditioned by different conductivity of ionosphere in summer and winter polar caps, not by quality of the PCN or PCS indices. Definitive PC indices have demonstrated the perfect agreement with preliminary PC indices validating the conclusions made in (Troshichev et al., 2021, https://doi.org/10.1029/2020JA028491). Key Points: Claims (Stauning, 2022) of using the unapproved polar cap indices in study (Troshichev et al., 2021) are baselessYearly QDC‐X and QDC_Y values demonstrated agreed alterations at stations Thule and Vostok in 1998–2019The relative yearly mean values derived via hourly median quantities used in analysis provide the same result as hourly mean quantities [ABSTRACT FROM AUTHOR]

Declaration (Stauning, 2022, https://doi.org/10.1029/2022JA030355) on “invalid polar cap south (PCS) index” is based on the following arguments: PCS index is calculated with use of incorrect “unified” PC derivation method; PCS index used in analyses is a preliminary index, which was not approved by IAGA and, therefore, it cannot be regarded as a correct index; PCN and PCS indices demonstrate, intermittently, large difference in value, which should be treated as evidence of the PCS index invalidity. The paper presents comments to these arguments. Conclusion is made that criticism of the PCS index, presented in Stauning (2022, https://doi.org/10.1029/2022JA030355), is based on groundless arguments. [ABSTRACT FROM AUTHOR]

The paper investigates the strengthening of magnetospheric activity related to geosynchronous magnetopause crossings (GMCs). We make a list of GMC events using the empirical magnetopause model (Lin et al., 2010, https://doi.org/10.1029/2009ja014235) and hourly averaged OMNI data and find which solar wind and magnetospheric conditions accompany and follow the GMCs. The GMCs are mostly caused by the impact of interplanetary coronal mass ejections (ICMEs) and/or interplanetary shocks often with a strong increase in the density and a moderate increase in velocity. The average solar wind density during the first GMC hour is higher than 20 cm−3 in 70% cases, while the velocity is higher than 500 km/s in 56% cases. The hourly interplanetary magnetic field (IMF) BZ is negative in 87% cases. The average over all events SMU (SML), Kp, and PC indices reach maxima (minima) in 1 h after the GMC beginning, while the delay of the minimum of the Dst index is usually 3–8 h. These average time delays do not depend on the strength of the storms and substorms. The SML (Dst) minimum is less than −500 nT (−30 nT) in the next 24 h in 95% (99%) cases, that is, the GMC events are mostly followed by magnetic storms and substorms. We compare solar wind and magnetospheric conditions for GMCs connected with ICMEs and stream interaction regions (SIRs). Our study confirms that the ICME‐related events are characterized by stronger ring current and auroral activity than the SIR‐related events. The difference might be explained by the different behavior of the solar wind velocity. Plain Language Summary: With an ever‐increasing number of satellites in geostationary orbit, it is crucial to predict when they will be located outside of the Earth's magnetosphere which shields satellites from the interplanetary medium, including the low‐energy part of the solar energetic particle spectra which can potentially damage exposed satellite systems. Here, using a well‐established empirical model, we show that such satellites are outside of the protective magnetic shield in rare cases of extreme magnetospheric compression, as only 100 cases were identified over 24 years. We demonstrate that most of the cases occur during the times when the Sun is most active and are associated with Interplanetary Coronal Mass Ejections, giant bubbles of plasma occasionally emitted from the Sun and propagating through space. The disturbances of the near‐Earth space associated with the times of extreme magnetospheric compression are analyzed. It is shown that such compressions are followed by local geomagnetic activity at the high‐latitudes in the nightside ("magnetospheric substorm") with an average time delay of 1 h and by global magnetic disturbances ("magnetic storm") with the maximum effect observed by 3–8 h later. Key Points: Geosynchronous magnetopause crossings are studied in connection with solar wind drivers and accompanying geomagnetic activityDst index often reaches a minimum in 3–8 h after geosynchronous magnetopause crossingsAverage SML (SMU) index reach minimum (maximum) in one hour after geosynchronous magnetopause crossings [ABSTRACT FROM AUTHOR]

To study further the factors and mechanisms controlling 10-150 keV particle fluxes in the inner magnetosphere, we investigate empirically their behavior in the nightside transition region (6-14 Re) depending on solar wind parameters taken at different time lags. We aim to establish the hierarchy of predictors (V, N, Pd, Ekl = VByz sin²(θ/2), etc.) and the optimal range of their time delays, both depending on the distance and local time. We use THEMIS 5-min averaged observations of energetic proton and electron fluxes in 2007-2018 near the plasma sheet midplane and build regression models exploring the combination of predictors, taken at time delays up to 24 h. The model obtained shows that protons and electrons are controlled differently by solar wind parameters: electrons are influenced equally by Vsw and Ekl, whereas protons are controlled mostly by Vsw and Pd and less by Ekl. We found that a wide range of time delays is involved depending on distance and particle energy. Specifically, the Ekl affects the energetic fluxes with time delays up to 24 h (or more), exhibiting the long delays in the innermost regions. As regards the mechanism of Vsw influence, the Vsw-related flux changes are large and, to a large extent, established on the route of the energy flow from solar wind to the plasma sheet and, eventually, the inner magnetosphere. We also identified a new parameter, NBL = VByz cos²(θ/2), which helps to reveal the loss processes in the plasma sheet transition region. [ABSTRACT FROM AUTHOR]

The polar cap magnetic activity PC index 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 PC index and such indicators as the magnetic activity indices (AE and 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 PC index 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 PC index perfectly correlate with the yearly values of Vsw and |B| and their product electric field EKL, as well as with the yearly values of AE and Dst indices, testifying that PC index serves as the ground-based indicator of the solar wind energy input into the magnetosphere. The yearly values of EKL, PC, AE, Dst correlate with the total IMF field |B| much better that with the IMF Bz component, 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. Plain Language Summary The PC index was introduced as a characteristic of magnetic activity in the polar caps. At present, the PC index 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 PC index 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 BZ and total value |B|, as well as in relation to magnetic storms (Dst-index) and magnetospheric substorms (AE index), and in relation to the solar activity characteristics: sunspot number, coronal mass ejections, and high-speed streams. It is shown that the annual PC values perfectly correlate with the solar wind velocity Vsw and total interplanetary magnetic field (IMF) field |B|, as well as with annual values AE and Dst indices. 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. [ABSTRACT FROM AUTHOR]

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. Key Points: MMS observed the enhancement of O+/H+ flux ratio in the magnetosphere during the recovery phase of a small stormDrift‐bounce resonance with low‐m Pc5 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 [ABSTRACT FROM AUTHOR]

Abstract: A Pc5 wave is observed by the Exploration of energization and Radiation in Geospace Arase satellite in the inner magnetosphere (L ~5.4–6.1) near postmidnight (L‐magnetic local time ~1.8–2.5 hr) during the storm recovery phase on 27 March 2017. Its azimuthal wave number (m‐number) is estimated using two independent methods with satellites and ground observations to be −8 to −15. The direct measurement of the m‐number enables us to calculate the resonance energy. The flux oscillations of H+ and O+ ions at ≥ 56.3 keV are caused by drift resonance and those of O+ ions at ≤ 18.6 keV by bounce resonance. Resonances of O+ ions at multiple energies are simultaneously observed for the first time. The enhancement of the O+/H+ flux ratio at ≤ 18.6 keV indicates selective acceleration of O+ ions through bounce resonance. [ABSTRACT FROM AUTHOR]