Physical Science International Journal

Solar wind–magnetosphere coupling governs the transfer of energy from the solar wind into the Earth's magnetosphere and plays a fundamental role in space weather processes. This study investigates the occurrence of different geomagnetic activity classes defined by Legrand & Simon (1989) and quantifies the power transferred to the inner magnetosphere during Solar Cycle 24 (2008–2018). Solar wind and interplanetary magnetic field data were analyzed using the coupling function proposed by Wang et al. (2014) to estimate the energy input into the inner magnetosphere. The variability of the parameter E_in is studied during periods of disturbed geomagnetic activity in comparison with quiet periods; the B_Z component of the IMF, the clock angle and the dynamic pressure. We also use pixel diagrams to study the occurrences of different days of geomagnetic activity. The results show that quiet days dominated Solar Cycle 24, accounting for approximately 73% of the observed period, whereas disturbed conditions represented about 27% of days. The distribution of disturbed days in terms of geomagnetic activity is as follows: 25.03% fluctuating days, 1.42% recurrent days and less than 1% shock days. This strong predominance of quiet days indicates the generally calm state of the magnetosphere during Solar Cycle 24. The highest frequencies of calm days are recorded at the phase minimum; those of shock days, recurrent days and fluctuating days during the descending phase. Furthermore, from the phase minimum to the descending phase, the frequency of calm days decreases continuously, falling from 84.81% at the phase minimum to 62.90% in the descending phase.  The perfectly in-phase diurnal variations between the power transferred to the inner magnetosphere during shock days and fluctuating days, and the geometric efficiency of solar wind–magnetosphere coupling, suggest that the clock angle is the parameter that directly controls the transfer of energy from the solar wind to the inner magnetosphere on days of geomagnetic activity during such events. The average power transmitted to the inner magnetosphere is minimal during quiet days (316 GW) and maximal during shock activity days (4864 GW). The average power transmitted during recurrent days (2518 GW) is approximately double that transmitted during fluctuating days (1200 GW). Regardless of the nature of the disturbance, the power transmitted to the inner magnetosphere during disturbed periods is 1800 GW. The power transmitted to the inner magnetosphere is correlated with the Aa geomagnetic index, estimated from the level of disturbance in the geomagnetic field caused by solar activity.