Numerical Study On The Solar Wind Energy Transfer Into The Geospace

The magnetospheric activities are largely controlled by the external driver,the solar wind, and the interplanetary magnetic ?eld(IMF). The solar wind energy transfers into the magnetosphere mainly by magnetic reconnection and viscouslike process and lead to the major disturbances of the magnetosphere,such as magnetic storms and substorms. The solar wind- magnetosphere(SWM) system energy coupling is always the frontier and hot issue in space physics.To understand the coupling process of the SW-M system more deeply, we conduct the numerical study on the solar wind energy transfer into the geospace based on the 3D global magnetohydrodynamics(MHD) model to understand the control e?ect of solar wind conditions on the SW-M energy coupling function.1. The SW-M energy coupling functionQuantitatively estimating the energy input from the solar wind into the magnetosphere on a global scale is still an observational challenge. We simulate the solar wind energy input of 240 quasi-steady states with di?erent solar wind conditions and 37 quasi-steady states with di?erent IMF clock angle and same solar wind conditions using the global MHD model and ?t a new solar wind energy coupling function based on the dimensional analysis Ein=3.78 × 107n0.24V1.47B0.86T[sin2.70(θ/2) + 0.25][W]. The energy coupling function includes two parts: one part is a sine power function controlled by the IMF clock angle representing the electromagnetic energy input involving the dayside magnetic reconnection and the other is a constant independent of the IMF clock angle representing the energy input caused by other processes, such as highlatitude magnetic reconnection, viscouslike. When the IMF is purely southward,the energy input is mainly from electromagnetic energy coupling; when the IMF is northward, the mechanical energy input will be dominant. In addition, the energy input is more sensitive to the solar wind velocity and the IMF clock angle than other parameters according to the energy coupling function.Furthermore, We also study the correlations between the energy couplingfunction and some indices of magnetospheric activity, such as the indices of Dst,Kp, ap, AE, AU, AL, the polar cap index, and the hemispheric auroral power. The results show that this new energy coupling function works better than the traditional ε function. This result is also applied to a storm event under northward IMF conditions, which indicates that the energy coupling function can supply more energy for the magnetosphere than the ε function. The analysis also shows that about 13% of the solar wind kinetic energy is transferred into the magnetosphere and about 35% of the input energy is dissipated in the ionosphere.2. Invariant modulation of IMF clock angle on energy input processTo validate the ?tting process, we simulate seven groups of quasi-steady states with di?erent solar wind conditions and the same range of the IMF clock angle to study the e?ect of solar wind parameters(velocity, number density)and IMF magnitude on the variation pattern of the energy input with the IMF clock angle G(θ). The simulation results indicate that the variation pattern of the energy input with the IMF clock angle is not a?ected by the solar wind parameters and IMF magnitude. It can be explained as follows: the energy input process mainly occurs in the dayside of the magnetopause and the near magnetotail by the magnetic reconnection process. However, the solar wind parameters and IMF magnitude can only a?ect the magnetopause con?guration of far magnetotail but do not change the magnetopause con?guration signi?cantly in near magnetotail and dayside region. Therefore, that the dayside magnetopause con-?guration and the near magnetotail magnetopause con?guration do not change signi?cantly determine the unchanged mode of the solar wind energy input into the magnetosphere process.3. Energetics characteristics of the super magnetic stormIn addition to the quasi-steady states, a super storm caused by the famous magnetic cloud(MC) is also simulated and studied. We analyse the energy budget of magnetosphere during the super storm and ?nd that about 23% solar wind kinetic energy is transferred into the magnetosphere, about 9.50 × 1017 J,14 times of a moderate storm. The input e?ciency(IE) of the intense storm and the super storm are(13.2%,9.0%,12.4%) and(3 4.3%,14.7%,23.3%),respectively, for main phase, recovery phase and entire storm. Di?erent from the previous studies, which use the ε function to determine the energy input, we calculate the energy input based on the MHD model. And other studies results indicate that the ε function always underestimate or overestimate the energy input and do not consider the important e?ect of solar wind number density on the energy input process. Our results also indicate that the MHD results correlate better with the indices of magnetospheric activity by considering the e?ect of solar wind number density and can estimate the energy input and dissipation on global scale. Therefore, it is more reasonable using the MHD model to determine the energy input and can exert the advantage of the MHD model.In addition, the energy coupling e?ciencies(CEs) of ring current and polar cap ionosphere are also calculated with 3.1%, 23.6% for intense storm and 40.2%,23.1% for super storm. The simulation results indicate that the proportion of the ring current to total energy output increases with the increase of the magnetospheric activity level. The energy dissipation via ring current injection is about twice of the energy dissipation via high-latitude ionosphere suggested by the simulation results. About 63.3% of the transferred energy is dissipated via ring current and high-latitude ionosphere and 36.7% of the transferred energy is consumed by other energy dissipation channels in the tail, such as plasma sheet heating, plasmoid ejection returning to the solar wind.