The present understanding is that the coupling of the solar wind and the Earth's magnetosphere occurs via magnetic reconnection ( Dungey, 1961) and viscous processes ( Axford and Hines, 1961) such as the Kelvin–Helmholtz instability (e.g., Nykyri and Otto, 2001) and diffusion ( Johnson and Cheng, 1997). However, care must be taken in interpreting these results, as the CPCP is also sensitive to the simulation resolution. The cross-polar cap potential (CPCP) saturation is shown to depend on the Alfvén–Mach number of the upstream solar wind. The same appeared to be true when the event was run with another global simulation. However, the local values, especially during high driving, show more variation: such extreme conditions do not reproduce local measurements made deep inside the magnetosphere. We show that on the large scale, and during moderate driving, the GUMICS-4 results are in good agreement with the reference values. We examine how the Earth's space environment dynamics evolves during both ICME events from both global and local perspectives, using well-established empirical models and in situ measurements as references. The 2014 event was a moderate one, with the corresponding upstream values of 30 particles cm −3, 320 km s −1 and 10 nT, indicating a Mach number of 5.8. During the strong 2012 event, the solar wind upstream values reached up to 35 particles cm −3, speeds of up to 694 km s −1, and an interplanetary magnetic field of up to 22 nT, giving a Mach number of 2.3. We study the response of the Earth's magnetosphere to fluctuating solar wind conditions during interplanetary coronal mass ejections (ICMEs) using the Grand Unified Magnetosphere-Ionosphere Coupling Simulation (GUMICS-4).
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