The evolution of Earth's magnetosphere during the solar main sequence

As a star spins-down during the main sequence, its wind properties are affected. In this work, we investigate how Earth's magnetosphere has responded to the change in the solar wind. Earth's magnetosphere is simulated using 3D magnetohydrodynamic models that incorporate the evolving local properties of the solar wind. The solar wind, on the other hand, is modelled in 1.5D for a range of rotation rates Omega from 50 to 0.8 times the present-day solar rotation (Omega(circle dot)). Our solar wind model uses empirical values for magnetic field strengths, base temperature, and density, which are derived from observations of solar-like stars. We find that for rotation rates similar or equal to 10 Omega(circle dot), Earth's magnetosphere was substantially smaller than it is today, exhibiting a strong bow shock. As the Sun spins-down, the magnetopause standoff distance varies with Omega(-0.27) for higher rotation rates (early ages, >= 1.4 Omega(circle dot)) and with Omega(-2.04) for lower rotation rates (older ages, <1.4 Omega(circle dot)). This break is a result of the empirical properties adopted for the solar wind evolution. We also see a linear relationship between the magnetopause distance and the thickness of the shock on the subsolar line for the majority of the evolution (<= 10 Omega(circle dot)). It is possible that a young fast rotating Sun would have had rotation rates as high as 30-50 Omega(circle dot). In these speculative scenarios, at 30 Omega(circle dot), a weak shock would have been formed, but for 50 Omega(circle dot), we find that no bow shock could be present around Earth's magnetosphere. This implies that with the Sun continuing to spin-down, a strong shock would have developed around our planet and remained for most of the duration of the solar main sequence.