2019MNRAS.489.5784C

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 Ω from 50 to 0.8 times the present-day solar rotation (Ω_☉). 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 ≃10 Ω_☉, 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 Ω^–0.27 for higher rotation rates (early ages, >=1.4 Ω_☉) and with Ω^–2.04 for lower rotation rates (older ages, <1.4 Ω_☉). 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 Ω_☉). It is possible that a young fast rotating Sun would have had rotation rates as high as 30-50 Ω_☉. In these speculative scenarios, at 30 Ω_☉, a weak shock would have been formed, but for 50 Ω_☉, 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.