# The Evolution of Earth's Magnetosphere During the Solar Main Sequence

## TL;DR

This study models how Earth's magnetosphere responded to the evolving solar wind during the Sun's main sequence, revealing size variations and shock presence linked to the Sun's rotation rate changes over time.

## Contribution

It introduces a combined 3D MHD and 1.5D solar wind model to simulate Earth's magnetosphere evolution across different solar rotation rates, highlighting the impact of stellar spin-down.

## Key findings

- Magnetosphere was smaller and had a strong bow shock at higher solar rotation rates.
- Magnetopause standoff distance scales with rotation rate as Omega^{-0.27} or Omega^{-2.04} depending on age.
- No bow shock would form at very high rotation rates (~50 Omega_sun).

## Abstract

As a star spins-down during the main sequence, its wind properties are affected. In this work, we investigate how the 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_sun). 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 Omega_sun, 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_sun), and with Omega^{-2.04} for lower rotation rates (older ages, < 1.4 Omega_sun). This break is a result of the empirical properties adopted for the solar wind evolution. We also see a linear relationship between magnetopause distance and the thickness of the shock on the subsolar line for the majority of the evolution (< 10 Omega_sun). It is possible that a young fast rotating Sun would have had rotation rates as high as 30 to 50 Omega_sun. In these speculative scenarios, at 30 Omega_sun, a weak shock would have been formed, but for 50 Omega_sun, 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.