Dr. Donna Rodgers-Lee

Trinity College Dublin, Ireland

The intensity of stellar and Galactic cosmic rays at the young Earth and young exoplanets

Abstract: Cosmic rays may have contributed to the start of life on Earth. Cosmic rays can also influence and contribute to atmospheric electrical circuits, cloud cover and biological mutation rates which are important for exoplanetary systems. The flux of stellar and Galactic cosmic rays present at the time when life is thought to have begun on the young Earth or in other young exoplanetary systems is largely determined by the properties of the stellar wind.

The spectrum of Galactic cosmic rays that we observe at Earth is modulated, or suppressed, by the magnetised solar wind and thus differs from the local interstellar spectrum observed by Voyager 1 and 2 outside of the solar system. Solar cosmic rays are accelerated in flares and coronal mass ejections. Upon reaching 1au, solar and Galactic cosmic rays subsequently interact with the Earth’s magnetosphere and some of their energy is deposited in the upper atmosphere. The properties of the solar wind, such as the magnetic field strength and velocity profile, evolve with time. Generally, young solar-type stars are very magnetically active and are therefore thought to drive faster stellar winds.

Here I will present our recent results which simulate the propagation of stellar and Galactic cosmic rays through the astrosphere to the location of Earth as a function of a solar-type star’s life, from 600 Myr to 6 Gyr. I will specifically focus on the flux of stellar and Galactic cosmic rays present at the time when life is thought to have started on Earth (~1 Gyr). I will show that the intensity of Galactic cosmic rays which reached the young Earth, by interacting with the solar wind, would have been greatly reduced in comparison to the present day intensity. I will also discuss the effect that the Sun being a slow/fast rotator would have had on the flux of cosmic rays reaching Earth at early times in the solar system’s life.