Increased and varied radiation during the Sun’s encounters with cold clouds in the last 10 million years

Abstract

Abstract Recent research raises the possibility that 2–3 and 6–7 million years ago, the Sun encountered massive clouds that shrank the heliosphere —the solar cocoon protecting our solar system— exposing Earth to its interstellar environment, in agreement with geological evidence from  60 Fe and 244 Pu isotopes. Here we show that during such encounters Earth was exposed to increased radiation in the form of high-energy particles. During periods of Earth’s immersion in the heliosphere, it received particle radiation that we name Heliospheric Energetic Particles (HEPs). The intensity of < 10 MeV protons was at least an order of magnitude more intense than today’s most extreme solar energetic particle (SEP) events. SEPs today last minutes to hours, but HEP exposure then lasted for extensive periods of several months, making it a prolonged external driver. During Earth’s excursion outside the heliosphere, it was exposed to a galactic cosmic ray radiation with the intensity of < 1 GeV protons at least an order of magnitude more intense than today. Therefore, the space surrounding Earth was permeated by a variable high-energy radiation. We discuss the implications for Earth’s climate and biodiversity.