Terrestrial Impact from the Passage of the Solar System through a Cold
  Cloud a Few Million Years Ago
  It is expected that as the Sun travels through the interstellar medium (ISM),
there will be different filtration of Galactic Cosmic Rays (GCR) that affect
Earth. The effect of GCR on Earth's atmosphere and climate is still uncertain.
Although the interaction with molecular clouds was previously considered, the
terrestrial impact of compact cold clouds was neglected. There is overwhelming
geological evidence from 60Fe and 244Pu isotopes that Earth was in direct
contact with the ISM 2 million years ago, and the local ISM is home to several
nearby cold clouds. Here we show, with a state-of the art simulation that
incorporate all the current knowledge about the heliosphere that if the solar
system passed through a cloud such as Local Leo Cold Cloud, then the
heliosphere which protects the solar system from interstellar particles, must
have shrunk to a scale smaller than the Earth's orbit around the Sun (0.22).
Using a magnetohydrodynamic simulation that includes charge exchange between
neutral atoms and ions, we show that during the heliosphere shrinkage, Earth
was exposed to a neutral hydrogen density of up to 3000cm-3. This could have
had drastic effects on Earth's climate and potentially on human evolution at
that time, as suggested by existing data.
