Consequences of a past encounter of the earth with an interstellar cloud

In its motion through the Galaxy the sun may have suffered a number of encounters with dense interstellar clouds for which the number density of molecular hydrogen is in excess of 1000 per cu cm. Several authors have shown that this is sufficient density to stop the solar wind inside earth's orbit. Earth's atmosphere would be subjected to an interstellar H2 flux of more than 7 billion per sq cm/sec for periods of the order of 100,000 years. Simple scaling arguments are used to examine several consequences for earth's atmosphere: (1) the ionospheric F region would largely disappear; (2) the water-vapor content of the middle atmosphere would be greatly enhanced, reducing the mesospheric ozone concentrations and thereby lowering the average temperature and altitude of the mesopause; (3) as a result of (2), widespread mesospheric ice clouds would occur, increasing the planetary albedo; and (4) the resultant radiative cooling at the surface may have been sufficient to 'trigger' an ice age.