Very long-term changes in the ionosphere and upper atmosphere

Because direct "archeological" traces are nor. available, long term changes of the ionosphere can only indirectly be inferred from those of solar radiation and atmospheric composition as have occurred on a Giga-year (Ga) time scale, On that scale the appearance of free oxygen 2.6 Ga ago marked the beginning of a most important change, Solar activity is known to modulate the solar radiation in the EUV and X-ranges causing a strong solar cycle effect in ionospheric electron density. Direct solar cycle observations exist since more than 2000 years, may, however, be extended by indirect, archeological information, The dynamo-theory of the solar interior suggests more or less regular changes occurring in conjunction with the sometimes strong, sometimes weak cycles of solar activity and with changing solar wind sectors. Since the global ionization distribution is controlled by the magnetic dipole field it should have been distinct from now whenever this field was almost absent, This happened in the rather short (kilo ka) switching intervals of irregularly occurring magnetic field reversals. (C) 1997 COSPAR. Published by Elsevier Science Ltd.