We investigate the geomagnetic field variations recorded by INTERMAGNET geomagnetic observatories. We confirm that the effect of solar eclipse can be seen over an interval of 180 minutes centered at the time of maximum eclipse on a site of a geomagnetic observatory. It is found that the effect of the solar eclipse on the geomagnetic field becomes conspicuous as the magnitude of a solar eclipse becomes larger. The effect of solar eclipses is more evident in the second half of the path of Moon’s shadow. We also find that the effect can be overwhelmed, more sensitively by geomagnetic disturbances than by solar activity of solar cycle.

New Thellier-Coe archeointensity determinations have been measured on 15 potsherds from the Oaxaca Valley belonging to three of the four Formative Periods (Pre-Classical) of Mesoamerica, spanning 1400–700 BC. Seven of these are considered to be reliable and indicate a geomagnetic field strength of about 30 μT. This value is some 75% of the present geomagnetic field strength but is in agreement with the absolute intensities predicted from global models for this time and location, and consistent with coeval published determinations. These data thus provide significant evidence for the geomagnetic field strength in an area and for a time that was previously poorly constrained, thus providing an important contribution towards establishing a local master curve for the last 3500 yr. When established, such a curve would be a useful dating tool and also enable establishing for field strength correlations with climatic events and civilization evolutions in a region that is particularly strong in archeological and geological features. Such potential is examined for aridity events, although such observations can only be considered tentative at this stage.

The Earth is embedded in the solar wind, this ever-streaming extremely tenuous ionized gas emanating from the Sun. It is the geomagnetic field which inhibits the solar wind plasma to directly impinge upon the terrestrial atmosphere. It is also the geomagnetic field which moderates and controls the entry of energetic particles of cosmic and solar origin into the atmosphere. During geomagnetic polarity transitions the terrestrial magnetic field decays down to about 10% of its current value. Also, the magnetic field topology changes from a dipole dominated structure to a multipole dominated topology. What happens to the Earth system during such a polarity transition, that is, during episodes of a weak transition field? Which modifications of the configuration of the terrestrial magnetosphere can be expected? Is there any influence on the atmosphere from the intensified particle bombardment? What are the possible effects on the biosphere? Is a polarity transition another example of a cosmic cataclysm? A review is provided on the current understanding of the problem. A first, illustrating model is also discussed to outline the complexity of any biospheric reaction on polarity transitions.