RELATION TO SOLAR-ACTIVITY OF INTENSE AURORAE IN SUNLIGHT AND DARKNESS

The oldest documented(1,2) relationship between the number of sunspots (the solar cycle) and terrestrial effects is the increased frequency of aurorae in the period immediately after the solar maximum (the peak of the number of sunspots). This correlation is, however, based only on observations of the relatively rare events of 'great aurorae', whic h are those that reach mid-latitudes or lower. The overwhelming majori ty of intense aurorae, and therefore most of the energy put into the i onosphere, occurs at high latitudes, where aurorae appear nightly. Her e we report the global frequency of aurorae as a function of solar cyc le, determined by data from the US Air Force Defense Meteorological Sa tellite Program. We find that, contrary to expectations, the total num ber of intense aurorae is uncorrelated with solar activity in darkness , and is negatively correlated with solar activity in sunlit condition s. These findings imply a causal relationship between aurorae and iono spheric conductivity (the latter is maximal at solar maximum) and ther efore indicate that the occurrence of intense aurorae is a discharge p henomenon, similar to lightning.