Relation to solar activity of intense aurorae in sunlight and darkness

The oldest documented1,2 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’, which are those that reach mid-latitudes or lower. The overwhelming majority of intense aurorae, and therefore most of the energy put into the ionosphere, occurs at high latitudes, where aurorae appear nightly. Here we report the global frequency of aurorae as a function of solar cycle, determined by data from the US Air Force Defense Meteorological Satellite Program. We find that, contrary to expectations, the total number of intense aurorae is uncorrelated with solar activity in darkness, and is negatively correlated with solar activity in sunlit conditions. These findings imply a causal relationship between aurorae and ionospheric conductivity (the latter is maximal at solar maximum) and therefore indicate that the occurrence of intense aurorae is a discharge phenomenon, similar to lightning.