Auroral observations in Finland: Results from all-sky cameras, 1973-1997

Analyses of time-space behavior of auroral occurrence were carried out using quick-look data of Finnish all-sky cameras operated in 1973-1997 using seven stations in Finland and one in Svalbard. This unique data set consists of 25 years of continuous auroral observations, or about 100,000 hours of all-sky pictures. The basic parameter derived from quick-look data is the auroral occurrence index (AO), which gives the probability (in percentage) of the occurrence of auroras during clear nights in the dark season (January to April and August to December for each year). We have analyzed the time variations of AO in diurnal, semiannual, and solar cycle timescales. The daily and semiannual occurrence of auroral frequency in Finland are consistent with the results of earlier studies carried out in other countries. There is no clear correlation between annual values of sunspot numbers and auroral occurrence within a solar cycle, but the AO values show a rather systematic increase for 1973-1993. A comparison between AO:data with synoptic auroral observations made in Finland and worldwide from 1881 to 1979 reveals that both data series exhibit an increasing trend of auroral activity since the beginning of the twentieth century, similar to the global magnetic activity index series aa and sunspot number. We interpret that the increase of AO values is associated with the long-term increase of solar magnetic activity, as has been suggested recently.