Secular Variation of the Aurora for the past 500 Years

Knowledge of solar variability and its effects on the Earth is essential since the Sun affects almost every aspect of our lives. Direct observations of the Sun, usually of sunspots, with some continuity, exist only since about 1700. Understanding of long-term solar variability must then depend on proxy data, such as visual auroral observations, measurements of magnetic activity, and the radiocarbon record. These also give us information on the interaction between the solar wind, the interplanetary field, and the terrestrial magnetosphere, as well as, for the radiocarbon record, hellospheric onditions. This paper uses a data base of visual auroral observations for a period of about 500 years, from 1450 to 1948, comprising about 45,000 observations, in addition to the well-known sunspot series and the magnetic activity index aa, from 1868 to 1990. The secular variation of the aurora is examined and compared, where possible, to sunspot data and magnetic activity data. Blackman-Tukey power spectra are used to determine periodicities. The study confirms the variability of the periodicities in both frequency and amplitude. In particular, the well-known 11.l-year cycle disappears during the Maunder minimum and at the end of the eighteenth and beginning of the nineteenth century. While the 11.l-year period is normally strongly dominant for sunspots, other shorter periods become important, and even dominant, for auroras and magnetic activity. Consequently, the temporal behavior of these three variables differs. Prolonged solar activity minima are clearly evident. In addition to the known Sptrer, Maunder, Dalton, and 1901-1913 minima, a previously unrecognized minimum about 1765 is clearly evident in the data. Comparison of the depth of these minima shows that the Dalton minimum may be the deepest, or at least rivals the Maunder minimum in importance. This minimum clearly deserves further study. Combining the polar data base with that of mid-latitudes provides for the first time a globally comprehensive historical record of auroral occurrence. The data provide confirmation of the anticorrelation of auroral occurrence in the polar regions with sunspot activity, as a result of displacement of the auroral oval with changes in solar and magnetic activity. Assuming the validity of some current models of the solar origin of geomagnetic activity, the data provide a basis for understanding the variation over time of the general magnetic field of the Sun, in particular the polar field.