M.: Secular variation of the aurora for the past 500 years

Abstract. 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 sun-spots, with some continuity, exist only since about 1700. Understanding of long-term solar variability must then de-pend on proxy data, such as visual auroral observations, measurements of magnetic activity, and the radiocarbon re-cord. These also give us information on the interaction be-tween 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 variatio