Solar cycle correlations of substorm and auroral occurrence frequency

Abstract

Recent results of Newell et al. [1998a] suggest that the polar cap integrated auroral occurrence frequency is independent of the solar cycle phase. This paper utilizes auroral all‐sky camera observations from Northern Finland during 1974–1996 to show that although the total number of auroral observations shows only a weak solar cycle dependence, the number of quiet auroral arcs is anticorrelated with the sunspot number, indicating that active auroral forms are less common during sunspot minima. A local index for geomagnetic substorm occurrence frequency and intensity is developed. It is shown that in long‐term averages this index is highly correlated with the planetary aa index, and that both show strong correlation with the sunspot number. We further demonstrate a clear correlation between the active auroral forms and the local geomagnetic disturbances.