22-year cycle in the frequency of aurora occurrence in XIX century: Latitudinal effects.

The 22-year variation in the frequency of aurora occurrence is found through an analysis of data of the Russian network of meteorological stations from 1837-1909. This variation is obtained in a form of asymmetry between even and odd solar cycles. We found that the nature of the 22-year variation depends on the latitude of the observation station. The annual number N of midlatitude auroras (geomagnetic latitudes Φ < 56°) for about three years at the end of the descending part of solar cycles is larger for the even cycles than for the odd. For high-latitude auroras (Φ ≥ 56°), the pattern is opposite: at the descending part of the solar cycle, N is larger in the odd cycles than in the even. For the high-latitude sector, asymmetry of the polar sun cycles (the period between two magnetic field reversals) is clearly observed: an increased N is observed during the whole odd polar cycle (which starts approximately at the maximum of the odd Schwabe cycle) as compared to the even cycle. Extrapolation of the modern picture of alternation of the sign of the global solar magnetic field back in time leads to the conclusion that the most geoeffective polar cycles in cycles 8-14 were those in which the polar magnetic field in the northen hemisphere was negative. [ABSTRACT FROM AUTHOR]