GEOMAGNETIC-ACTIVITY DURING SOLAR-CYCLE 9 (1844-1856)

Ten-minute values of declination observations carried out at the Helsinki observatory were used to analyze magnetic activity and storminess during 1844-1856 spanning solar cycle 9. Three-hour K-indices and daily Ak-amplitudes were derived numerically from the data and compared with corresponding indices from the present Nurmijarvi magnetic observatory nearby. Helsinki magnetic activity data are based on homogeneous and reliable observations being thus utilizable for extending long activity series (like aa since 1868 onwards) backwards in time. Magnetic activity during 1844-1856 has been, on the average, on the same level as the activity at Nurmnijarvi during the last four sunspot cycles in 1953-1992 but the occurrence of very large magnetic storms has been more frequent at Nurmijarvi. There have been about 300 magnetic storm days (Ak greater-than-or-equal-to 30) during cycle 9. The distribution of storm occurrence was double-peaked within the cycle: in the rising (1847-48) and falling (1851-54) parts of the sunspot cycle substantially more magnetic storms appeared than during the years in between.