Abstract. The geomagnetic field declination (D) and horizontal component (H) were observed visually at the Helsinki magnetic observatory between 1844–1912. About 2.0 million single observations of the magnetic components are available. The observing equipment and observation methods were the same for almost 70 years. The Helsinki data series is thus rather homogeneous and suitable for magnetic field analysis of both internal and external origin for about five sunspot cycles (sunspot cycles 9–13). Due to disturbances from nearby electric tramway traffic, most of the observations after 1897 are very noisy and unreliable for magnetic activity studies. Observations of D (1844–1897) have been converted into an absolute scale but H refers to variation values only. Observations of D have been previously analyzed and published for the time interval 1844–1880. In this paper we present new results of D for 1881–1897 and H for 1844–1897. The annual rate of the secular variation of D has been rather stable between 1844–1909, showing a mean eastward increase of +0.11°/year, which is about twice as large as the mean secular variation rate for the past 50 years at the same latitude in Finland. Around 1875 there was a sudden change in the secular variation rate resembling the famous jerk of 1970. Magnetic activity indices (K, Ak) for 1844–1897 were calculated from hourly values of D- and D-series separately using the IAGA K-index algorithm (the FMI-method). Comparisons with other relevant activity series from other sources (aa, u, RI, C9, auroral occurrence rate) show that the Helsinki index series gives an important contribution to the index family. By extending the Mayaud's aa-index series with Helsinki Ak-values (1844–1868), it is possible to reconstruct a (pseudo) aa-series that covers almost 160 years. Magnetic activity (space weather) was appreciably greater during the first three cycles (9–11) than during the two last ones (12–13). The largest magnetic storm occurred in September 1859. Other stormy periods were in 1847, 1852, and 1870–1872. Mean magnetic activity remained exceptionally low in years 1875–1881 and 1887–1891. In an FFT-analysis of the activity index series, the solar cycle, annual, semiannual, solar rotation and half solar rotation spectral lines are well established.

Key words. Geomagnetism and paleomagnetism (time variations, diurnal to secular) – History of geophysics (planetology) – Magnetospheric physics (solar wind-magnetosphere interactions)7