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Ann. Geophys., 22, 1691-1704, 2004
www.ann-geophys.net/22/1691/2004/
© European Geosciences Union 2004
Results of the Helsinki magnetic observatory 1844-1912
H. Nevanlinna
Finnish Meteorological Institute, Geophysical Research Division, P.O.Box 503, FIN-00101 Helsinki, Finland
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
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