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Ann. Geophys., 24, 3411-3419, 2006
www.ann-geophys.net/24/3411/2006/
© European Geosciences Union 2006
Correcting the geomagnetic IHV index of the Eskdalemuir observatory
D. Martini1,* and K. Mursula1
1Department of Physical Sciences, P.O. Box 3000, FIN-90014 University of Oulu, Finland
*permanent address: Geod. and Geoph. Res. Inst., Sopron, Hungary
Abstract. We study here the recently proposed measure of local geomagnetic activity
called the IHV (Inter-Hour Variability) index calculated for the Eskdalemuir
(ESK) station. It was found earlier that the ESK IHV index depicts an
artificial, step-like increase from 1931 to 1932. We show here that this
increase is due to the fact that the values of the magnetic field components
of the ESK observatory stored at the World Data Center are two-hour running
averages of hourly data stored in ESK yearbooks. Two-hour averaging greatly
reduces the variability of the data which leads to artificially small values
of the IHV index in 1911–1931. We also study the effect of two-hour averaging
upon hourly mean and spot values using 1-minute data available for recent
years, and calculate the correction factors for the early years, taking into
account the weak dependence of correction factors on solar activity. Using
these correction factors, we correct the ESK IHV indices in 1912–1931, and
revise the estimate of the centennial change based on them. The effect of
correction is very significant: the centennial increase in the ESK IHV-raw
(IHV-cor) index in 1912–2000 changes from 73.9% (134.4%) before
correction to 10.3% (25.3%) thereafter, making the centennial increase
at ESK quite similar to other mid-latitude stations. Obviously, earlier
long-term studies based on ESK IHV values are affected by the correction and
need to be revised. These results also strongly suggest that the ESK yearbook
data should be digitized and the hourly ESK data at WDC should be replaced by
them.
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