Geomagnetic secular variation in Borok
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Figure 7
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[16] The geomagnetic secular variation originates in the Earth's outer core, where fluid flows
generate the main magnetic field through a dynamo process. A simple (although not perfect)
way to average out external field variations in the observatory recordings consists in calculating
monthly means and annual means. The resulting series for the Borok observatory are shown
in Figure 7, after combining the series from the IZMIRAN observatory (before 2001)
and those from the INTERMAGNET one (after 2004). Fortunately the data discontinuity between the
two series is not large, thanks to the low magnetic field gradient on the Borok site. It is worth
noting that IPGP is currently developing a publicly available database of monthly means from all
INTERMAGNET observatories [Chulliat and Telali, 2007].
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Figure 8
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[17] One of the main features of the geomagnetic secular variation is the existence of so-called
"geomagnetic jerks'', that is, abrupt changes in the secular variation
[Courtillot et al., 1978],
that occur several times per century [Mandea et al., 2000].
The 1990 geomagnetic jerk is present in the
Y component recorded in Borok, as can be seen on Figure 8.
The secular variation values in this figure have been computed by taking first differences of
monthly means and then applying a one-year sliding average. However, the interruption of the
observatory between 2001 and 2004 makes it difficult to properly detect the 1999 jerk
[Mandea et al., 2000] and impossible to detect the 2003 one
[Olsen and Mandea, 2007].
This example shows the importance of having uninterrupted observatory data series to
study the geomagnetic secular variation in general, and geomagnetic jerks in particular.
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