The main part of the geomagnetic field – produced by a dynamo process in the Earth's outer core – changes its direction and strength in time, over timescales from months to centuries, even millennia. Its temporal variations, known as secular variation and secular acceleration, are crucial ingredients for understanding the physics of the deep Earth. Very long series of measurements therefore play an important role. Here, we provide an updated series of geomagnetic declination in Paris, shortly after a very special occasion: its value has reached zero after some 350 years of westerly values. Indeed, during October and November 2013, the declination at the Chambon la Forêt geomagnetic observatory changed from westerly to easterly values, the agonic line then passing through this place. We take this occasion to emphasize the importance of long series of continuous measurements.
The observed Earth's magnetic field is the sum of several internal and
external contributions. The core field is more than 1 order of magnitude
stronger than the other contributions. This main part of the geomagnetic
field is believed to be generated by convective motions in the Earth's
iron-rich, electrically conducting fluid outer core, by a process known as
the geodynamo. This geodynamo-generated field is named the core field or main
field, and its temporal variation, over timescales from months to centuries,
is named secular variation. The core field morphology at the Earth's surface
is relatively simple, being dominated by a centered dipole-like field which
accounts for some 90 % of the total field. The lithospheric magnetic field,
with its origin in the remanent and induced magnetization of the crust
and upper mantle, is not only weaker and much less
variable in time, but also of a much smaller spatial scale when compared to
the large-scale core field. Its complexity comes from its geological and
tectonic origins. The Earth's magnetic external fields stem from the
interaction of the solar wind with the magnetosphere, and have as direct
sources electric currents in the ionosphere and magnetosphere. In addition,
an external current system independent of the solar wind–magnetosphere
interaction exists, the so-called quiet-time daily variation (
Observing the Earth's magnetic field, since the time of the first compasses, can be regarded as the oldest branch of modern geoscience and core physics. Nowadays, measuring the magnetic field is more focussed on answering fundamental questions about the planetary deep interior and its near-space environment than on practical navigation matters. Since 1832 the Earth's magnetic field direction and strength have been continuously measured at various locations around the world, and these data represent the most important body of measurements to analyse the time evolution of the field morphology. The geomagnetic observatory network is still a crucial source of data in producing secular variation models in the past, and now, together with satellite data, enables us to better describe the internal and external field contributions.
In this paper, we first describe some key steps in measuring the geomagnetic field, in France, and then we underline the role of French magnetic observations in our knowledge of the magnetic field.
Measurements of the Earth's magnetic field have been taken over more than a century and a half on the ground (geomagnetic observatories and magnetic surveys) and over some decades from space (low Earth orbit magnetic satellites).
The geomagnetic field, at any particular location and time, is defined in
terms of the three components (
The declination was the very first measured geomagnetic field element, due to
the early use of compasses. This instrument has been known in Europe since
the 12th century
At times when navigation became more important, the geographic north
direction was linked to measurements of latitude. Generally, the astrolabe
was used for determining latitude by measuring the angle between the horizon
and Polaris (pole star). The position of Polaris might be located within less
than 1
The discovery of declination in the European area has often been ascribed to
Christopher Columbus in 1492, but there is evidence from ancient sundials and
compasses that declination had been known in Europe since at least the early
15th century. The oldest declination value given by a magnetic compass known
to us is dated 1451. This instrument was made by Peuerbach in Vienna.
However, it is not clear whether Peuerbach understood the deviation from the
geographic north as purely a property of the magnetic field or as one of the
instrument. Three more compasses made by him at the same location
between 1451 and 1456 indicate different declination values, although the
discovery of change in declination with time (i.e. secular variation) is
generally assumed to have taken place only in the early 17th century. It was
probably first noticed by Edmund Gunter in 1622 and was fully described by
Henry Gellibrand in 1634
There was considerable interest in explaining the direction of the magnetic
field from the very beginning of the 17th century and onwards.
William Gilbert published his book
From the 17th century, declination and inclination measurements started to be made on a more regular basis, giving the direction of the local geomagnetic vector, more or less continuously, in different places around the globe. Relative measurements of the intensity (or the magnitude) of this vector were made from the 1790s, by comparing the “swing time” of a needle at the current location with that measured at a reference site. It is only in 1825 and 1830 that Denis Poisson and Carl Friedrich Gauss developed along somewhat different lines the theory of an absolute measurement of the magnetic field intensity, or strength. Simultaneously, Gauss built a magnetometer capable of providing reliable measurements of intensity. Gauss also established the first magnetic observatory in Göttingen in 1832. Moreover, Gauss and Wilhelm Weber founded the Magnetischer Verein (Magnetic Union) which, from 1834 to 1841, supported the growth of a geomagnetic network through Europe; in this context, magnetometers were installed at sites such as Berlin (1836), Dublin (1838), Greenwich (1838), Prague (1839), and Munich (1841).
In France, following a long activity in the field of Earth's magnetism, the national magnetic observatory started its activity in 1883. Before that date, declination and inclination measurements had been taken since 1541 and 1676 respectively. In the following, we focus on the Paris declination series only.
Early declination data can be found in published catalogues or time series.
For French catalogues, we can quote the Guillaume de Nautonier one
Another category of sources is made of time series at a given location; for a
mere handful of sites, series with more or less regular observations,
spanning a few centuries, do exist. We can quote the time series of
declination and sometimes inclination which have been compiled by
The Paris declination series starts as early as in the 16th century, with the
first measurement performed by Künstler Bellarmatus in 1541, giving a
value of 7
Paris declination series: annual means of declination corrected and
adjusted to Chambon la Forêt observatory (see
.
Two epochs are important from a historical point of view, separated by some
350 years, when declination reaches a null value. The first one is
around 1663 (considering the mean of measurements in Paris and that in
Issy-les-Moulineaux, i.e. measurements which are not reduced to the
present-day location of the Chambon la Forêt observatory). From 1658
to 1667 we note eight values of declination around zero, in 1658, 1660 (two
measurements), 1663, 1664, 1666, and 1667 (two measurements) – see also
Table 4 in
The second period with a nearly null value of declination is October–November 2013. Figure 2 shows the daily variation of declination over the last 4 months of 2013, indicating a change in the declination sign over October–November 2013.
To give a flavour of the early interest in variation of declination in Paris,
and a possible approach to its estimation and null values, we mention the
note left by a French astronomer who tried to represent the declination in
Paris by a polynomial. In M. Burckardt, an ingenious French astronomer, invented a formula to
represent the magnetic declination observed at Paris; thus if
Chambon la Forêt declination: daily means from 1 September 2013 to 31 December 2013.
To analyse the temporal variations of the core magnetic field, it is of
course essential to have available long series of data. To the slowly varying
secular variation, we note additional characteristics. One specific feature
of the declination variation we are interested in are the so-called
“geomagnetic jerks”, defined as abrupt changes in the secular variation and
completed in a short time (see more details in
To enhance rapid events, the first time derivative is computed after applying
an 11-year smoothing. The Paris declination curve clearly exhibits a number
of changes in the secular variation, as shown in Fig. 3. These changes are
more rounded as a result of the use of the filter; nevertheless, the
geomagnetic jerks can still be clearly identified. The figure clearly shows
that, prior to the 20th century, one of the most prominent geomagnetic jerks
appears around 1870. This event is also observed in four other European
locations
Considering the origin of geomagnetic jerks in the fluid outer core, their signature on the measured magnetic field at the Earth's surface may differ from place to place, which explains why there is no perfect temporal coincidence between the different declination series.
Paris declination series: secular variation of the declination computed with an 11-year smoothing filter.
In this paper we updated the Paris series with the last 20 years of data,
considering the event of zero declination through the Chambon la Firêt
observatory. This is not a unique incident in Europe and an image of the
declination observations at other observatories and repeat stations to
demonstrate the westerly drift of the agonic line is provided by the
declination map of Europe for the epoch 2006, a product of the MagNetE group
The evolution of the Earth's magnetic field is intimately linked to the history of the Earth, allowing insights into the inner workings of our planet. Furthermore, the magnetic field is an important component in shielding the Earth's surface from solar emissions. Hence, understanding its behaviour is crucial, even if long-term accurate predictions are not presently possible. It is inevitable that the geomagnetic field will continue to exhibit secular variation at all timescales, and its strength continues to change, producing changes in the structure and dynamics of the magnetosphere. One can expect a corresponding change in the geometry of the magnetosphere, which is controlled to first order by a balance between static pressure generated by the geomagnetic field and the dynamic pressure of the solar wind. The possible consequences for our planet are not yet completely acknowledged.
Understanding how the future geomagnetic field varies is strongly dependent on how well we know the past magnetic field. And for this, geomagnetic data, represented by the declination curve, remain a unique dataset.
Data are available as following:
The annual means 1541–1994: from Alexandrescu et al. (1996); 1995–2014: from The daily means from
The Paris declination series, reduced to the current location of Chambon la
Forêt observatory, is available on request.
Mioara Mandea and Jean-Louis Le Mouël contributed equally to this work, which is a legacy of the time they spent in Chambon la Forêt observatory.
This “null-declination” event is dedicated to so many observers, who, with constant patience, measured the geomagnetic field over centuries. Special thanks are given to the operators of Chambon la Forêt observatory. We gratefully acknowledge constructive suggestions from two anonymous reviewers. Edited by: M. G. Johnsen Reviewed by: two anonymous referees