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Geomagnetic jerks


The term "geomagnetic jerk” commonly denotes an abrupt change in the slope of secular variation of any of the elements of the geomagnetic field. Geomagnetic jerks occur on a time scale of the order of 1 year and it is observable in the secular variations of many worldwide observatories. In the last century many events have occurred, some involving observatories on global scale others on regional scale.
The first jerk has been discovered at the end of 70s’. Since then, by using different analysis techniques, many other jerks have been pointed out, namely in the years: 1901, 1913, 1925, 1932, 1949, 1958, 1969, 1978, 1986, 1991 and 1999. The figure below shows Y secular variation for two observatories; geomagnetic jerks, i.e. rapid changes of the slope of secular variations, are clearly visible.

Y component secular variation as measured at Chambon la Foret and Tucson observatories. Straight lines point out time intervals of constant slope secular variation. In correspondence of the intersections, abrupt slope changes occur, in other words geomagnetic jerks take place.

At present the hypothesis most widely accepted  for the nature of geomagnetic jerk is the one considering that they take origin inside the Earth thus depending on the dynamics of fluid motion inside the fluid outer core. In the past some authors have criticised this hypothesis, assigning to geomagnetic jerk an external origin, possibly related to effects induced by the solar cycle.
Therefore, assuming the internal origin for geomagnetic jerks, it is easy to understand why their study plays an important role in geomagnetism for understanding the mechanisms generating the geomagnetic field and also for the study of mantle conductivity. Indeed, if this phenomenon were of internal origin, it would closely constraint values for the conductivity of the mantle across which the field propagate.
The study of global jerks of 1969, 1978 and 1991 has shown that, generally, this phenomenon is observed first in magnetic data measured by observatories located in the Northern hemisphere and then, after 1-2 years, in those recorded in the Southern hemisphere.

 

 

The 1969 geomagnetic jerk

The 1969 geomagnetic jerk has been the first event to be detected. Its discovery dates back to the end of the 70’s when a group of French scientists realised that the recordings of magnetic data made at some French observatories presented, around 1969, an abrupt change in the slope of secular variation (i.e. the first time derivative of the geomagnetic field). Since then, many studies aimed at understanding this phenomenon from both qualitative and quantitative point of view.
The figure below shows a map with the spatial distribution of the 1969 event delay times. The pattern of this distribution, observed also for other events, represents one of the most intriguing features of geomagnetic jerks, namely to be detected first in the Northern hemisphere and then in the Southern.

Map representing the spatial distribution of delay times in the occurrence, at the observatories marked with green stars, of 1969 geomagnetic jerk. In white areas jerk has been observed in 1970, in red area sit has been observed before 1970 and in blue area sit has been observed after 1970 (for more details De Michelis P. & R. Tozzi, A local intermittency measure (LIM) approach to the detection of geomagnetic jerks, Earth Planet. Sci. Lett., 235, 261-272, 2005).

  


 

The 1978 geomagnetic jerk

As in the cases of 1969 and 1991 geomagnetic jerks the spatial distribution of the delay times of 1978 event is clearly characterised by an early occurrence in the Northern hemisphere and a late occurrence in the Southern hemisphere.

Map representing the spatial distribution of delay times in the occurrence, at the observatories marked with green stars, of 1978 geomagnetic jerk. In white areas jerk has been observed in 1979, in red area sit has been observed before 1979 and in blue area sit has been observed after 1979 (for more details De Michelis P. & R. Tozzi, A local intermittency measure (LIM) approach to the detection of geomagnetic jerks, Earth Planet. Sci. Lett., 235, 261-272, 2005).

  

 

 


 

The 1991 geomagnetic jerk

Time analysis of the geomagnetic field variations, measured at worldwide observatories, has evidenced the possible occurrence of a geomagnetic jerk in 1991. The global character and the spatial distribution of this event have been studied by analysing secular variation of the three Cartesian components of the geomagnetic field (X, Y and Z) at 27 geomagnetic observatories. Results clearly shown the non-simultaneous global occurrence of this event.

Map representing the spatial distribution of delay times in the occurrence, at the observatories marked with green stars, of 1991 geomagnetic jerk. In white areas jerk has been observed around 1992, in red area sit has been observed before 1992 and in blue area sit has been observed after 1992 (for more details De Michelis P. & R. Tozzi, A local intermittency measure (LIM) approach to the detection of geomagnetic jerks, Earth Planet. Sci. Lett., 235, 261-272, 2005).

 

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