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Geomagnetic jerk amplitudes recovered by time-dependent flows |
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Pinheiro, K. J. (1), Amit, H. (2) and Terra-Nova, F. (2) |
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(1) Geophysics Department, Observatório Nacional, 20921-400 Rio de Janeiro, Brazil. (2) CNRS, Université de Nantes, Nantes Atlantiques Universités, UMR CNRS 6112, Laboratoire de Planétologie et de Géodynamique, 2 rue de la Houssinière, F-44000 Nantes, France. |
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The geomagnetic field generated in the outer core varies on a wide range of timescales, from the geomagnetic secular variation (SV) over months to hundreds of years to paleomagnetic SV over longer timescales such as reversals and chron durations. Abrupt changes of the SV termed geomagnetic jerks represent the shortest observed timescales of the core field. A jerk is commonly defined as a ``V-shape'' of the geomagnetic SV or more generally as a change of sign in the secular acceleration (SA). Neither the physical mechanism producing such abrupt changes nor their spatio-temporal characteristics at the Earth's surface are well understood. We used a set of synthetic core flow models to solve the radial magnetic induction equation in order to reproduce geomagnetic jerk characteristics. Steady flow models may reproduce important characteristics of geomagnetic jerks, such as non-simultaneous behaviour, non-global pattern, spatial variability of amplitudes and strongest jerks in the radial component. However, secular acceleration changes of sign induced by the steady flows produce too weak amplitudes compared to geomagnetic jerks. Flow models with steady patterns but a time-dependent amplitude produce jerk amplitudes about 70 times larger than steady flow models, comparable to jerk amplitudes found in magnetic observatories. We present results of ten magnetic observatories from various regions of the Earth to demonstrate the typical geomagnetic jerk amplitudes and temporal characteristics. Because our large-scale flows yield smooth synthetic SV timeseries and magnetic jerks which are better fitted by a third order polynomial than by straight-line segments, we also examine the polynomial fit to the data. We compare the two fits and jerk amplitudes in our synthetic models and in geomagnetic data. |
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Applications |
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oral |
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