amplitudes recovered by time-dependent flows
Pinheiro, K. J. (1), Amit, H. (2) and Terra-Nova, F. (2)
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.
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.