Geomagnetic jerk

amplitudes recovered by time-dependent flows

Pinheiro, K. J. (1), Amit, H. (2) and Terra-Nova, F. (2)

(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.


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.