TY  - JOUR
A1  - Sanchez, S.
A1  - Wicht, J.
A1  - Baerenzung, Julien
A1  - Holschneider, Matthias
T1  - Sequential assimilation of geomagnetic observations
BT  - perspectives for the reconstruction and prediction of core dynamics
JF  - Geophysical journal international
N2  - High-precision observations of the present-day geomagnetic field by ground-based observatories and satellites provide unprecedented conditions for unveiling the dynamics of the Earth’s core. Combining geomagnetic observations with dynamo simulations in a data assimilation (DA) framework allows the reconstruction of past and present states of the internal core dynamics. The essential information that couples the internal state to the observations is provided by the statistical correlations from a numerical dynamo model in the form of a model covariance matrix. Here we test a sequential DA framework, working through a succession of forecast and analysis steps, that extracts the correlations from an ensemble of dynamo models. The primary correlations couple variables of the same azimuthal wave number, reflecting the predominant axial symmetry of the magnetic field. Synthetic tests show that the scheme becomes unstable when confronted with high-precision geomagnetic observations. Our study has identified spurious secondary correlations as the origin of the problem. Keeping only the primary correlations by localizing the covariance matrix with respect to the azimuthal wave number suffices to stabilize the assimilation. While the first analysis step is fundamental in constraining the large-scale interior state, further assimilation steps refine the smaller and more dynamical scales. This refinement turns out to be critical for long-term geomagnetic predictions. Increasing the assimilation steps from one to 18 roughly doubles the prediction horizon for the dipole from about  tree to six centuries, and from 30 to about  60 yr for smaller observable scales. This improvement is also reflected on the predictability of surface intensity features such as the South Atlantic Anomaly. Intensity prediction errors are decreased roughly by a half when assimilating long observation sequences.
KW  - Magnetic field variations through time
KW  - Core dynamics
KW  - Dynamo: theories and simulations
KW  - Inverse theory
KW  - Probabilistic forecasting
Y1  - 2019
U6  - https://doi.org/10.1093/gji/ggz090
SN  - 0956-540X
SN  - 1365-246X
VL  - 217
IS  - 2
SP  - 1434
EP  - 1450
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Pick, Leonie
A1  - Korte, Monika
T1  - An annual proxy for the geomagnetic signal of magnetospheric currents on Earth based on observatory data from 1900–2010
JF  - Geophysical Journal International
N2  - We introduce the Annual Magnetospheric Currents index as long-term proxy for the geomagnetic signal of magnetospheric currents on Earth valid within the time span 1900–2010. Similar to the widely used disturbance storm time and ‘Ring Current’ indices, it is based on geomagnetic observatory data, but provides a realistic absolute level and uncertainty estimates. Crucial aspects to this end are the revision of observatory crustal biases as well as the implementation of a Bayesian inversion accounting for uncertainties in the main field estimate, both required for the index derivation. The observatory choice is based on a minimization of index variance during a reference period spanning 1960–2010. The new index is capable of correcting observatory time series from large-scale external signals in a user-friendly manner. At present the index is only available as annual mean values. An extension to hourly values for the same time span is in progress.
KW  - Magnetic field variations through time
KW  - Satellite magnetics
KW  - Inverse theory
KW  - Statistical methods
KW  - Time-series analysis
Y1  - 2017
U6  - https://doi.org/10.1093/gji/ggx367
SN  - 1365-246X
SN  - 0956-540X
VL  - 211
IS  - 2
SP  - 1223
EP  - 1236
PB  - Oxford Univ. Press
CY  - Oxford
ER  -