Main field and secular variation candidate models for the 12th IGRF generation after 10 months of Swarm measurements

Research areas:
Earth, Planets, and Space
We describe the main field and secular variation candidate models for
the 12th generation of the International Geomagnetic
Reference Field model. These two models are derived
from the same parent model, in which the main field
is extrapolated to epoch 2015.0 using its associated
secular variation. The parent model is exclusively
based on measurements acquired by the European Space
Agency Swarm mission between its launch on
11/22/2013 and 09/18/2014. It is computed up to
spherical harmonic degree and order 25 for the main
field, 13 for the secular variation, and 2 for the
external field. A selection on local time rather
than on true illumination of the spacecraft was
chosen in order to keep more measurements. Data
selection based on geomagnetic indices was used to
minimize the external field
contributions. Measurements were screened and
outliers were carefully removed. The model uses
magnetic field intensity measurements at all
latitudes and magnetic field vector measurements
equatorward of 50° absolute quasi-dipole magnetic
latitude. A second model using only the vertical
component of the measured magnetic field and the
total intensity was computed. This companion model
offers a slightly better fit to the
measurements. These two models are compared and
discussed.We discuss in particular the quality of
the model which does not use the full vector
measurements and underline that this approach may be
used when only partial directional information is
known. The candidate models and their associated
companion models are retrospectively compared to the
adopted IGRF which allows us to criticize our own