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\begin{document}

\title{The signature of geomagnetic field external drivers in Swarm data}



\author[1]{Diana Saturnino}%
\author[2]{Maria Alexandra Pais}%
\author[2]{João Domingos}%
\affil[1]{LPG UMR-6112}%
\affil[2]{University of Coimbra}%


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  \date{June 15, 2024}


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\begin{abstract}
The separation of contributions from different sources in the magnetic
field signal measured at satellite altitude is an open challenge. An
approach to this problem, using Principal Component Analysis, is here
applied to geomagnetic external field series at Virtual Observatories
(VO). These series are computed from an enlarged dataset of Swarm data
covering all local times and all geomagnetic activity levels between
January 2014 and December 2019. For each 30-days time window, the
Equivalent Source Dipole technique is used to reduce all measurements
inside a cylinder to one single `observation' at its axis and 500 km
altitude. Our results reveal a first principal mode with dipolar
geometry and time variation following very closely the RC-index of
geomagnetic activity. They display a resolved second principal mode with
annual periodicity and of approximately zonal quadrupolar radial
pattern, reminiscent of results in a previous study using VO series from
a filtered satellite dataset and with lower time resolution. We resort
to the recent comprehensive model CM6 to identify a possible source for
this second mode. We propose that the dipolar mode is the expression of
the magnetospheric ring current dynamics, at 30-day time resolution, and
the quadrupolar mode is the expression of the annual asymmetry between
local summer and winter Sq current vortices. Two fainter modes could be
related to the equinoctial amplification of Sq vortices and the
ionospheric dynamo modulation by nonmigrating tides. We show that a more
uniform local time sampling could contribute to better resolve
ionospheric structures.%
\end{abstract}%



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