Abstract
We present geomagnetic main field and secular variation time series, at 300 equal-area distributed locations and at 490 km altitude, derived from magnetic field measurements collected by the three Swarm satellites. These Geomagnetic Virtual Observatory (GVO) series provide a convenient means to globally monitor and analyze long-term variations of the geomagnetic field from low-Earth orbit. The series are obtained by robust fits of local Cartesian potential field models to along-track and East–West sums and differences of Swarm satellite data collected within a radius of 700 km of the GVO locations during either 1-monthly or 4-monthly time windows. We describe two GVO data products: (1) ‘Observed Field’ GVO time series, where all observed sources contribute to the estimated values, without any data selection or correction, and (2) ‘Core Field’ GVO time series, where additional data selection is carried out, then de-noising schemes and epoch-by-epoch spherical harmonic analysis are applied to reduce contamination by magnetospheric and ionospheric signals. Secular variation series are provided as annual differences of the Core Field GVOs. We present examples of the resulting Swarm GVO series, assessing their quality through comparisons with ground observatories and geomagnetic field models. In benchmark comparisons with six high-quality mid-to-low latitude ground observatories we find the secular variation of the Core Field GVO field intensities, calculated using annual differences, agrees to an rms of 1.8 nT/yr and 1.2 nT/yr for the 1-monthly and 4-monthly versions, respectively. Regular sampling in space and time, and the availability of data error estimates, makes the GVO series well suited for users wishing to perform data assimilation studies of core dynamics, or to study long-period magnetospheric and ionospheric signals and their induced counterparts. The Swarm GVO time series will be regularly updated, approximately every four months, allowing ready access to the latest secular variation data from the Swarm satellites.
Highlights
The geomagnetic field undergoes gradual change, evolving year by year in a process known as geomagnetic secular variation
Note the small difference in the time scales shown at the bottom left of these two figures; the secular variation (SV) of the 1-monthly Geomagnetic Virtual Observatory (GVO)-CORE time series begins in 2015.5, since the GVOCORE time series starts only in 2015 due to the principal component analysis (PCA) processing, while the SV of the 4-monthly GVO-CORE begins in 2014.7 since no PCA is not performed on these
Given that the requirement for a good standard (INTERMAGNET) ground observatory is an accuracy of 5 nT this indicates that the GVO method yields results comparable on these time scales with good ground observatories
Summary
The geomagnetic field undergoes gradual change, evolving year by year in a process known as geomagnetic secular variation. These changes are thought to result primarily from motions of liquid metal in the Earth’s outer core but this process is not yet well enough understood to allow accurate predictions of future behavior, even a few years ahead (e.g., Alken et al 2020a). Hammer et al Earth, Planets and Space (2021) 73:54 models, such that the processes underlying secular variation can be better understood. For both monitoring long-term geomagnetic variations, and for data assimilation applications, it is an advantage to have processed satellite magnetic field data available on a well organized grid, with a regular sampling rate in space and time. The Geomagnetic Virtual Observatory (GVO) method is one approach to obtain such a dataset
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