  We use 20 years of magnetic field measurements from the Oersted, CHAMP and
Swarm satellite missions, supplemented by calibrated platform magnetometer data
from the CryoSat2 satellite, to study time variations of the Earth's core field
at satellite altitude and at the core-mantle boundary (CMB). From the satellite
data we derive composite time series of the core field secular variation (SV)
with 4month cadence, at 300 globally distributed Geomagnetic Virtual
Observatories (GVO). GVO radial SV series display regional fluctuations with
5-10 years duration and amplitudes reaching 20 nT/yr, most notably at low
latitudes over Indonesia (2014), over South America and the South Atlantic
(2007, 2011 and 2014), and over the central Pacific (2017). Applying the
Subtractive Optimally Localized Averages (SOLA) method, we map the SV at the
CMB as a collection of locally averaged SV estimates. We demonstrate that using
2-year windows of CryoSat2 data, it is possible to reliably estimate the SV and
its time derivative, the secular acceleration (SA), at the CMB, with a spatial
resolution, corresponding to spherical harmonic degree 10. Along the CMB
geographic equator, we find strong SA features under Indonesia from 2011-2014,
under central America from 2015 to 2019, and sequences of SA with alternating
sign under the Atlantic during 2004-2019. We find that data from CryoSat2 make
a valuable contribution to the emerging picture of sub-decadal core field
variations. Using 1 year windows of data from the Swarm satellites, it is
possible to study SA changes at low latitudes on timescales down to 1 year,
with spatial resolution corresponding to spherical harmonic degree 10. We find
strong positive and negative SA features appearing side-by-side in the Pacific
in 2017, and thereafter drift westward.
