  We present local time-series of the magnetic field gradient tensor elements
at satellite altitude derived using a Geomagnetic Virtual Observatory (GVO)
approach. Gradient element timeseries are computed in 4-monthly bins on an
approximately equal-area distributed worldwide network. This enables global
investigations of spatio temporal variations in the gradient tensor elements.
Series are derived from data collected by the Swarm and CHAMP satellite
missions, using vector field measurements and their along-track and east-west
differences, when available. We find evidence for a regional secular variation
impulse (jerk) event in 2017 in the first time derivative of the gradient
tensor elements. This event is located at low latitudes in the Pacific region.
It has a similar profile and amplitude regardless of the adopted data selection
criteria and is well fit by an internal potential field. Spherical harmonic
models of the internal magnetic field built from the GVO gradient series show
lower scatter in near-zonal harmonics compared with models built using standard
GVO vector field series. The GVO gradient element series are an effective means
of compressing the spatio temporal information gathered by low-Earth orbit
satellites on geomagnetic field variations, which may prove useful for core
flow inversions and in geodynamo data assimilation studies.
