Detecting low-latitude outer core-surface waves with 25 years of satellite secular variation data

Abstract

Fluid motion in the Earth’s liquid outer core generates most of the geomagnetic field, and its time changes over timescales of one year or longer, the secular variation (SV). Data from the satellite missions Ørsted, CHAMP, CryoSat-2 and Swarm, together with data from ground observatories, were combined to yield a SV dataset spanning from late 1997 to early 2023. These SV data were inverted for time-varying core surface fluid velocity assuming it is purely advective, with the main field specified by the CHAOS-7.16 field model. The inversion was regularised both in time and in space. In time, the difference in velocity between individual epochs was minimised. In space, small-scale velocity structures were penalised. Flow acceleration was then calculated from first differences of velocities at successive epochs. Time-longitude diagrams of azimuthal acceleration show sloping features at low latitudes, interpreted as signatures of propagating waves. Waves propagating both eastwards and westwards were observed, with propagation velocities of approximately 1700 km/yr which is in agreement with previous inferences of fast core waves. Power spectral density plots reveal that the energy is concentrated in modes with periods of 6–7 years, and azimuthal wavenumbers −5, −2 and 2, where negative wave numbers indicate westward motion. There is a higher energy content in the westward propagating waves than in those travelling eastwards. Finally, we find intermittent low-latitude standing waves, which coincide with times of recent equatorial geomagnetic jerks, consistent with inferences of magneto-Coriolis and Alfvén waves from other studies.