  Ground observatory and satellite-based determinations of temporal variations
in the geomagnetic field probe a decadal to annual time scale range where
Earth's core slow, inertialess convective motions and rapidly propagating,
inertia-bearing hydromagnetic waves are in interplay. Here we numerically model
and jointly investigate these two important features with the help of a
geodynamo simulation that (to date) is the closest to the dynamical regime of
Earth's core. This model also considerably enlarges the scope of a previous
asymptotic scaling analysis. Three classes of hydrodynamic and hydromagnetic
waves are identified in the model output, all with propagation velocity largely
exceeding that of convective advection: axisymmetric, geostrophic Alfv\'en
torsional waves, and non-axisymmetric, quasi-geostrophic Alfv\'en and Rossby
waves. The contribution of these waves to the geomagnetic acceleration amounts
to an enrichment and flattening of its energy density spectral profile at
decadal time scales, thereby providing a constraint on the extent of the
$f^{-4}$ range observed in the geomagnetic frequency power spectrum. The flow
and magnetic acceleration energies carried by waves both linearly increase with
the ratio of the magnetic diffusion time scale to the Alfv\'en time scale,
highlighting the dominance of Alfv\'en waves in the signal and the stabilising
control of magnetic dissipation at non-axisymmetric scales. Extrapolation of
the results to Earth's core conditions supports the detectability of Alfv\'en
waves in geomagnetic observations, either as axisymmetric torsional
oscillations or through the geomagnetic jerks caused by non-axisymmetric waves.
In contrast, Rossby waves appear to be too fast and carry too little magnetic
energy to be detectable in geomagnetic acceleration signals of limited
spatio-temporal resolution.
