  Evidence of fast variations in the Earth's core field are seen both in
magnetic observatory and satellite records. We present here how they have been
identified at the Earth's surface from ground-based observatory records and how
their spatio-temporal structure is now characterised by satellite data. It is
shown how their properties at the core mantle boundary are extracted through
localised and global modelling processes, paying particular attention to their
time scales. Finally are listed possible types of waves in the liquid outer
core, together with their main properties, that may give rise to these observed
fast variations.
