  Context: The fast rotator, pre-main sequence star AB Dor A is a strong and
persistent radio emitter. The extraordinary coronal flaring activity is thought
to be the origin of compact radio emission and other associated phenomena as
large slingshot prominences. Aim: We aim to investigate the radio emission
mechanism and the milliarcsecond radio structure around AB Dor A. Methods: We
performed phase-referenced VLBI observations at 22.3 GHz, 8.4 GHz, and 1.4 GHz
over more than one decade using the Australian VLBI array. Results: Our 8.4 GHz
images show a double core-halo morphology, similar at all epochs, with emission
extending at heights between 5 and 18 stellar radii. Furthermore, the sequence
of the 8.4 GHz maps shows a clear variation of the source structure within the
observing time. However, images at 1.4 GHz and 22.3 GHz are compatible with a
compact source. The phase-reference position at 8.4 GHz and 1.4 GHz are
coincident with those expected from the well-known milliarcsecond-precise
astrometry of this star, meanwhile the 22.3 GHz position is 4$\sigma$ off the
prediction in the north-west direction. The origin of this offset is still
unclear. Conclusions: We have considered several models to explain the
morphology and evolution of the inner radio structure detected in AB Dor A
which include emission from the stellar polar caps, a flaring,
magnetically-driven loop structure, and the presence of helmet streamers. A
possible close companion to AB Dor A has been also investigated. Our results
confirm the extraordinary coronal magnetic activity of this star, able to
produce compact radio structures at very large heights, so far only seen in
binary interacting systems.
