  The inner regions of protoplanetary disks are the locations where most of
planets are thought to form and where processes that influence the global
evolution of the disk, such as MHD-winds and photoevaporation, originate.
Transition disks (TDs) with large inner dust cavities are the ideal targets to
study the inner tens of au of disks, as the central emission can be fully
disentangled from the outer disk emission. We present a homogeneous
multi-wavelength analysis of the continuum emission in a sample of 11 TDs. We
investigate the nature of the central emission close to the star,
distinguishing between thermal dust and free-free emission. Spatially resolved
measurements of continuum emission from archival ALMA data are combined with
literature cm-wave observations to study the spectral indices of the inner and
outer disks separately. While the emission from the outer disks is consistent
with thermal dust emission, 10/11 of the spectral indices estimated for the
central emission close to the star suggest that this emission is free-free
emission, likely associated with an ionized jet or a disk wind. No correlation
between the free-free luminosity and the accretion luminosity or the X-ray
luminosity is found, arguing against the photoevaporative wind origin. A
sub-linear correlation between the ionized mass loss rate and the accretion
rate onto the star is observed, suggesting an origin in an ionized jet. The
relative lack of mm-dust grains in the majority of inner disks in transition
disks suggests that either such dust grains have drifted quickly towards the
central star, grain growth is less efficient in the inner disk, or grains grow
rapidly to planetesimal sizes in the inner disk. The observed correlation
between the ionized mass loss rate and the accretion rate suggests the outflow
is strictly connected with the stellar accretion and that accretion in these
disks is driven by a jet.
