  This study uses multi-frequency Very Long Baseline Interferometry (VLBI) to
study the radio emission from 10 radio-quiet quasars (RQQs) and four radio-loud
quasars (RLQs). The diverse morphologies, radio spectra, and brightness
temperatures observed in the VLBI images of these RQQs, together with the
variability in their GHz spectra and VLBI flux densities, shed light on the
origins of their nuclear radio emission. The total radio emission of RQQs
appears to originate from non-thermal synchrotron radiation due to a
combination of active galactic nuclei and star formation activities. However,
our data suggest that the VLBI-detected radio emission from these RQQs is
primarily associated with compact jets or corona, with extended emissions such
as star formation and large-scale jets being resolved by the high resolution of
the VLBI images. Wind emission models are not in complete agreement the VLBI
observations. Unlike RLQs, where the parsec-scale radio emission is dominated
by a relativistically boosted core, the radio cores of RQQs are either not
dominant or are mixed with significant jet emission. RQQs with compact cores or
core-jet structures typically have more pronounced variability, with flat or
inverted spectra, whereas jet-dominated RQQs have steep spectra and
unremarkable variability. Future high-resolution observations of more RQQs
could help to determine the fraction of different emission sources and their
associated physical mechanisms.
