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This study uses multi-frequency Very Long Baseline Interferometry (VLBI) tostudy the radio emission from 10 radio-quiet quasars (RQQs) and four radio-loudquasars (RLQs). The diverse morphologies, radio spectra, and brightnesstemperatures observed in the VLBI images of these RQQs, together with thevariability in their GHz spectra and VLBI flux densities, shed light on theorigins of their nuclear radio emission. The total radio emission of RQQsappears to originate from non-thermal synchrotron radiation due to acombination of active galactic nuclei and star formation activities. However,our data suggest that the VLBI-detected radio emission from these RQQs isprimarily associated with compact jets or corona, with extended emissions suchas star formation and large-scale jets being resolved by the high resolution ofthe VLBI images. Wind emission models are not in complete agreement the VLBIobservations. Unlike RLQs, where the parsec-scale radio emission is dominatedby a relativistically boosted core, the radio cores of RQQs are either notdominant or are mixed with significant jet emission. RQQs with compact cores orcore-jet structures typically have more pronounced variability, with flat orinverted spectra, whereas jet-dominated RQQs have steep spectra andunremarkable variability. Future high-resolution observations of more RQQscould help to determine the fraction of different emission sources and theirassociated physical mechanisms.

VLBI Observations of a sample of Palomar-Green quasars II: characterising the parsec-scale radio emission