With the exception of some pioneering mm-VLBI observations, the present-day VLBI cannot resolve the most compact components in active galaxies, nor measure their brightness temperatures. In addition, the observing frequencies are too low: the innermost components are still self-absorbed and often completely undetectable at standard VLBI frequencies. However, studies of continuum spectra and variations of AGN can give us estimates of what will eventually be seen when VLBI techniques reach sufficient resolutions and frequencies. These estimates are also useful in considering what capabilities will be required from future interferometry for studying AGN.
We have used multifrequency continuum monitoring data together with standard shock-in-jet models to derive flux, size and brightness temperature distributions for the innermost core components in a complete 2 Jy sample of AGN. The millimeter spectra and its variations can be satisfactorily explained by just two types of high-frequency components, a relativistic inhomogeneous jet and evolving shocks in this jet (Valtaoja et al. 1988). Moreover, with extended multifrequency flux monitoring the contributions from these two components can be separated, which makes possible the extraction of limited amounts of structural data (average sizes, brightness distributions, etc.) just from continuum data.
We have used two main methods. Cm-to-mm spectra can reveal the existence of persistent high frequency components and give estimates of their fluxes and turnover frequencies. Continuum monitoring gives fluxes and variability timescales of transient components, and further their sizes and brightness temperatures.