It is clearly of interest to image AGN with high resolution, to obtain information about regions close to the central energy source and base of the jets. An extremely powerful tool for studies of radio-loud AGN is Very Long Baseline Interferometry (VLBI), in which radio telescopes around the world are used in synchrony to obtain images with angular resolutions of the order of a milliarcsecond on the sky, or about a parsec (3.26 light years) at the cosmological distances characteristic of AGN. This is roughly 50-100 times better than could be achieved with the Hubble Space Telescope at optical wavelengths.
On the parsec scales probed by VLBI, the radio emitting regions of radio-loud AGN usually display a one-sided "core+jet" structure. The intrinsic structure is believed to be two-sided, with the two oppositely directed relativistic jet outflows both being highly "beamed" in the forward direction of their motion. If the jet axis is oriented at a small angle to the line of sight, as we believe to be the case for these objects, the observed brightness of the approaching jet is boosted, while that of the receding jet is "deboosted", giving rise to the observed one-sided structure. This geometry can also explain the apparent "superluminal" motions that are observed for individual outflowing knots in the radio jets. It is thought that the radio jets are probably ejected along the rotational axis of the central black hole, roughly perpendicular to the plane of an accretion disk surrounding the black hole.
The intrinsic linear polarization of synchrotron radiation can reach about 75% in the case of a uniform magnetic field and a random pitch-angle distribution for the radiating electrons. VLBI polarization measurements are a unique source of information about the jet B fields, as well as the densities of relativistic and thermal electrons in and around the radio-emitting regions.