We present results of simultaneous dual-frequency (2 GHz and 8 GHz) very long baseline interferometry (VLBI) observations of 12 active galactic nuclei with prominent jets. Spectral properties of the jets and evolution of their brightness temperature are discussed. Measured sizes and brightness temperatures of VLBI features are found to be consistent with emission from relativistic shocks dominated by adiabatic losses. Physical scenarios with different magnetic field orientation in the jets are discussed.
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