The radio emission of weakly active galactic nuclei is in most cases composed of extended emission from radiojets and a compact radio core with an inverted spectrum. Optically thin synchrotron emission of high-energy electrons is considered as one possible explanation and is applied to the galactic center source Sgr A* and to the radio and infrared emission from the cores of M81 and NGC1068.

An alternative explanation is given in terms of optically thick synchrotron emission of an advection dominated accretion flow into a supermassive black hole. The general relativistic description of viscous accretion flows close to the horizon of a black hole is discussed. The thermal balance given by viscous heating, radiation cooling and energy advection for a two-temperature plasma is solved using an approximate treatment of the radial structure of the flow. The resulting synchrotron and free-free emission is compared with the spectrum of Sgr A*.

The advection dominated accretion mode is considered as an explanation for the distinction between active and inactive galactic nuclei. Only a few intermediate objects are suggested.

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