The low-frequency structure of powerful radio sources and limits to departures from equipartition

Recent interplanetary scintillation and spectral data have been combined to derive information about source structures at frequencies below 100 MHz. For an appreciable fraction of sources there is evidence of extended components which are prominent at low frequencies but which have not been detected in higher frequency interferometric observations. Sources having low-frequency spectral turnovers are studied in detail and in several cases it is shown that the total energy is within a factor 30 of the energy of equipartition between the particles and magnetic fields. In no case is there conclusive evidence of departure from equipartition.