The spin modulation of the high-frequency noise from one electrical antenna of the Viking satellite has been used to determine for the first time the direction of the polarization plane of the auroral kilometric radiation (AKR) inside the sources of the radiation (i.e., at the local frequency of generation). The observations show a clear difference between the modulation patterns obtained inside the AKR sources and the ones obtained far from them. The chi-square goodness of fit test has been used to compare the measurements made during 36 AKR source crossings with a theoretical model accounting for the specific Viking antenna equipment. This statistical study shows that, inside the sources, the wave electric field is directed perpendicular to the static magnetic field, within 10¿. This observation, consistent with a wave generation by the cyclotron maser instability, is also a fundamental constraint on any model attempting to relate the wave observations far from the sources to the physical-characteristics of the sources. ¿ American Geophysical Union 1992