The dynamic structure of large amplitude alfvenic disturbances of the interplanetary magnetic field (MF) is examined by transforming one-hour intervals of Explores 33 and 35 magnetometer data from the solar ecliptic coordinate system to a coordinate system defined by the principal axes of the variance matrix. Graphic displays of the rotated magnetometer data, in the form of linear time series for the field component in the direction of minimum variance and hodographs for the magnetic field vector component in the plane of maximum variance. reveal order structure to field components which appear almost random when displayed as time series in the solar ecliptic coordiante system. When there is a well defined minimum variance direction, the component of the magnetic field in the plane normal to the direction of minimum variance tends to oscillate with constant magnitude so that the tip of the vector lies on a circular arc. However, the angular motion of the vector appears to be unstrained. This type of behavior for the IMF satisfies the necessary local conditions for plane large amplitude Alfven waves. Residual variability of the magnetic field in the direction of minimum variance may be due to the fully three-dimensional oscillations of the nearly plane waves or a small compressional component associated with fluctuations in the magnetic field magnitude. The different types of angular motion of the magnetic field component normal to the direction of minimmum variance may be indicative of the detailed conditions of the solar coronal plasma in the regions generating the Alfven waves, or some aspect of local generation.