Unusual subpulse modulation in PSR B0320+39

We report on an analysis of the drifting subpulses of PSR B0320+39 that indicates a sudden step of ~180 degrees in subpulse phase near the centre of the pulse profile. The phase step, in combination with the attenuation of the periodic subpulse modulation at pulse longitudes near the step, suggests that the patterns arise from the addition of two superposed components of nearly opposite drift phase and differing longitudinal dependence. We argue that since there cannot be physical overlap of spark patterns on the polar cap, the drift components must be associated with a kind of "multiple imaging" of a single polar cap "carousel" spark pattern. One possibility is that the two components correspond to refracted rays originating from opposite sides of the polar cap. A second option associates the components with emission from two altitudes in the magnetosphere.