Unusual profile variations in pulsar PSR J1022+1001 - Evidence for magnetospheric "return currents"?

We report a detailed multi-frequency study of significant instabilities observed in the average pulse profile of the 16-millisecond pulsar PSR J1022+1001. These unusual profile variations which are seen as a function of time and of radio frequency are clearly different from classical profile mode-changing. We also note discrete jumps in the polarisation position angle curve of this pulsar which are remarkably coincident with the unstable profile component. We propose that these jumps, as well as the instability of the pulse profile, are due to magnetospheric return currents. This would allow us to measure the basic properties of the magnetospheric plasma for the very first time.