  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.
