Velocity fluctuations associated with polar cap patches

The digital ionosonde technique has been used to measure the convection in the vicinity of polar cap patches. Two types of velocity fluctuations were seen: (1) parallel fluctuations which are a slowing down and speeding up of the convection without a change in direction, and (2) perpendicular fluctuations which are perpendicular to the average convection. The parallel type of fluctuation was most often seen when the patch density ratio was large, and the patch tended to be overhead at the time of the minimum of the velocity fluctuation. The perpendicular fluctuation behavior tended to be seen when the patch density ratio was small and the patch tended to be overhead at an extremum of the velocity fluctuation. Two mechanisms that can possibly explain the behavior are discussed: (1) Convection changes are produced ''internally'' because of electric field fluctuations caused by the enhanced conductivity of the patch interacting with the cross polar cap current system. (2) The electric field fluctuations are imposed ''externally'' by fluctuations in the induced electric field from the solar wind. It was found that the internal mechanism seems to be most consistent with the properties of the parallel fluctuations, and the external mechanism seemed most consistent with the perpendicular fluctuations.