Eddy

The Fifth Doctor, when using the Ionograph to detect the ion trail of a spaceship or spacepod, likened this trail to that of an eddy, and indeed dubbed their iono-trail a "space eddy". (AUDIO: Hexagora)

Behind the scenes

An eddy is a term used to describe loops of turbulence that come from an object moving through a fluid. On Earth, for example, it can be observed when a water current flows around a rock as seen on a beach or a coastal shore.