The motion of depleted plasma regions in the equatorial ionosphere


  • D.N. Anderson,

  • G. Haerendel


Depleted plasma regions in the equatorial ionosphere and their associated motions have been observed by a variety of ground-based, rocket-borne, and satellite instruments. Various theories, based primarily on the Rayleigh-Taylor instability mechanism, have been proposed for the formation and motion of these bubbles. Numerical studies in the past have incorporated local electron density depletions, with Pedersen conductivities involving local ion-neutral collision frequencies. Realizing that bubbles are actually depleted magnetic flux tubes, we investigate the vertical E × B motion of these depleted regions, incorporating flux tube integrated quantities of electron content and Pedersen conductivity. A simple expression for the polarization electric field E1 inside the depleted flux tube is used. The resulting calculations show that the vertical bubble velocity as a function of time critically depends on the background ionospheric electric field and that this dependence extends to much greater heights than was previously thought. Bubbles which are initiated at 350-km altitude (1900LT) with a 5% depletion in electron content attain an upward velocity of 200 m/s at 1920 LT when the background electric field is 0.6 mV/m. Bubble altitude at this time is 447 km with an 88% depletion in electron content. In the absence of an ambient electric field, 1 hour is required for the vertical bubble velocity to reach 200 m/s.