• storm-enhanced densities;
  • equatorial fountain;
  • data assimiliation

[1] The October 2003 geomagnetic storm (often called the Halloween storm) was one of the largest storms (as measured by Dst) yet recorded. The storm-induced synoptic-scale changes in the ionosphere's plasma content and density can be viewed through space weather maps created by objective analysis algorithms. For this study, these maps, which specify the electron density in altitude, latitude, and longitude, are created by the ionospheric data assimilation three dimensional (IDA3D), a three-dimensional variation algorithm of the ionospheric electron density. These maps, representing the average conditions in the ionosphere over a 15 min sampling time, show how dramatically the ionosphere changed during the Halloween storm. Following the southward turning of the interplanetary magnetic field, the dayside electron content is significantly reduced in the equatorial ionosphere between ±18° magnetic latitude and is enhanced poleward of this latitude. This is the expected behavior when the equatorial fountain is enhanced by a strong penetration electric field. In addition, the electron content is significantly increased in the dayside midlatitude ionosphere, which corresponds to a storm-enhanced density (SED) plume. Above 40° magnetic latitude, the dayside plasma content is significantly reduced in the regions adjacent to the SED structure, which enhances the electron content gradient. Electron density maps in the altitude–magnetic latitude plane show an increase in the topside electron densities within an SED plume.