Ionospheric observations during the geomagnetic storm events on 24–27 July 2004: Long‐duration positive storm effects

Abstract

Ionospheric storms represent large global disturbances of the ionospheric F region electron density in response to geomagnetic storms. In this study, we use a combination of ionospheric total electron content (TEC) global maps and data from in‐situ satellite measurements, such as solar wind data from the Advanced Composition Explorer (ACE), the Defense Meteorological Satellite Program (DMSP), and TOPographic EXplorer (TOPEX) and JASON‐1 satellites, to investigate the ionospheric response during the geomagnetic storm event on 24–27 July 2004. A chain of ground‐based Global Positioning System (GPS) stations and ionosonde measurements across South Africa have been used to give a comprehensive coverage over this midlatitude location. The most pronounced ionospheric effects of the storm occurred at low‐ and midlatitudes in the Southern hemisphere, with the most significant enhancements, observed on 25 and 27 July, presented here. The DMSP F15 satellite observed a sharp density enhancement over the midlatitudes. Over South Africa, the enhancement on 25 July was about twice as large as that observed on 27 July. The positive storm enhancements on 25 and 27 July both lasted over 7 hours, and can be classified as long‐duration positive storm effects. Also, IMF Bz had southward orientation for an extended number of hours (exceeding 9 hours) and could have been the means by which energy was continuously fed into the magnetosphere and ionosphere. In addition, the F region critical frequency (foF2) values observed at two ionosonde stations showed marked positive responses that were associated with an increase in the ionospheric peak height (hmF2).