Equatorial F region irregularity morphology during an equinoctial month at solar minimum

A large number of instruments was used in October 1996 to record activities in the equatorial ionosphere above South America. In a month at solar minimum, data were obtained at various levels of magnetic activity and various levels of ionospheric irregularity development. With this multi-instrumented study, it was possible to utilize optical data, radar, GPS transmissions, and ionosondes at various sites in the equatorial region. The concept of this paper is to review the plethora of events which occurred during this month with a view to describing the interrelationship of the wide variety of irregularity developments. Data were obtained on nights when no irregularities were observed at any location in the equatorial region across South America. There were nights when only localized irregularity structures with relatively narrow latitudinal and longitudinal effects were noted close to the magnetic equator. We noted the occasional presence in the 02–06 local time period of plume structures with data available from optical observations as well as from phase and amplitude scintillation. During a major magnetic storm on one night, October 22–23, a long lasting high altitude plume was detected by the Jicamarca radar. On this night, irregularities were noted all across South America and even beyond the western and eastern coasts. This plume produced ionospheric effects which could be traced to turbulence at over 2000 km above the magnetic equator. With additional data from high latitude stations and from Guam and Kwajelein, it was possible to link and compare irregularity development in the same time period over a large portion of the globe. The aim of this paper is to give a day-to-day picture of the occurrence and intensity of equatorial irregularity development over a month-long period rather than a short case study or the converse, long term statistics over several seasons. Using this database and the modeling of total electron content as a function of solar flux, we outline the possibilities and limitations for forecasting irregularity activity in this region for a period of low solar flux. Forecasting is limited and calls for experimental data for necessary and sufficient gradients and wind conditions for plumes to fully develop. This revised version was published online in June 2006 with corrections to the Cover Date.