The delayed occurrence of equatorial ionospheric F2 layer post-sunset height decreases following auroral-zone substorm onsets

Nighttime F₂ layer height decreases have been examined for post-sunset intervals at the equatorial station, Huancayo. The analyses involved mainly R_z max years, (1957–1960), although R_z min years (1974–1977) were also used. The data were obtained from tabulations of the ionogram parameters h'F, f_○F₂ and f_○E_s. The height reductions are delayed by more than 7 h following geomagnetic substorm onsets for locations at longitudes to the east. The reduced occurrence of spread-F and f_○E_s enhancements is found to be associated. These enhancements are also recorded at the European station, Dourbes about 4.5 h before the Huancayo enhancements. It is proposed that an LS-TID which propagates in the 80 km sound channel may be involved. Also, the experimental evidence suggests that the westward propagation of the LS-TIDs allows equatorial disturbances to occur at local times (before midnight) which are similar to the times when the LS-TIDs are generated.