Equatorial ionospheric response to isolated auroral substorms over a solar cycle (1980−85): evidence of longitudinal anomaly

Abstract. The equatorial ionospheric response to 228 isolated, rapid-onset auroral substorms (as defined from the auroral electrojet index AE) was found from enhancements of the virtual (minimum) height of the F-region (∆h$^\prime$F) in the declining phase of a solar cycle (1980–85). The responses, found for three longitudinal sectors at the equator: Africa (Ouagadougou and Dakar), Asia (Manila) and America (Huancayo), were compared with the response close to the auroral source region at Yakutsk (northern Siberia). The auroral substorm onsets were centered at 17 and 15 UT at sunspot maximum (1980–82) and minimum (1983–85), preceding by 3–5 h the period of post-sunset height rise in the African sector whereas other sectors were in the early afternoon (Huancayo) and morning (Manila). The African response, particularly at Ouagadougou, was distinctly different from other sectors. In the sunspot maximum years (1980–81) the auroral surges were followed after about 3 h by a sharp depression (∆h$^\prime$F<0) in the post-sunset height rise, with a period of little or no response (∆h$^\prime$F=0) in 1982. A response polarity reversal (∆h$^\prime$F>0) was noted in this sector for sunspot minimum (1983–85) when large h$^\prime$F enhancements were observed at the sunset region. The responses in the Asian and American sector were positive except for a case in Huancayo when response was negative, following an auroral surge before the sunset at this station. It appears that the aurorally generated large-scale travelling ionospheric disturbances (LSTIDs), which first cause positive height enhancements in a sub-auroral location (Yakutsk), subsequently affect the unstable post-sunset ionosphere in the equatorial Africa.