Deduction of the faint travelling ionospheric structures using topside sounding data

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Asymmetric response of the topside ionosphere to large-scale IGW generated during the November 30, 1979, substorm

We used bottomside ground observations and topside sounding data from the Intercosmos-19 satellite to study a Travelling Ionospheric Disturbance (TID) that occurred in response to Large-Scale Internal Gravity Wave (LSIGW) propagation during a substorm on November 30, 1979. We built a global scheme for the wavelike ionospheric variations during this medium substorm (AEmax 800 nT). The area where the TID was observed looks like a wedge since it covers the nighttime hours at subauroral latitudes but contracts to a 02 h local sector at low latitudes. The ionospheric response is strongly asymmetric because the wedge area and the TID amplitude are larger in the winter hemisphere than in the summer hemisphere. Clear evidence was obtained indicating that the more powerful TID from the Northern (winter) hemisphere propagated across the equator into the low latitude Southern (summer) hemisphere. Intercosmos-19 observations show that the disturbance covers the entire thickness of the topside ionosphere, from hmF2 up to at least the 1000 km satellite altitude at post-midnight local times. F-layer lifting reached 200 km, Ne increases in the topside ionosphere by up to a factor of 1.9 and variations in NmF2 of both signs were observed. Assumptions are made concerning the reason for the IGW effect at high altitudes in the topside ionosphere. The relationship between TID parameters and source characteristics determined from a global network of magnetometers are studied. The role of the dayside cusp in the generation of the TID in the daytime ionosphere is discussed. The magnetospheric electric field effects are distinguished from IGW effects. Research Highlights The global scheme for the large-scale TID propagation during a substorm is built. TID covers the entire thickness of the topside ionosphere. The area where the TID is observed looks like a wedge directed to the equator. TID area and amplitude are larger in the winter hemisphere than in the summer hemisphere. The more powerful TID from the winter hemisphere propagates into the summer hemisphere. Keywords: Substorm; Topside ionosphere; Internal gravity wave; IGW; Travelling ionospheric disturbance; TI