Abstract
A study of the formation and movement of sequential Sporadic-E layers observed during the night-time hours at two Indian low-latitude stations, SHAR (dip 10°N) and Waltair (dip 20°N) shows that the layer are formed around 19:00 h. IST at altitudes of ~180 km. They descend to the normal E-region altitude of about 100 km in three to four hours and becomes blanketing type of Es before they disappear. However, the absence of these descending layers at an equatorial station, Trivandrum (dip 2°N) gives the experimental evidence for wind shear theory. The meridional neutral wind derived from the height variation of the F-layer showed significant poleward wind during the descent of these layers. Hence it is inferred that these layers are formed as a consequence of the convergence of plasma by the poleward wind and the equatorward propagating gravity waves (inferred from the height fluctuations of F-layer).Key words. Ionosphere (active experiments; equatorial ionosphere · ionospheric irregularities)
Highlights
The term Sporadic-E (Es), which is an important example of the irregular structure of the lower ionosphere seen on ionograms encompasses the processes such as particle precipitation, plasma instabilities and gravity waves
The layer is not observed at Trivandrum, the equatorial station
The results presented describe the eect of meridional neutral wind and the gravity waves (TIDs) in contributing to the formation of descending Sporadic-E layers at night-time in the low-latitude ionosphere
Summary
The term Sporadic-E (Es), which is an important example of the irregular structure of the lower ionosphere seen on ionograms encompasses the processes such as particle precipitation, plasma instabilities and gravity waves. The dierent processes which can form the Es layers are the horizontal convergence of ionization due to the vertical shears of horizontal neutral. The virtual height of F-layer (h¢F) is scaled at a frequency of 3 MHz so that the group retardation due to underlying ionization are negligible. The vertical drift velocity is obtained from the scaled values of h¢F by computing the value of Dh¢F/Dt for every 15 min interval. At the bottom side of the F-region, the electron loss manifests as a rise in the altitude of constant level of electron density and results in an apparent vertical drift. The correction for this apparent vertical drift due to
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