The availability of similar ionosondes at four locations in India covering the regions from the equator to the northern crest of the equatorial ionization anomaly (EIA) has provided a unique opportunity to study the role of the EIA and related processes in the occurrence of spread F. The study is conducted during equinoctial months of high solar activity period (Rz=145), when the probability of occurrence of spread F is maximum. This study also deals with vertical movement of the F layer due to both electric field and neutral winds and the occurrence of spread F. The present study shows that a well‐developed anomaly beyond the latitude of 18°N is one of the base‐level conditions for the generation of spread F. The total integrated vertical movement is the controlling factor for the onset of spread F and not the peak value of h′F alone. There seems to be no threshold value in the altitude of the F layer (h′F) for the occurrence of spread F at all stations. The direction and magnitude of meridional winds that are crucial in the generation of spread. F showed higher values of poleward wind on non‐spread‐F days, indicating that the irregularity growth is suppressed due to the poleward wind.
Abstract. A study of the formation and movement of sequential Sporadic-E layers observed during the nighttime 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 signi®cant 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¯uctuations of F-layer).
Abstract. The characteristics of VHF radiowave scintillations at 244 MHz (FLEETSAT) during a complete solar cycle (1983–93) at a low-latitude station, Waltair (17.7°N, 83.3°E), are presented. The occurrence of night-time scintillations shows equinoctial maxima and summer minima in all the epochs of solar activity, and follows the solar activity. The daytime scintillation occurrence is negatively correlated with the solar activity and shows maximum occurrence during the summer months in a period of low solar activity. The occurrence of night-time scintillations is inhibited during disturbed days of high solar activity and enhanced during low solar activity.
In any supersonic intake, the flow decelerates from supersonic to subsonic speed through a constant or divergent channel "isolator" by a series of bifurcated compression shock waves referred to as a shock train. It is important to understand the characteristics of the shock train which occur inside the isolator to improve the performance of scramjet engines. In the present work, numerical simulations were carried out to investigate the characteristics of the shock train occurring in the divergent channels using coupled implicit Reynolds Averaged Navier-Stokes (RANS) equations along with the two-equation k-w SST turbulence model. Results show that the downstream pressure variation causes the shock train length to decrease and the shock structure phenomenon varies from Mach reflection to Regular reflection. The variation of the inlet Mach number has less influence on the shock train length and the location of the shock train is determined by the area ratio. In comparison with the constant area duct, the shock train structure phenomena varies from Mach reflection to regular reflection in the divergent channel. Also, the increase in divergent angle raises the total pressure loss.
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