An MST radar operating at 53 MHz with an average power aperture product of 7 × 108 W m2 has been established at Gadanki (13.5°N, 79.2°E), India. The radar development has been accomplished in two phases. In the first phase it was commissioned in ST mode using a partial system comprising one quarter (16 × 16)of the Yagi antenna array and 16 driver units of the transmitters providing an average power aperture product of 4.8 × 106 W m2. In this part we present the radar system description, including off‐line data processing, and some sample high‐resolution vector wind measurements made in ST mode operation.

Variations of ionospheric ionization (represented by ionospheric electron content (IEC)) and related solar fluxes with the 10.7‐cm solar flux index (F10.7) are studied for the intense solar cycle 21 when F10.7 was as high as 367. The IEC data collected at several stations during 1980‐1985, the solar EUV (50‐1050 Å) fluxes obtained from the EUV91 solar EUV flux model, and the measured values of Lyman α (1216 Å) flux and He I (10,830 Å) equivalent width (EW) are used for the study. It is shown that daily values of diurnal maximum IEC (IECmax) saturate (remain constant) when F10.7 (or its 81‐day running average) exceeds a threshold (approximately 160‐200) which depends slightly on season and latitude. Variations of the model values of the solar EUV fluxes reveal that when F10.7 exceeds the threshold: (1) the integrated solar EUV (50‐1050 Å) flux increases at a very low rate, and (2) the fluxes of the important (for thermospheric heating) chromospheric lines and intervals generally saturate (remain constant), while those of the coronal lines and intervals increase at a reduced rate. Lyman α flux and He I EW, which are used as input data in the solar EUV flux model, also increase at a very low rate when F10.7 exceeds the threshold. The saturation of ionospheric ionization, observed for high values of F10.7 during the last three solar cycles 19‐21, is the result of the nonlinear variation of the solar EUV and Lyman α fluxes with F10.7. IECmax increases linearly with the integrated solar EUV flux, Lyman α flux and He I EW.

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Latitudinal variations of the various characteristics of nighttime anomalous enhancements in total electron content (TEC) are presented by considering TEC data from the ATS series of satellites for a 2‐month period from a number of stations in the northern hemisphere extending from 2° to 63° dip latitude. The latitudinal variations of the various TEC enhancement characteristics are found to be strikingly similar, and they reveal a pattern of cyclic variation with two distinct minima, one at 30°N and the other at 60°N (dip latitude). The TEC enhancements show a strong positive correlation with geomagnetic activity at middle and high latitudes and no significant correlation at low latitudes. The observed pattern of the latitudinal vaiations of the TEC enhancements is discussed in the light of the potential source mechanisms.

The relative importance of height, vertical drift velocity, and electron density gradient of the postsunset bottomside (5.5 MHz) equatorial Fregion for the onset of spread Fis studied using simultaneous HF Doppler radar and ionosonde observations. The study conducted for the periods January-March of 1984 and 1985 shows that the height of the F layer, determined by the time history of the prereversal enhancement of the drift velocity, is the deciding factor for the onset of equatorial spread F (ESF) with little contribution from the electron density gradient. Maximum growth rate of linear collisional Rayleigh-Taylor instability occurs at the time of peak height rather than at the time of peak velocity confirming that, for the onset of ESF, the layer should attain a threshold height. The threshold (group) height of the 5.5 MHz layer falls from ~450 km in 1984 (mean F10.7 equals 120) to ~350 km in 1985 (mean F10.7 equals 70); the corresponding evening peak upward drift velocities decrease from about 30 m s -1 in 1984 to about 20 m s -1 in 1985. The significant fall of the thresholds with the declining solar activity is due to the decrease in the ion-neutral collision frequency with declining solar activity; the fall of the thresholds is reflected in large decreases in the intensity and duration of the spread F. the hydrodynamic instability associated with a heavy 13,741 13,742 •AYACHANDRAN ET AL.' ONSET OF EQUATORIAL SPREAD F fluid resting on top of a light fluid) causes linear growth of irregularities in the bottomside F region, which, in turn, cause plasma density depletions or plasma bubbles. The bubbles then rise nonlinearly to the topside by the enhanced E x B drift. The bubble development is in accord with many observations [Woodman and La Hoz, 1976; Aggson et al., 1992].