The growth in altitude/latitude of equatorial plasma bubbles was monitored, using simultaneous recordings of VHF scintillations at five locations situated between 3° and 23°N magnetic latitudes along a common meridian (84°E) during February 1980. The onsets of postsunset scintillation were mostly abrupt in character, and their occurrence at higher latitudes was conditional on their prior appearance at lower latitudes, indicating a causal link to irregularities associated with rising equatorial plasma bubbles. The day‐to‐day occurrence and the latitudinal, and effectively altitudinal, growths are examined in relation to the prereversal enhancement in h′F during sunset hours and its rate of rise, the onset of a postsunset secondary maximum (PSSM) in ionospheric electron content (IEC), and equatorial electrojet strength (EEJ) variations. It is observed that the bubble and associated irregularities, after its onset over the magnetic equator, reached the highest altitudes/latitudes only on those days when a prior PSSM in IEC is observed there in addition to high values of h′F, dh′F/dt and bubble rise velocity; otherwise it will be confined to near equatorial latitudes only. Also, the equatorial h′F, dh′F/dt, magnitude of PSSM and intensity of 4 GHz scintillations at low latitude are all showing positive correlation with daytime EEJ strength variations. It is concluded that, after the initial development of a bubble, the ExB drift and the PSSM play an important role in the subsequent growth and evolution, and EEJ is a useful parameter for the prediction of the development.

VHF nighttime scintillations, recorded during a high solar activity period at a meridian chain of stations covering a magnetic latitude belt of 3°–21°N (420 km subionospheric points) are analyzed to investigate the influence of equatorial spread F irregularities on the occurrence of scintillation at latitudes away from the equator. Observations show that saturated amplitude scintillations start abruptly about one and a half hours after ground sunset and their onset is almost simultaneous at stations whose subionospheric points are within 12°N latitude of the magnetic equator, but is delayed at a station whose subionospheric point is at 21°N magnetic latitude by 15 min to 4 hours. In addition, the occurrence of postsunset scintillations at all the stations is found to be conditional on their prior occurrence at the equatorial station. If no postsunset scintillation activity is seen at the equatorial station, no scintillations are seen at other stations also. The occurrence of scintillations is explained as caused by rising plasma bubbles and associated irregularities over the magnetic equator and the subsequent mapping of these irregularities down the magnetic field lines to the F region of higher latitudes through some instantaneous mechanism; and hence an equatorial control is established on the generation of postsunset scintillation‐producing irregularities in the entire low‐latitude belt.

Abstract. Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinates: 28.63°N, 77.22°E; geomagnetic co-ordinates: 19.08°N, 148.91°E; dip Latitude 24.8°N), India, for the period 1975±80 and 1986± 89 belonging to an ascending phase of solar activity during ®rst halves of solar cycles 21 and 22 respectively have been used to study the diurnal, seasonal, solar and magnetic activity variations. The diurnal variation of seasonal mean of IEC on quiet days shows a secondary peak comparable to the daytime peak in equinox and winter in high solar activity. IEC max (daytime maximum value of IEC, one per day) shows winter anomaly only during high solar activity at Delhi. Further, IEC max shows positive correlation with F 10.7 up to about 200 ux units at equinox and 240 units both in winter and summer; for greater F 10.7 values, IEC max is substantially constant in all the seasons. IEC max and magnetic activity (A p ) are found to be positively correlated in summer in high solar activity. Winter IEC max shows positive correlation with A p in low solar activity and negative correlation in high solar activity in both the solar cycles. In equinox IEC max is independent of A p in both solar cycles in low solar activity. A study of day-to-day variations in IEC max shows single day and alternate day abnormalities, semi-annual and annual variations controlled by the equatorial electrojet strength, and 27-day periodicity attributable to the solar rotation.

Abstract. Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinates: 28.63°N, 77.22°E; geomagnetic co-ordinates: 19.08°N, 148.91°E; dip Latitude 24.8°N), India, for the period 1975±80 and 1986± 89 belonging to an ascending phase of solar activity during ®rst halves of solar cycles 21 and 22 respectively have been used to study the diurnal, seasonal, solar and magnetic activity variations. The diurnal variation of seasonal mean of IEC on quiet days shows a secondary peak comparable to the daytime peak in equinox and winter in high solar activity. IEC max (daytime maximum value of IEC, one per day) shows winter anomaly only during high solar activity at Delhi. Further, IEC max shows positive correlation with F 10.7 up to about 200 ux units at equinox and 240 units both in winter and summer; for greater F 10.7 values, IEC max is substantially constant in all the seasons. IEC max and magnetic activity (A p ) are found to be positively correlated in summer in high solar activity. Winter IEC max shows positive correlation with A p in low solar activity and negative correlation in high solar activity in both the solar cycles. In equinox IEC max is independent of A p in both solar cycles in low solar activity. A study of day-to-day variations in IEC max shows single day and alternate day abnormalities, semi-annual and annual variations controlled by the equatorial electrojet strength, and 27-day periodicity attributable to the solar rotation.

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