R. G. Rastogi

Spread-F and ionization anomaly belt

The present investigation attempts to bring out the dynamics of the F-region at magnetic equatorial and low latitudes in the American zone. Data are examined for two sets of nights, one with strong range-type spread at Huancayo another with complete absence of spread-F. A prominent bulge of the F-region was observed within and below a latitude 10°N in the evening hours of the spread-F nights. Contours of electron distribution during post-sunset hours at the equatorial latitude, Huancayo (Dip 2°N); low latitude, Talara (dip 13°N); and a location near the anomaly crest location, Panama (dip 38°N), indicated a much steeper gradient in electron density at fixed heights on spread-F nights compared to a rather low gradient on the nonspread-F nights. Enhanced concentration of electrons at the anomaly crest location Panama, and a lower density at the equatorial location Huancayo, were observed on spread-F present nights. This is attributed to the phenomena of an evening plasma fountain in operation at equatorial latitudes on spread-F nights.

Geomagnetic field variations at low latitudes and ionospheric electric fields

The solar cycle, seasonal and daily variations of the geomagnetic H field at an equatorial station, Kodaikanal, and at a tropical latitude station, Alibag, are compared with corresponding variations of the E-region ionization densities. The solar cycle variation of the daily range of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-region of the ionosphere. The seasonal variation of the ΔH at equatorial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of ΔH at the equatorial station is attributed to the combined effects of the electron density with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum at about 1110 LT. These results are consistent with the ionosphere E-region electron horizontal velocity measurements at the equatorial electrojet station, Thumba in India.

Amplitude scintillations of ATS-6 radio beacon signals within the equatorial electrojet region (Ootacamund, dip 4° N)

The recordings of the amplitudes of radio beacon signals on 40, 140 and 360 MHz from ATS-6 (at 34° E longitude) recorded at Ootacamund, India (11.43° N, 76.70°E, dip 4°N, elevation angle 41°) have revealed largest occurrence of scintillations for about 60% of cases around 2200 hr during the nighttime, and two secondary peaks (25% of cases) around 0900 hr and 1400 hr during the daytime.During the daytime, the scintillation decreases approximately as the inverse of the frequency for higher frequencies while for lower frequencies the law is valid till scintillation index at 40 MHz does not exceed 0.9. The temporal variation of daytime scintillation shows impulsive character, the duration of activity lasts for 1–2 hours at a time.During the nighttime, the scintillation decreases inversely with frequency for weak and moderate scintillation activity. The scintillation index at 360 MHz becomes independent of that at 140 MHz when the index at 140 MHz exceeds 0.85. For the set of frequencies 40–140 MHz, on some occasions scintillation index at 40 MHz is seen to be less than that at 140 MHz. The nighttime scintillations are in general stronger and remain so for extended length of time.The daytime scintillations are suggested to be due to blanketing or some other non-q type of sporadicE layer. The nighttime scintillations are most probably due to spreadF condition and the abnormal frequency variation of the scintillations may be due to multiple scattering layer during periods of intense spreadF.

Geomagnetic disturbance effects on equatorial spreadF

Geomagnetic disturbance effects on the occurrence of range and frequency spread at Huancayo are studied for the period 1957–74. The occurrence of frequency spread is decreased on disturbed days forD-months andE-months while duringJ-months an increase is noted in the post-midnight period. The occurrence of range spread is decreased on disturbed days in the pre-midnight hours duringD andE-months. Post-midnight hours ofD andE-months and all hours of night duringJ-months show an increase of range spread on disturbed days, which is most prominent during low sunspot years. The mean occurrence of frequency spread (2300–0100 LT) and range spread (2000–2300 LT) decrease with increasing ΣKp. On the other hand, occurrence of range spread (0300–0500 LT) increases with increasing ΣKp, and is suggested as the consequence of the changes of the horizontal electric field in theF-region associated with the geomagnetic storms.

Dissimilar forms of the ionospheric equatorial anomaly observed in East Asia and India

A longitudinal comparison has been undertaken of electron content-latitude profiles at low latitudes obtained in East Asia and India from measurements of the Faraday rotation of transiting beacon satellite signals. Very appreciable differences in the extents of the development of the equatorial anomaly have been found (1) between the East Asia and India regions (longitude difference 40°) for two particular months and (2) between similar months in two consecutive years for the India region alone. A comparison of the daily ranges of the horizontal magnetic field component (ΔH) measured near the magnetic equator in these two regions showed that ΔH was less (by about 25γ) during the period of poor development of the equatorial anomaly in the Indian region. It is considered that changes in ΔH are related to changes in the magnitude of the eastward electric field E overhead the magnetic equator and these in turn affect the development of the ionospheric equatorial anomaly via corresponding variations of the magnitude of the ionisation uplifting, (E × B) mechanism. The correlation between ΔH and the equatorial anomaly development was found to be poor in the northern summer but throughout the rest of the year it is thought that ionospheric predictions at low latitudes could be improved by considering the noon values of H monitored at appropriate longitudinally-sited stations. The day-to-day and longitudinal variabilities of ΔH and the equatorial anomaly development are thought to have their origin in the lower atmosphere. The upward propagation of semi-diurnal tides and planetary waves are affected by the temperature-height profiles in the mesosphere and also the structure of the middle atmosphere jet, these being most variable in local winter. These tides in turn would produce a variability of local winds in the upper atmosphere and by dynamo action create electric fields in the vicinity of the magnetic equator affecting the electrojet and the development of the ionospheric equatorial anomaly. It is proposed that a search should be made for planetary and tidal waves in both the lower and higher atmosphere at low latitudes.

On the equatorial spreadF

The post-sunset maximum in virtual height of theF region near the magnetic equator is associated with the general rise of the wholeF region from the base to the height of peak ionisation with little change in the semi-thickness of the layer. This rise ofF region is accentuated on days with large evening peak in the vertical drift velocity or the horizontal electric field in theF region. The range type of equatorial spreadF first occurs only if theF region drift velocity remains significantly upwards after sunset but the maximum intensity of spreadF occurs when the drift velocities are low or even downwards. The range spread first appears at or below the base of theF layer and later spreads into theF layer due to downward movement of the layer and/or upward movement of the irregularity. SpreadF seen on VHF backscatter records corresponds to the range type of spreadF seen on normal ionograms. The frequency type of spreadF does not produce VHF echoes. A strong peak in the electric field seems to be a necessary condition for the generation of equatorial spreadF.

Total electron content and F-region electron density distribution near the magnetic equator in India

Abstract Total electron content derived from the group delay measurements of ATS-6 radio beacons received at Ootacamund (India) are compared with the electron-density vs height distributions derived from the ionosonde data of the nearby station Kodaikanal. The daily variation of equivalent vertical total electron content (NTV) does not show the midday bite out which is so prominently present in the corresponding daily variation of NmF2, the peak electron density. The topside electron content (Na) continues to increase from sunrise to a maximum value around 1500LT, while the bottomside electron content (Nb) reaches a maximum value around 0500 LT. Daily variations of these as well as other parameters, e.g. the vertical slab thickness, the bottomside semi-thickness, the height of theF2 peak have been also studied for a geomagnetically quiet and a disturbed day.

Multifrequency studies of equatorial ionospheric scintillations at Ootacamund

Abstract Intensity scintillations of 40 MHz, 140 MHz and 360 MHz radio beacons on board the geostationary satellite ATS-6, (at 34°E longitude), recorded at Ootacamund (magnetic dip latitude 3°N) during the ATS-6 phase II (1975–1976) are described. The scintillation index S4, which is the normalised RMS value of the intensity fluctuations, is found to be very large at 2100-0200 h associated with equatorial spread-F, and in the forenoon and afternoon hours associated with a blanketing type of sporadic E-layer. At 40 MHz there seems to be a weak scintillation (S4 The frequency exponent, n, (with S 4 ∝ ƒ −1 , whereƒ is the signal frequency), was found to decrease monotonically with increasing intensity of scintillations, approaching a value of zero for saturated scintillations. The relationship between the frequency exponent and the scintillation index was found to be independent of the time of the day or the season, in spite of different kinds of irregularities being involved in the scintillation processes.

Solar and lunar variations in TEC at low latitudes in India

Abstract Total electron content data at Ootacamund (dip 6°N) during the second phase of the ATS-6 program are analysed for solar and lunar variations. Both the lunar semi-monthly and lunar semi-diurnal tides at Ootacamund are smaller in magnitude than at stations near the Appleton anomaly crest. The phases of the lunar oscillations however are almost the same as that at tropical latitudes. Thus the lunar tides in total electron content and in maximum F2-region electron density at the equatorial station are not in phase and present problems for the simple electrodynamic theory.

The equatorial anomaly in ionospheric total electron content and the equatorial electrojet current strength

Faraday Rotation of 40 and 41 MHz signals from the satellite BE-B (Explorer 22) recorded simultaneously at Ahmedabad (dip 34° N) and Kodaikanal (dip 3·4° N) during the years 1964–69 are used to derive the latitudinal profiles of Total Electron Content (TEC) over the Indian equatorial anomaly region. From these profiles the diurnal development of the equatorial anomaly and its correlation with equatorial electrojet strength are studied. The anomaly is found to maximise around 1400 LT,i.e., two-three hours after the electrojet attains its peak. The anomaly parameters such as the dip latitude of the anomaly peak,φ, the normalised depth,d, of the anomaly and the strength of the anomaly defined asS=ϕxd are found to be well correlated with the electrojet strength.