Abstract: The variability of ambient ionization near the crest of the equatorial anomaly on the days of counter electrojet (CEJ) is studied using a long-term (1980–1990) database of total electron content (TEC) obtained from Calcutta (geographic: longitude, 88.38°E; latitude, 22.58°N; dip: 32°N), situated virtually below the northern crest of the equatorial anomaly, in conjunction with the equatorial electrojet (EEJ) data of the Indian subcontinent and foF2 data from the equatorial stations. On the days of CEJ events the diurnal variations of TEC exhibit deviations (decreases) from monthly mean values, but the feature is not regular throughout the observing period. A larger percentage of CEJ days show decreases in the descending phase (1980–1985) of solar cycle compared to the ascending one (1986–1990). During low to moderate solar activity years the ionosphere near the anomaly crest is found to be more susceptible to CEJ related equatorial electrodynamics than in the high solar activity periods. On the seasonal basis, though the solstitial months of ascending phase exhibit higher percentage occurrence of decreases, the feature is not so prominent in the descending epoch. An attempt is made to relate the decreases in TEC with the prevailing electrodynamics at the magnetic equator for which EEJ, under quiet geomagnetic conditions, may be treated as a proxy index. A statistical analysis reveals positive correlations of TEC decreases with strength and duration of CEJ but anticorrelations with rising slope, strength, and time-integrated value of EEJ as well as initiation time of CEJ. To represent better correspondence of the TEC variability with the equatorial electrodynamics a function combining various EEJ parameters has been generated, which seems to exhibit significant association with TEC decreases on the days of CEJ events.
Abstract: A long-term (1978–1990) database of total electron content (TEC) from a location (Calcutta: 22.58° N, 88.38° E geographic, dip: 32° N) near the northern crest of the equatorial ionization anomaly has extensively been studied to characterize the contribution of fountain effect in the maintenance of ambient ionization. The equatorial electrojet (EEJ) data obtained from ground magnetometer recording are used to assess the contribution of equatorial fountain. Analysis made with instantaneous values, day's maximum values and time-integrated values of EEJ strength exhibit more or less similar features. When instantaneous values of EEJ are considered TEC variations exhibit two maxima in correlation, one around 10:00–12:00 IST and the other around 18:00–20:00 IST. The later maximum in correlation coefficient is conspicuously absent when integrated values of EEJ are considered. An impulse-like feature is reflected in the diurnal TEC variation during the time intervals (09:00–10:00 IST) and (18:00–19:00 IST). The statistical analysis reveals greater correspondence with high level of significance between diurnal TEC and EEJ in the descending epoch of solar cycle than in the ascending one. On the seasonal basis, TEC in the summer solstitial months are observed to be more sensitive to the changes in EEJ strength than in the equinoctial and winter solstitial months. Combining the effects of solar flux, season, local time and EEJ an empirical formula for monthly mean diurnal TEC has been developed and validated using observed TEC data. An estimation of the relative contributions of the several terms appearing in the formula reveals much more solar flux contribution (~50–70%) in the maintenance of ambient ionization around the present location than the EEJ effects (maximum~20%).
Abstract: The storm time responses of the ionosphere near equatorial anomaly crest in the Indian longitude zone during super magnetic storm on 13-14 March 1989 have been studied using (i) total electron content (TEC) and VHF/UHF scintillation data from location near the equatorial anomaly crest (Calcutta), (ii) h/F, foF2 data from an equatorial station Kodaikanal, and (iii) ion density data from Defense Meteorological Satellite Program (DMSP) satellites. Some distinctive features in the storm time variation of the Indian low latitude ionosphere compared to those of the East Asian and western longitude sectors are revealed through the analysis of DMSP ion density data. The h/F and foF2 data of Kodaikanal exhibit some unusual features, i.e. abnormal height rise and corresponding drop of plasma density at equatorial F-layer in the midnight-post-midnight period of both the main and recovery phases of the storm. From Calcutta large depressions in diurnal variation of TEC for two consecutive days marked the disturbed conditions. An abnormal post-midnight enhancement of TEC is also detected. A remarkable feature of storm time responses of the equatorial low latitude ionosphere is the preferential occurrence of scintillation in the path of trans-ionospheric signals from a particular satellite out of two (ETS-2 and FSC) with a longitude separation of 50. VHF/UHF scintillation observations in the post-midnight period from Calcutta exhibit the longitudinally confined nature of the storm induced ionospheric irregularities. A distinctive feature of scintillation occurrence near the anomaly crest of the Indian and East Asian longitude sectors justifies typical local time dependence of disturbance electric field components. Combined studies on scintillation, TEC, ion density, h/ F, foF2 data produce a coherent picture of the storm time equatorial ionosphere over Indian longitude sector.
Abstract: Long-term (1978–1990) total electron content (TEC) data have been analyzed to show the dependence of ambient ionization on EUV radiation from the Sun. TEC observations were made at Calcutta (22.58° N, 88.38° E geographic, dip: 32° N), situated virtually below the northern crest of the equatorial ionization anomaly. Day-to-day changes in TEC at different local times do not show any significant correlation with F10.7 solar flux. A good correlation is, however, observed between the F10.7 solar flux and the monthly mean TEC when both are considered on a long-term basis, i.e. either in the ascending (1986–1990) or in the descending (1979–1985) phase. In the early morning hours the correlation coefficient maximizes around the 08:00–10:00 h IST interval. The flux independent nature of diurnal TEC is evident around the noon time hours of only a few months in the descending phase for F10.7 values greater than 150 unit. Variation of TEC for the whole time period (1979–1990) also exhibits a prominent hysteresis effect. The remarkable feature of the hysteresis effect is its local time dependence, leading to a temporal flip-over. Solar flux-normalized TEC values show a clear seasonal dependence with asymmetrical variations in the two equinoxes. The amplitudes of the equinoctial peaks reveal a prominent local time dependence. A further normalization leads to a typical local time variation of TEC. Based on solar flux, seasonal and local time dependent features of TEC, an empirical formula has been developed to represent the TEC variation in the early morning hours. It yields a quantitative estimate of the solar flux dependent nature of the TEC variation. The formula has been validated using the available TEC data and data from the neural network.
