First results from ground-based daytime optical investigation of the development of the equatorial ionization anomaly

A meridional scanning OI 630.0-nm dayglow photometer was operated from Ahmedabad (17.2^°N dip lat.) scanning a region towards the south in the upper atmosphere extending over \sim5^° in latitude from 10.2^°N to 15.2^°N dip latitude. From the spatial and temporal variabilities of the dayglow intensity in the scanning region we show for the first time, evidence for the passage of the crest of the equatorial ionization anomaly (EIA) in the daytime by means of a ground-based optical technique. The relationship between the daytime eastward electric field over the dip equator in the same longitude zone as inferred from the equatorial electrojet strength and the evolutionary pattern of EIA is clearly demonstrated. The latter as inferred from the dayglow measurements is shown to be consistent with our present understanding of the electrodynamical processes in the equatorial region. The present results reveal the potential of this ground-based optical technique for the investigation of ionospheric/thermospheric phenomena with unprecedented spatial and temporal resolution.     

Ionospheric effects near the magnetic equator and the anomaly crest of the Indian longitude zone during a large number of intense geomagnetic storms

Ionospheric effects of a large number (51) of severe geomagnetic storms are studied using total electron content (TEC) and VHF/UHF scintillation data from Calcutta, situated near the northern crest of equatorial ionization anomaly and equatorial spread-F (ESF) data from Kodaikanal. The susceptibility of the equatorial ionosphere to develop storm time plasma density irregularities responsible for ESF and scintillation is found to be largely modulated by the local times of occurrences of main and recovery phases as seen in the D_st index. While inhibition of premidnight scintillation for lower TEC values compared to the quiet day averages is omnipresent, occurrence of scintillation for enhancements of TEC is largely dependent on initiation time and amplitude of the said deviations. An overall reduction in threshold values of h′F for observing storm induced ESF and scintillation compared to reported quiet time values is noted. The results are discussed in terms of storm time variabilities in electric fields, neutral wind system and composition changes.     

HF radar observations of equatorial spread-F over West Africa

New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9°24N, 5°37W, dip 4°S). Range-time-intensity maps of the radar echoes have been analyzed to identify the signatures of density depletions and bottomside spread-F. Density depletions are well known features of equatorial spread-F, and are believed to emerge after the development of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density depletions over the radar field of view. The simulation permits an interpretation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitative arguments are presented to identify bottomside spread-F. Using the computational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset.     

Equatorial temperature and wind anomaly (ETWA)—a review

We review the observational evidence for describing the characteristics of the equatorial temperature and zonal wind anomaly (ETWA) in the low-latitude thermosphere in solar maximum and minimum periods. In spite of some new results on ETWA in the last decade, including its discovery, there is no satisfactory explanation in our understanding of the phenomenon. The two suggested mechanisms for heating at the crests of the equatorial ionization anomaly (EIA) to form the equatorial temperature anomaly (ETA) are due to (1) the ion-drag on the zonal winds resulting in the transfer of kinetic energy into heat energy and (2) the exothermic chemical reactions involved in the dominant O⁺ion re-combinations. To verify which of the two suggested mechanisms is the most effective in causing ETA, it is necessary to measure simultaneously a few parameters in situ by the satellite-borne instruments. They are (1) the electron density (N_e) and temperature (T_e), (2) the molecular and atomic ion densities (N_ij) and ion temperatures (T_i), (3) the gas temperatures (T) and densities of the gas constituents, (4) the vector winds or at least the zonal (Z) and vertical (V) wind components and (5) the drift velocities of the ionization. These together with the simultaneous ground-based measurements, will resolve identifying not only the dominant mechanism(s) for ETWA, but also the processes responsible for the enigmatic phenomena, such as the equatorial spread-F (ESF), the midnight temperature maximum (MTM) and the possible role of the EIA in their occurrences.     

Letter to the editor Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

At dip equatorial stations in the Indian zone, spread-F conditions are known to develop preferentially around midnight during the June solstice (northern summer) months of low solar activity, in association with a distinct increase in F layer height. It is currently held that this onset of spread-F far away from the sunset terminator is due to the generalised Rayleigh-Taylor instability mechanism, with the gravitational and cross-field instability factors (and hence F layer height) playing important roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N; 77.5°E; dip 4°N) for the northern summer months (May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions against which the post-midnight onset of spread-F takes place. A data sample of 38 nights with midnight onset of spread-F and 34 nights without spread-F is used for the purpose. It is found that a conspicious increase in F layer height beginning around 2100 LT occurs on nights with spread-F as well as without spread-F. This feature is seen in the nocturnal pattern of F layer height on many individual nights as well as of average F layer height for the two categories of nights. The result strongly suggests that the F layer height does not play a pivotal role in the midnight onset of spread-F during the June solstice of solar minimum. The implications of this finding are discussed.     

First in situ measurement of electric field fluctuations during strong spread F in the Indian zone

An RH-560 rocket flight was conducted from Sriharikota rocket range (SHAR) (14°N, 80°E, dip 14°N) along with other experiments, as a part of equatorial spread F (ESF) campaign, to study the nature of irregularities in electric field and electron density. The rocket was launched at 2130 local time (LT) and it attained an apogee of 348 km. Results of vertical and horizontal electric field fluctuations are presented here. Scale sizes of electric field fluctuations were measured in the vertical direction only. Strong ESF irregularities were observed in three regions, viz., 160/190 km, 210/257 km and 290/330 km. Some of the valley region vertical electric field irregularities (at 165 km and 168 km), in the intermediate-scale size range, observed during this flight, show spectral peak at kilometer scales and can be interpreted in terms of the image striation theory suggested by Vickrey et al. The irregularities at 176 km do not exhibit any peak at kilometer scales and appear to be of a new type. Scale sizes of vertical electric field fluctuations showed a decrease with increasing altitude. The most prominent scales were of the order of a few kilometers around 170 km and a few hundred meters around 310 km. Spectra of intermediate-scale vertical electric field fluctuations below the base of the F region (210/257 km) showed a tendency to become slightly flatter (spectral index n = –2.1 ± 0.7) as compared to the valley region (n = –3.6 ± 0.8) and the region below the F peak (n = –2.8 ± 0.5). Correlation analysis of the electron density and vertical electric field fluctuations suggests the presence of a sheared flow of current in 160/330 km region.     

Letter to the editor Electric field fluctuations (25–35 min) in the midnight dip equatorial ionosphere

Measurements with a HF Doppler sounder at Kodaikanal (10.2°N, 77.5°E, geomagnetic latitude 0.8°N) showed conspicuous quasi-periodic fluctuations (period 25/35 min) in F region vertical plasma drift, V_z in the interval 0047/0210 IST on the night of 23/24 December, 1991 (A_p = 14, K_p < 4^–). The fluctuations in F region vertical drift are found to be coherent with variations in B_z (north-south) component of interplanetary magnetic field (IMF), in geomagnetic H/X components at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.     

Long term ionospheric electron content variations over Delhi

Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinates: 28.63°N, 77.22°E; geomagnetic co-ordinates: 19.08°N, 148.91E; dip Latitude 24.8°N), India, for the period 1975/80 and 1986/89 belonging to an ascending phase of solar activity during first 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 flux 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. IECmax 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.     

The equatorial ionospheric anomaly in electron content from solar minimum to solar maximum for South East Asia

Median hourly, electron content-latitude profiles obtained in South East Asia under solar minimum and maximum conditions have been used to establish seasonal and solar differences in the diurnal variations of the ionospheric equatorial anomaly (EIA). The seasonal changes have been mainly accounted for from a consideration of the daytime meridional wind, affecting the EIA diffusion of ionization from the magnetic equator down the magnetic field lines towards the crests. Depending upon the seasonal location of the subsolar point in relation to the magnetic equator diffusion rates were increased or decreased. This led to crest asymmetries at the solstices with (1) the winter crest enhanced in the morning (increased diffusion rate) and (2) the same crest decaying most rapidly in the late afternoon (faster recombination rate at lower ionospheric levels). Such asymmetries were also observed, to a lesser extent, at the equinoxes since the magnetic equator (located at about 9○N lat) does not coincide with the geographic equator. Another factor affecting the magnitude of a particular electron content crest was the proximity of the subsolar point, since this increased the local ionization production rate. Enhancements of the EIA took place around sunset, mainly during the equinoxes and more frequently at solar maximum, and also there was evidence of apparent EIA crest resurgences around 0300 LST for all seasons at solar maximum. The latter are thought to be associated with the commonly observed, post-midnight, ionization enhancements at midlatitudes, ionization being transported to low latitudes by an equatorward wind. The ratio increases in crest peak electron contents from solar minimum to maximum of 2.7 at the equinoxes, 2.0 at the northern summer solstice and 1.7 at northern winter solstice can be explained, only partly, by increases in the magnitude of the eastward electric field E overhead the magnetic equator affecting the [E×B] vertical drifts. The most important factor is the corresponding increase in ionization production rate due to the increase in solar radiation flux. The EIA crest asymmetries observed at solar maximum were less significant, and this is probably due to the corresponding increase in ionization densities leading to an increase of the retarding effect of ion-drag on the daytime meridional winds.     

Equatorial spread and dynamics in the F layer over West Africa from ionogram analysis,during the declining solar flux year 1994–1995

Equatorial spread-F (ESF) dynamics is studied based on quarter-hourly series of ionograms recorded in West Africa during the year December 1994 to November 1995. The time evolution of F layer at the typical equatorial site OUAGADOUGOU (Burkina Faso) differs slightly from that at DAKAR (Senegal), located at the northern edge of the magnetic equatorial belt. Comparing ESF occurrences at both stations with the pre-sunrise ones at FORTALEZA (Brazil), we find generally antisymmetric variations on both sides of the Atlantic, except for a common December solstice maximum. We stress the convergence of the multiple-scale morphologies drawn from new radars with those from high-quality ionosonde density (h′,t) plots. This revives the analysis of ESF dynamics and points toward long-term network studies of ESF coupling phenomena.     

Onset location of scintillation-producing spread-F plume over Sanya

Equatorial spread-F (ESF) backscatter plumes are often observed in radar range-time-intensity (RTI) maps at low latitude. Except case studies, few statistical investigations on the onset locations of scintillation-producing ESF plumes at given sites have been conducted. In this study, a statistical analysis is carried out on onset locations of ESF backscatter plumes observed at a low-latitude location Sanya (18.4°N, 109.6°E; dip lat 12.8°N) during equinoctial months of 2013. By employing a tracing method to locate backscatter plumes, we estimate the onset longitudes of periodic plumes obtained from the Sanya VHF radar five-beam steering measurements. The results show that the inter-plume distances (in longitude) are mostly confined within 200–600 km, and the ESF plumes producing ionospheric scintillations over Sanya are almost exclusively generated at the longitudes of 94°–110°E. The results indicate the necessity to monitor ESF plume initial generation in the longitude region of 94°–110°E to better understand the day-to-day variability in the occurrence of ionospheric scintillations over Sanya.     

Equatorial electrojet as part of the global circuit: a case-study from the IEEY

Geomagnetic storm-time variations often occur coherently at high latitude and the day-side dip equator where they affect the normal eastward Sq field. This paper presents an analysis of ground magnetic field and ionospheric electrodynamic data related to the geomagnetic storm which occured on 27 May 1993 during the International Equatorial Electrojet Year (IEEY) experiment. This storm-signature analysis on the auroral, mid-latitude and equatorial ground field and ionospheric electrodynamic data leads to the identification of a sensitive response of the equatorial electrojet (EEJ) to large-scale auroral return current: this response consists in a change of the eastward electric field during the pre-sunrise hours (0400–0600 UT) coherently to the high-, mid-, and equatorial-latitude H decrease and the disappearance of the EEJ irregularities between the time-interval 0800–0950 UT. Subsequent to the change in hF during pre-sunrise hours, the observed foF2 increase revealed an enhancement of the equatorial ionization anomaly (EIA) caused by the high-latitude penetrating electric field. The strengthening of these irregularities attested by the Doppler frequency increase tracks the H component at the equator which undergoes a rapid increase around 0800 UT. The H variations observed at the equator are the sum of the following components: S_R, DP, DR, DCF and DT.     

Characteristics of L-band (1.5 GHz) and VHF (244 MHz) amplitude scintillations recorded at Kolkata during 1996–2006 and development of models for the occurrence probability of scintillations using neural network

L-band (1.5 GHz) and VHF (244 MHz) amplitude scintillations observed from Kolkata (22.58°N lat, 88.38° E long, 32°N dip), over a solar cycle 1996–2006 are presented in this paper. Situated near the northern crest of the equatorial anomaly, it is an excellent platform for scintillation studies. Based on 11 years’ data, an attempt is made to develop models of hourly percentage occurrence of scintillations for the rising and declining phases of solar cycle using Neural Network. The relation between fading rate at VHF with the S4 index at L-band is also investigated.     

Characteristics of VHF scintillations in the equatorial anomaly crest region in India

The characteristics of ionospheric scintillations at Rajkot in the equatorial anomaly crest region in India are described for the years 1987–1991 by monitoring the 244-MHz transmission from the satellite FLEETSAT. This period covers the ascending phase of solar cycle 22. Scintillations occur predominantly in the pre-midnight period during equinoxes and winter seasons and in the post-midnight period during summer season. During equinoxes and winter, scintillation occurrence increases with solar activity, whilst in summer it is found to decrease with solar activity. Statistically, scintillation occurrence is suppressed by magnetic activity. The characteristics observed during winter and equinoxes are similar to those seen at the equatorial station, Trivandrum. This, coupled with the nature of the post-sunset equatorial F-region drift and h‘F variations, supports the view that at the anomaly crest station, scintillations are of equatorial origin during equinox and winter, whilst in summer they may be of mid-latitude type. The variations in scintillation intensity (in dB) with season and solar activity are also reported.     

Equatorial ionization anomaly and thermospheric meridional winds during two major storms over Brazilian low latitudes

The equatorial ionosphere responses over Brazil to two intense magnetic storms that occurred during 2001 are investigated. The equatorial ionization anomaly (EIA) and variations in the zonal electric field and meridional winds at different storms phases are studied using data collected by digisondes and GPS receivers. The difference between the F layer peak density (foF2) at an equatorial and a low latitude sites was used to quantify the EIA; while the difference between the true heights (h_F) at the equatorial and an off-equatorial site was used to calculate the magnetic meridional winds. The vertical drift was calculated as dh_F/dt. The results show prompt penetration electric fields causing unusual early morning development of the EIA, and disturbed dynamo electric field producing significant modification in the F region parameters. Variations to different degrees in the vertical drift, the thermospheric meridional winds and the EIA developments were observed depending on the storm phases.     

High resolution 2-D maps of OI 630.0 nm thermospheric dayglow from equatorial latitudes

The first-ever high resolution 2-D maps of OI 630.0 nm dayglow obtained from equatorial latitudes clearly reveal the movement as a large-scale feature of the equatorial ionization anomaly (EIA). These also show the presence of wave-like features classified as gravity waves presumably originating at the crest of the EIA, similar to the equatorial electrojet acting as a source of these waves. These results are presented and discussed.     

Meridional movement of northern and southern equatorial ionization anomaly crests in the East-Asian sector during 2002–2003 SSW

The present paper investigates the asymmetrical variability of the location of the north and the south equatorial ionization anomaly (EIA) crests in the East-Asian sector, along with their association with simultaneous observations of equatorial electrojet (EEJ) strength, geomagnetic activity index, and solar flux index during the 2002–2003 sudden stratospheric warming (SSW) event. Analysis of these observations indicates the existence of a large-scale quasi 16-day periodic meridional movement in both EIA crests, and also reveals a strong correlation between the quasi 16-day oscillation in geomagnetic latitudes of the EIA crest and EEJ strength. The latitude of the northern/southern EIA crest and the EEJ strength indicate that obvious synchronous periodic oscillations were in-phase in the northern and southern hemisphere when the SSW occurred. In addition, it is also found that both the EIA crest location and amplitude of the periodic movement of the EIA locations exhibit hemispheric asymmetry. The amplitude of the periodic movement of the EIA location in the southern hemisphere is larger than that of the northern hemisphere, and the southern EIA crest is further off from the equator than the north one. Understanding these asymmetries requires a combination of mechanisms that involve at least trans-equator meridional winds and the position of a sub-solar point; however, potential disturbances in neutral winds associated with the SSW may additionally complicate the equatorial ionospheric dynamics.     

Equatorial and low latitude spread-F irregularity characteristics over the Indian region and their prediction possibilities

To study the occurrence characteristics of equatorial spread-F irregularities and their latitudinal extent, simultaneous digital ionosonde data (January–December 2001) from Trivandrum (8.2°N), Waltair (17.7°N) and Delhi (28.6°N) and 4 GHz scintillation data from Sikandarabad (26.8°N) and Chenglepet (10.4°N), and 250 MHz scintillation data from Bhopal (23.2°N) for equinoxes period are analysed. It is noted that except summer months, occurrence of spread F is always maximum at Trivandrum, minimum at Delhi and moderate at Waltair. During equinoxes and winter months. Their occurrences at higher latitude station are always conditional to their prior occurrences at lower latitudes indicating their association with the generation of equatorial plasma bubble and associated irregularities. Scintillation occurrences also follow the similar pattern. During the summer months, the spread-F occurrences are highest at equatorial location Trivandrum, moderate at Delhi and minimum at Waltair and seem to be caused by irregularities generated locally especially over Delhi.To gain forecasting capability, night-to-night occurrences of spread-F/scintillation at these locations are examined in relation to post sunset rise of h’F and upward ExB drift velocity over the magnetic equator using Trivandrum ionosonde data. It is noted that except the summer months, the spread-F at Trivandrum, Waltair and Delhi are observed only when equatorial ExB (h’F) is more than about 15 m/s (325 km), 20 m/s (350 km) and 25 m/s (375 km), respectively. With these threshold values their corresponding success rate of predictions are more than 90%, 50% and 15% at the respective locations. Whereas in the case of GHz scintillations near equator are observed only when ExB (h’F) is more than 15 m/s (325 km), whereas for low latitude, the same should be 30 m/s (400 km) and their success rate of prediction is about 90% and 30%, respectively. The intensity of 4 GHz scintillation at low latitude is also found to be positively correlated with equatorial upward ExB drift velocity values, whereas correlation is poor with that of equatorial scintillations. In conclusions, near magnetic equator threshold values of ExB or h’F can be successfully used for the night-to-night prediction of spread-F/scintillations occurrences, whereas these are necessary but not sufficient for their prediction at higher latitudes. For that some other controlling parameters like background electron density, neutral winds, gravity waves, etc. should also be examined.     

Specific propagation of radiowaves from the Intercosmos-19 satellite in the region of the nighttime equatorial anomaly crest

It has been indicated how a complex ionogram of topside sounding near the outer slope of the winter southern crest of the equatorial anomaly, where a large NmF2, gradient and a deep hmF2, minimum are observed, is formed. The model latitudinal cross-sectio n of the ionosphere, used to perform trajectory calculations, has been constructed based on the corrected Intercosmos-19 data. The ray trajectories have been modeled using the method of characteristics. It has been indicated that a complex Intercosmos-19 ionogram is formed by an oblique reflection from the equatorial anomaly crest slope (the main trace) and by a strongly oblique reflection from the crest bottom as a result of the wave capture by a large-scale inhomogeneity (the additional trace).     

Equatorial spread-F (ESF) and vertical winds

The Equatorial Spread-F (ESF) phenomenon is recorded in ionograms as a hierarchy of plasma instabilities in the F-layer of the equatorial ionosphere. The ESF is characterized by irregularities in the plasma (electron and ion) density and electric field distributions perpendicular to the Earth’s magnetic field. Large scale irregularities are generated by a primary plasma instability that develops in electric fields and plasma densities. Other secondary instabilities then develop and generate irregularities at several scale sizes that often produce a plasma ‘hole’ or ‘bubble’ that rises up with high E×B velocities. The ESF/plasma bubble phenomenon has been studied extensively with experimental techniques and modeling, which revealed important features. In the bottom side F-layer, near sunset, when the vertical density gradient steepens as the layer is supported by the horizontal (North–South) Earth’s magnetic field lines against the omnipresent Earth’s gravitational acceleration (g), the plasma conditions can give rise to Rayleigh–Taylor (RT) type instability. But the observed day to day variability of the ESF occurrence suggested that other agencies may also be involved in generating the instability. Sekar and Raghavarao (1987) with linear theory, and Raghavarao, Sekar and Suhasini (1992), with non-linear numerical modeling, suggested that vertical downward (upward) winds in the ambient gas have the potential to cause (inhibit) the ESF/bubble phenomenon. The presence of downward winds near the equator was reported earlier. In this paper, we show evidence for the presence of downward winds collocated with irregularities in electric fields and plasma densities as revealed by an unique combination of highly accurate measurements with instruments onboard the DE-2 satellite. The observations reported here are also consistent with the notion that the build-up of the equatorial ionization anomaly (EIA) prior to local sunset is important for the ESF instability.