The characteristics of different types of Sporadic E (ES) layers and the associated plasma density irregularities over the magnetic equator have been studied in a campaign mode using VHF backscatter radar, digital ionosonde, and ground magnetometer data from Trivandrum (dip latitude 0.5°N, geographic latitude 8.5°N, geographic longitude 77°E), India. The presence of blanketing type ES (ESb) in the ionograms with varying intensity and duration were observed in association with afternoon Counter Equatorial Electrojet (CEEJ) events. ESb was associated with intense backscatter returns and with either very low zonal electric field and/or with distortions present in the altitude profile of the drift velocity of the type II irregularities. The results of the coordinated study indicate the possible role of vertical electron density gradients in ESb layers in addition to providing evidence for the local winds to be responsible for the vertical gradients themselves.
 The Equatorial Electrojet (EEJ) consists of an enhanced flow of current in the eastward direction during daytime, near the magnetic equator driven basically by the eastward electric field (Ey), which has its origin in the global wind dynamo. The electrojet current together with the geometry of the east-west electric field and the horizontal component of the Earth's north-south magnetic field (H) over the dip equator leads to the development of instabilities in the electrojet plasma [Kato, 1973]. These instabilities cause electron density fluctuations or irregularities in the electrojet region and they can act as scattering centres for HF and VHF radio waves. The predominant equatorial E-region irregularity is sporadic E (ESq) as seen in the ionograms.
 Much of our understanding of the physics of ionization irregularities in the equatorial electrojet is based on radar observations involving Doppler spectral measurements of the scattered signals. Pioneering contributions on electrojet irregularities have been made from Jicamarca using VHF coherent backscatter radars [Cohen and Bowles, 1967; Balsley, 1969, 1973; Fejer et al., 1975; Farley, 1985]. Subsequent radar investigations in other longitude sectors were notably from India and Africa [Reddy et al., 1987; Crochet et al., 1979]. Recent observations on the VHF radar echoes from the electrojet irregularities from Pohnpei (0.5° dip) by Tsunoda and Ecklund  and from the Equatorial Space Observatory of INPE at São Luís (−0.5°dip) in Brazil by Abdu et al.  bring out the complexity of the processes associated with their generation. Basically there are two types of plasma instabilities in the electrojet region. The first one, i.e., the two stream instability gives rise to type I irregularities in the plasma when the electrojet current is very strong with large electron drift velocities [Farley, 1963]. Also the presence of a vertical electron density gradient in the electrojet plasma, perpendicular to the electrojet current flow can excite yet another plasma instability known as the ‘cross field’ or ‘gradient drift’ plasma instability and the irregularities associated with this type of instability are classified as type II [Farley and Balsley, 1973]. The condition for the onset of gradient drift plasma instability in the EEJ region is thus determined primarily by the direction of the vertical polarization electric field Ep (due to Ey) that is responsible for the east-west electron drift velocity and that of the vertical electron density gradient, whereas it is the magnitude of Ep that governs the onset of two stream instability and the associated type I irregularities in the EEJ region.
 The occurrence of normal daytime ESq as seen in the ionograms is caused by the gradient drift instability [Rastogi, 1972a, 1973a, 1973b] implying the prevalence of the above basic ionospheric conditions at that time. This is consistent with the disappearance of the ESq irregularities during certain times of the day when the electrojet currents reverse their direction causing a negative excursion in the horizontal component (ΔH) of the ground magnetic field [Krishna Murthy and Sen Gupta, 1972; Rastogi, 1972b, 1973a; Fambitakoye et al., 1973; Sen Gupta and Krishna Murthy, 1975]. These are known as Counter Equatorial Electrojet (CEEJ) events [Gouin and Mayaud, 1967; Hutton and Oyinloye, 1970]. At that time the reversed electric field, Ey (and consequently the downward Ep field) would not be able to destabilize the electrojet plasma when there is a vertically upward electron density gradient. Hence, if the vertical electron density gradient and the vertical polarization electric field are both upward, type II irregularities are generated and they can be observed by both the VHF backscatter radar and ionondes. During CEEJ events when the vertical polarization field becomes downward (negative) it can also give rise to type II irregularities, but only in the presence of negative (downward) electron density gradients. On occasions when the electron density profiles show large-scale vertical structure providing regions of negative electro density gradients, ESq type irregularities have been detected using Langmuir probe [Prakash et al., 1976] confirming the above. A similar occasion exists also at nighttime when, apart from the very low electron densities, the electron density profile is very irregular, with many regions of positive and negative vertical gradient again as shown by Prakash et al. . The gradient lengths are much shorter at night, increasing the strength of the gradient drift driving term and the direction in which the electrons are drifting becomes immaterial for the generation of irrregularities [Fejer et al., 1975]. During certain CEEJ events, simultaneous appearance of very thin layers of enhanced ionization with both positive and negative gradients effectively blanketing the ionosphere for the low frequency radio probing have been observed. These are known as blanketing ES (ESb) layers. The above facts imply that, just as the daytime electrojet is a seat of both type I and type II irregularities, the CEEJ, on certain occasions can also be considered as a seat of both type I and type II irregularities. However, the observations of type I irregularities under CEEJ conditions have been very rare and reported only once in Africa and once in India [Crochet et al., 1979; Somayajulu et al., 1994]. Recently, Woodman and Chau  have reported the first observations of type I irregularities from Jicamarca Radar observatory (JRO) under daytime CEEJ conditions.
 The generation mechanism of ESb layers at the magnetic equator has been proposed to be due to the local action of east west winds with large vertical shears on the electrojet plasma resulting in the generation of substantial wind induced polarization electric fields (Ew) perpendicular to the geomagnetic field in the magnetic meridional plane [Reddy and Devasia, 1981]. These wind generated electric fields can modify the vertical and latitudinal structure of the electrojet current and can also lead to ionization convergence and divergence and the eventual formation of ESb layers. The distinctly different wind patterns during CEEJ periods supporting the above suggestion has also been shown by Somayajulu et al. . The fact that ESb layers are generally observed during CEEJ periods when the E-region electron density profiles are jagged [Kato, 1973; Prakash and Pandey, 1979] provides the necessary condition for the growth processes of type II irregularities as observed by the VHF backscatter radar [Reddy and Devasia, 1977].
 With this background, a coordinated campaign was undertaken during the solstitial months of June and July 2000 to study the formation of different types of ES layers over the magnetic equator. During this campaign, it has been observed that ESb layers in the equatorial electrojet region give rise to strong backscattering of 54.95 MHz radar signals coinciding with their formation in the ionograms followed by a gradual decrease of radar returns even when the ESb continued in the former. In all the cases of ESb, they were invariably coincident with the presence of a CEEJ event. This could be seen in the negative excursion in the variation of ΔH (TIR) − ΔH (ABG) where ΔH (TIR) and ΔH (ABG) are respectively the daily variation in the geomagnetic field intensities (ΔH) above the night time levels at the equatorial location of Tirunelveli (geog. lat 8.5°N; geog. long 77°E; dip 0.5°N) [to where the geomagnetic observatory has been shifted from Trivandrum] and Alibag (geog. lat 18.6°N; geog. long 72.9°; dip 12.8°N), a station outside the electrojet. It may be noted that the geomagnetic coordinates of Trivandrum and Tirunelveli are nearly the same and hence the data from both the stations can be used interchangeably. All the previous studies on ESb have been emphasizing the possible role of zonal wind shears and the consequent electric fields in the irregularity generation down playing the role of electron density gradients associated with them. The present study highlights the possible crucial role of sharp electron density gradients associated with ESb layers in the explosive growth of gradient instabilities resulting in very large backscattered returns during CEEJ events when the zonal electric field is usually very weak and westward.
2. Data Presentation
 The data obtained under the ISTEP (Indian Solar Terrestrial Energy Program) campaign on ES, organized during 19 June to 7 July 2000 has been used for the investigation of different types of ES, their formation and sustenance under varied electrodynamic conditions prevailing in the equatorial electrojet region. The campaign involved the operation of a vertical incidence Ionosonde, 54.95 MHz VHF backscatter radar and 18 MHz HF radar at Trivandrum apart from the operation of various ground based experiments at different locations over India by other national institutes and Universities. The campaign provided a very good data set on various aspects related to the formation of ESq and ESb, which could be used to improve the current understanding of the electrodynamics of the equatorial ionospheric processes.
 The campaign period consisted of both quiet and moderately disturbed days, with and without the occurrence of CEEJ and blanketing ES. The data on H component of the Earth's magnetic field at Tirunelveli and Alibag corresponding to all the days of the campaign period were provided by the Indian Institute of Geomagnetism, Mumbai. The VHF backscatter radar operates at 54.95 MHz with, a peak power of 20 kW, pulse width of 20 μs and a pulse repetition frequency of 500 pps. The antenna system consists of a five element Yagi antenna array (4 × 16) with four antennae in the magnetic north south direction. The radar antenna beam is oriented at an elevation angle of 60° toward the west. The effective beam width of the antenna array is about 4° in the east west direction, which corresponds to a height resolution of about 6.0 km at the electrojet altitude. The backscattered signals were sampled at 20 μs intervals providing a range gate sampling at every 3 km. The detailed characteristics of the VHF backscatter radar at Trivandrum are given by Reddy et al. . For the antenna configuration the observed negative (positive) Doppler frequency variations of the VHF radar signals correspond to the westward (eastward) drift of electron density irregularities in the presence of an eastward (westward) electric field (Ey) in the electrojet region.
 The mean Doppler frequency (D) characterizing the mean east west drift of the electrojet irregularities is defined as
where P(fD) is the backscattered power per unit frequency interval at the Doppler frequency, fD. During the present campaign spectra at every 5 min interval have been recorded in the presence of ESq and at every 2 min interval during all the ESb events.
 In the case of type II irregularities observed by the radar, the mean Doppler frequency (D) variation of the backscattered signals is proportional to the phase velocity of the 2.7 m scale size irregularities (corresponding to half the radar wavelength λr) and this velocity, in turn, is proportional to the east west electric field in the electrojet region [Fejer and Kelley, 1980]. The parameter D is related to the east-west drift velocity (Vp) of the irregularities as,
where λr = 5.4 meters for the 54.95 MHz backscatter radar at Trivandrum. The error in the computed D is variable, since it depends on the power levels measured as well as the shape of the spectrum. Under strong signal conditions, typical uncertainty is ±1 Hz or equivalently ±2.7 m/s in velocity. In the rest of the text and in figures, only the east-west drift velocity Vp of the irregularities will be used, and will be denoted as the mean Doppler velocity.
 The mean Doppler velocity of type II irregularities is proportional to the cosine of the radar elevation angle [Balsley, 1969]. Measurement of the type II velocity has been extensively used to determine the drift velocity of electrons (Ve) in the electrojet and for the determination of the east-west electric field (Ey) which drives the electrojet [Balsley, 1969, 1973; Hysell and Burcham, 2000]. In yet another method, Hysell and Burcham  also used the Jicamarca antenna array and employed interferometry method to measure altitude profiles of the phase speeds of intermediate-scale primary gradient drift waves in the electrojet overhead. Electric fields were inferred from these profiles following a modelling approach. It is to be noted that the measured mean Doppler velocity (Vp) of type II irregularities has an important contribution due to the E-region neutral winds as shown below:
where Ve and Vi are electron and ion velocity
with νe, νi and Ωe, Ωi the collisional and gyro frequencies of ions and electrons. At altitudes above 100 km, the collisions are significantly less and hence α is negligibly small. Hence, over this height region, the drift velocity of the type II irregularities represents essentially the electron drift velocity (Ve). Below ∼100 km, because of large ion-neutral collisions, the ion motion (essentially due to neutral wind) can not be neglected while deriving the electron drift velocity or the electric field from the observed Doppler velocities.
3. Experimental Observation and Results
 A radar based study on the characteristics of type II irregularities associated with the appearance of ESb layers during a CEEJ event would need a comparison of the same with the observed characteristics associated with the ESq layers during normal daytime electrojet.
3.1. Daytime Type II Echo Characteristics on Non-CEEJ Days
Figure 1a shows the time variation of the mean Doppler velocity of the backscattered signals during daytime on 7 July 2000 corresponding to different height regions of the electrojet. The time variation of [ΔH (TIR) − ΔH (ABG)] during 0600–2000 hours on this day (bottom panel) showed the electrojet strength to have a normal pattern of smooth variation (with maximum strength ≈80 nT) and ionosonde showed the presence of ESq during 0625–1825 hours. The VHF radar showed the presence of backscattered signals during 0810–1840 hours with only type II echoes over the entire height region during 0810–1005 hours and 1555–1840 hours and with composite type I and type II echoes during the period 1010–1500 hours in the height regions above 103 km. In the lower height regions only type II echoes were present during this time. This is typical of a normal EEJ day. The mean Doppler velocities obtained at different heights from 93–106 km show significant temporal variations and also variations with height. As the mean Doppler velocity of the type II irregularities is a measure of the net electric field at each height, the height variations of the velocity in the electrojet region indicate the corresponding altitude variations in the vertical polarization field itself. Figure 1b (top) shows the plot of maximum backscattered power (Pmax) and the corresponding mean Doppler velocity along with [ΔH (TIR) − ΔH (ABG)] (bottom panel) during 1300–2000 hours. Figure 1c (bottom) shows the height profiles of the mean Doppler velocity of EEJ irregularities at different times (IST - Indian Standard Time corresponding to 82.5°E) along with a typical ionogram indicating the presence of ESq during this period (the time in SST is also shown in the ionograms). One could notice that the strength of the daytime EEJ as indicated by [ΔH(TIR) − ΔH(ABG)] variations are clearly reflected in the amplitude and height variations of the Doppler velocities during that period. Another notable feature of the velocity profiles is the comparatively larger height extent of the echoing region with larger velocity values mostly above the electrojet peak height in comparison to those observed on CEEJ days as would be shown and discussed in the following sections.
3.2. Daytime Type II Echo Characteristics on CEEJ Days
3.2.1. Case I: 22 June 2000 (AP = 11)—A Quiet Day Without the Occurrence of ESb Layers
 On similar lines as those of Figure 1a, Figure 2a represents the variation of Doppler velocity on 22 June 2000 corresponding to different height regions of the electrojet, and that of [ΔH (TIR) − ΔH (ABG)] which represents the strength of EEJ (bottom). It is clear that the daytime electrojet was weaker (ΔH ≈ 40nT) than that of 7 July 2000, and was followed by a moderate CEEJ (max ΔH ≈ −20 nT) during 1430–1800 hours. ESq was present in the ionograms all through 0800–1800 hours, contrary to the expectations that it would disappear with the reversal in the magnetic field [Rastogi, 1972b; Fambitakoye et al., 1973; Sen Gupta and Krishna Murthy, 1975] during 1430–1800 hours (Figure 2c). The VHF radar observations during this event also showed the presence of type II irregularities during 1430–1545 hours similar to those usually associated with the normal daytime EEJ (Figure 2b), but with different characteristics like, a sudden drop in scattered signal intensity coinciding with the CEEJ (Figure 2b, top) and the scattering region being confined to a narrow height extent between 96 and 104 km (Figure 2c, bottom).
 In Figure 2c, the height variation of the velocity of the type II irregularities at certain selected timings during the above period is shown along with the ionograms depicting the ES traces. This figure provides a comprehensive view of the gradual changes in the overall electrodynamics of the equatorial E region from that of a normal EEJ behavior to that of a CEEJ. Before the start of the CEEJ, the height extent of the scattering region was ∼14 km, broader than one normally expects during the daytime electrojet. With the onset and development of CEEJ, the scattering region becomes narrower without any change in the height location of maximum scattered power, until the CEEJ intensity becomes maximum at around 1545 hours. The narrowing of the scattering region to ∼8 km with structural changes appears to have progressively provided the proper electron density gradient (negative electron density gradient) in the presence of a westward electric field to produce the type II irregularities as observed during the initial phase of CEEJ event. However, the VHF radar signal disappeared at around 1545 hours, almost coinciding with the time of peak CEEJ whereas, the presence of ES in the ionogram continued till 1800 hours close to the recovery time of the CEEJ event. The absence of radar echoes after 1545 hours probably indicates that the electron density gradients (negative) associated with the ES layers at these times are not steep enough to sustain the gradient instabilities generating 2.7 m irregularities even though a significant westward electric field was present during the peak of CEEJ event. It is to be pointed out that although ESq was present in the ionograms even beyond 1500 hours (during the CEEJ), the radar signals during 1500–1545 hours were extremely weak to draw any conclusions.
3.2.2. Case II: CEEJ Event on 20 June 2000 (Ap = 6)—Strong Type II Signals and Strong Blanketing ES Occurrence
 20 June 2000 was also a magnetically quiet day (Ap = 6) with an eastward electrojet current upto 1545 hours (maximum ΔH intensity ∼ +55 nT). During 1545–1800 hours, the electrojet current reversed as indicated by the variation in [ΔH (TIR) − ΔH(ABG)] constituting the CEEJ (Figure 3a). The ionograms on this day showed the presence of Esq during 0630–1610 hours and the presence of ESb during 1615–0000 hours, even beyond the duration of CEEJ (i.e., beyond 1800 hours). The variations in the maximum backscattered power (Pmax) and corresponding mean Doppler velocity during 1300–2000 hours are also shown in the figure. The VHF radar showed the presence of radar returns from ESq during 0847–1525 hours only. The disappearance of type II irregularities took place well before the onset of CEEJ at 1545 hours as indicated by the ground magnetogram. However, the ESq was persisting in the ionograms even during this time and also during the build up time of the CEEJ event (1545–1610 hours). These ESq layers turned into strong ESb layers with multiple reflections beyond 1615 hours and persisted up to midnight (Figure 3b). On the other hand, there was a sudden build up of radar signals at 1629 hours coinciding with the appearance of ESb in the ionograms from 1615 hours onward (Figure 3a). Very strong radar signals with large and rapid changes with time accompanied by changes in height structure of Doppler velocity were observed during 1645–1827 hours. The variations in velocity during this period were very small and confined to almost within ±10 m/s indicating very weak east-west electric field. In spite of this, it is interesting to note that this weak field could sustain the gradient instabilities generating 2.7 m scale size irregularities giving very strong backscatter returns. After the CEEJ, i.e., beyond 1800 hours, ΔH became positive even though the magnitude was still very small. The radar returns during this time, which were from the persisting ESb layers, showed positive (eastward) velocity variations over the height region of 93–99 km. Figure 3b also shows the altitude profiles of the type II irregularity velocities at different times obtained from the VHF radar for the period (1520–1813 hours) before and during the CEEJ along with sample ionograms. It may be noted that, the Doppler velocity profiles during the presence of ESb layers have highly distorted altitude structures unlike the previous cases corresponding to ESq layers. The possible role of neutral wind shears is discussed later.
3.2.3. Case III: CEEJ Event on 4 July 2000 (Ap = 8)—Strong Type II Signals With Very Strong Blanketing ES Occurrence
 The ESb observations on this day shown in Figures 4a and 4b are unique in many respects. This happened to be a day with strong EEJ (∼+60 nT) followed by a very strong CEEJ event (∼−40 nT). Presence of very strong ESb with several multiple reflections in the ionograms and very large VHF backscattered power returns even exceeding the daytime peak value on this day with large fluctuations in the height structure of the irregularity velocities were the highlights. The CEEJ was observed during 1445–1830 hours. ESq during 0810–1530 hours and ESb during 1600–0000 hours, even beyond the duration of CEEJ were seen in the ionograms. During the initial phase of this event, ESq traces were seen in the ionograms from 1445–1530 hours, whereas the VHF radar indicated the presence of comparatively weaker signals with negative Doppler velocity (of fairly large value). The spectacular appearance of ESb with VHF radar power level even exceeding the day's peak value started at around 1616 hours and continued till 1909 hours, even after the duration of the CEEJ. The presence of positive Doppler velocity values predominantly all through the height regions of the ESb layer along with negative values of Doppler velocity confined to comparatively narrower height regions were the notable features observed during this event. Also, the radar signals with positive Doppler velocity and substantial power returns continued during the presence of the ESb layer even after the CEEJ event was over.
 A comparison of the VHF backscattered power (in dB) and Doppler velocity variations observed during the normal EEJ as well as during the CEEJ in the presence of blanketing ES is shown in Figure 5. The figure brings out the contrasting characteristics of the VHF radar echoes during daytime ESq and late after noon ESb in terms of the backscattered signal strength and associated mean Doppler velocity (indicating the persisting E-W electric field). Large power returns (maximum ∼42 dB) with large mean Doppler velocity (−160 to −320 m/s) during ESq and very large power returns (maximum ∼51 dB) with very low mean Doppler velocity (0 to 20 m/s) during ESb are the distinct features. The figure also illustrates the explosive nature of the radar signal build up during the ESb event. It may be noted that the mean Doppler velocity variations during the large power returns (mostly confined to 95–99 km region) associated with the onset of ESb are within 0 to +20 m/s indicating a very low value for the westward electric field at this time; whereas the gradual increase in magnitude of the west ward electric field as indicated by the increase in the mean Doppler velocity (in the range +20 m/s to +40 m/s) has resulted in the weakening of the gradient instabilities. The dominance of positive Doppler velocity variations of varying magnitudes over the entire region with correspondingly large changes in backscattered power returns during this period is clearly seen.
3.2.4. Case IV: CEEJ Event on 6 July 2000 (Ap = 5)—Weak Type II Signals With Strong Blanketing ES Occurrence
 6 July 2000 (Figures 6a and 6b) was characterized by a weak EEJ (+20 nT) followed by a relatively strong CEEJ (∼−30 nT). Ionograms indicated the presence of ESq during 0830–1540 hours and that of ESb during 1550–1800 hours. The CEEJ duration was from 1400–1730 hours. The initial phase of the CEEJ event showed the radar returns (though not very strong) with negative Doppler velocity till 1445 hours (Figure 6a) as well as the presence of ESq in the ionograms until 1540 hours (corresponding to the time of peak CEEJ intensity) (Figure 6b). The remarkable feature observed during the CEEJ event is the development of the ESq traces into very strong blanketing ES traces that persisted well beyond the CEEJ duration. In the case of radar observations, the weak backscatter signals with negative Doppler velocity disappeared at around 1445 hours and no signal condition continued till 1600 hours although ionograms indicated the onset of weak ESb even from 1550 hours onward. The appearance of very intense ESb was indicated in the ionograms from 1600 hours onward whereas the radar showed the presence of weak and intermittent signals with positive Doppler velocity during 1600–1702 hours. The altitude profiles of the velocity of the irregularities before and during the CEEJ are depicted in Figure 6b (bottom panel). A very notable feature of the Doppler velocity profiles during the blanketing ES layer on this day is its very low values without significant distortions and height reversals in its altitude structure unlike the earlier cases of 20.6.2000 and 4.7.2000. These aspects are discussed in the sections to follow.
4. Summary of Observations
 In the present study, four unique cases of CEEJ occurrences have been examined to understand the prevailing electrodynamics that could trigger and sustain gradient instabilities by proper combination of very sharp electron density gradients of the ESb layers and very weak zonal electric fields. The main features can be summarized as follows:
 1. The onset of ESb is seen to be mostly associated with the time of peak CEEJ intensity and it persists mostly for the entire duration of the CEEJ event. The layer sometimes continues to persist even after the CEEJ is over when the magnetic field intensity has become positive.
 2. Another noticeable feature is that the mean Doppler velocity is very small (when compared with normal EEJ and ESq conditions) during the explosive growth and sustenance of the type II irregularities of 2.7 m scale size associated with the ESb occurrence. The backscattered power is significantly larger and its peak value sometimes even exceeds the peak backscattered power from ESq observed during the daytime normal EEJ hours on the same day.
 3. The absence of radar signals on certain occasions even in the presence of ESb in the ionograms is indeed puzzling. This feature which is particularly observed during the initial phase of the ESb layer formation, is possibly due to the lack of proper gradient of the newly formed ESb layer or lack of a threshold level for the east-west electric field, or both, for the onset of gradient instabilities, which could result in 2.7 m irregularities.
 4. During a persisting ESb layer, the mean Doppler velocity at different heights changes from westward to eastward indicating large changes in the height structure of the drift velocity of the type II irregularities. Remarkable changes in the signal strength and Doppler frequency spectrum of the backscattered signals have been observed. Fast changes in the polarity and magnitude of the Doppler velocity at different altitudes in the electrojet region are also observed, highlighting the role of gravity wave induced electric field changes in the redistribution of ionization and its eventual destabilization.
5. Discussion and Conclusions
 The characteristic features of the post noon, type II echoes under discussion (Table 1) indicate that strong echoes are mostly associated with ESb layers appearing during CEEJ. These are generally preceded by echoes due to ESq layers and occasionally the type II echoes due to ESq could even continue well into the CEEJ duration.
Table 1. A Comparison of the Observed Characteristics of the ESq/ESb Associated With EEJ/CEEJ During the Campaign
Date and Ap
20.6.2000 Ap = 6
• EEJ: peak strength (+55 nT)
VHF radar: 0847–1525
• CEEJ: peak strength (−20 nT)
• No Signal in VHF during: 1525–1624; : 1735–1745
• Large changes in height structure of mean Doppler velocity during ESb
22.6.2000 Ap = 11
• Moderate EEJ: peak strength (+40 nT)
VHF radar: 0840–1430
• CEEJ: peak strength (−20nT)
• No Esb during CEJ; ES present
• No Signal in VHF during: 1545–1800
4.7.2000 Ap = 8
• EEJ: peak strength (+60 nT)
VHF radar: 0830–1445
• CEEJ very strong: peak strength (−40 nT)
• E region traces during: 1530–1600
• No signal in VHF during: 1525–1616
• Large changes in the height structure of mean Doppler velocity during ESb
6.7.2000 Ap = 5
• Weak EEJ: peak strength (+20 nT)
VHF radar: 0925–1445
• Strong CEEJ: peak strength (−30 nT)
• E region traces during: 1540–1550
• No signal in VHF during:1445–1600
• Very strong ESb in Ionosonde
• Very weak signals in VHF
7.7.2000 Ap = 5
• Strong EEJ: peak strength (+80 nT)
VHF radar: 0810–1825
• No CEEJ; No ESb
 In the case of Ionosonde observations, the ionogram traces corresponding to ESq and ESb layers are distinctly different, with strong ESb layers characterized by several multiple reflections and sometimes almost totally blanketing the F-layer. It has been illustrated in the previous sections that the characteristics of type II echoes due to ESq and ESb during CEEJ events are distinctly different in terms of their height of occurrence, signal strength and associated Doppler spectral characteristics. The onset and growth of ESb layers in general reveal narrow Doppler spectra that are nearly symmetric about the zero value of the Doppler frequency indicating the presence of near zero or very weak east-west electric field. On the contrary, type II echoes corresponding to ESq, during normal electrojet as well as on some occasions during the CEEJ were characterized by Doppler frequency spectra which are asymmetric with respect to the mean Doppler frequency (equivalently mean velocity of Type II irregularities) which is proportional to the east-west electric field. The velocity profiles from ESq irregularities in most cases resemble the smooth profile shape of the vertical polarization electric field (Ep) indicating the dominant role of the electric field in comparison to that of any wind generated polarization electric field. On the other hand, the velocity profiles of the type II irregularities from the blanketing ES layer show the presence of large velocity shears (or equivalently wind shears) [Reddy and Devasia, 1981]. The presence of very low value of east-west electric field at the time of onset of blanketing ES and the presence of large shears in the velocity profiles suggest the dominant role of wind induced polarization electric field generated by the height varying zonal winds [Reddy and Devasia, 1981]. Since ESb usually gets formed at lower heights typically ∼95 km, ion-neutral collisions become important and hence the neutral wind effects would become significant. The wind generated polarization field consequently could have strong vertical gradients and reversals. Such reversing electric fields in the E-region can give rise to ionization convergence and the formation of thin ionization layers (ESb layers) with sharp electron density gradients. Some of the Doppler velocity profiles shown in the present case study have a close similarity to the reversal of drift velocity with height as shown by Reddy .
 The description of the CEEJ events associated with varied forms of ESb given in the previous section brings into focus many aspects of the growth and sustenance of the gradient drift instabilities under conditions of weak/reversed electric field (eastward drift of electrons) and very sharp and variable electron density gradients. The role of the latter associated with the ESb layers is particularly important in the generation and explosive growth of type II irregularities of 2.7 m scale size especially when the drift velocity of the irregularities is very small. The generation of type II irregularities for normal westward drift of electrons in the presence of positive (upward) electron density gradient lengths (L) of a few kilometres or more (typical daytime EEJ conditions) is a regular phenomenon and the importance of the vertical electron density gradients in the VHF equatorial backscatter is well known [Fejer et al., 1975].
 The effects of the gradient term on the type II waves have been examined in the equatorial case by Farley and Balsley, 1973 and Fejer et al. . From linearized fluid theory, the equations for the oscillation frequency (or phase velocity) and growth rate of electrostatic waves propagating perpendicular to the Earth's magnetic field are given by
where λr = radar wavelength.
 The meaning and significance of various quantities in these equations are well known [Fejer and Kelley, 1980]. L is the electron density gradient scale length along the ambient electric field and Vd is the component of drift velocity of irregularities along the radar observing direction. In equation (6) it is assumed that both the destabilizing terms, i.e., electric field (E) and electron density gradient (∇⊥no) point the same direction. The electron density gradient scale length L = no−1 represents the component of density gradient perpendicular to the Earth's magnetic field (B) and parallel to E so that the product of L and E is positive (destabilizing). When Vd is very small, the growth rate is mainly controlled by L for a fixed value of λr (through k = 2π/λr) and hence the role of L can be extremely significant on occasions. The formation of a ESb layer with very sharp gradients during CEEJ events is a common feature over the magnetic equator during the summer solstitial months. However the type II irregularities could be generated at the top or bottom of a strong ESb layer, provided the combined action of an existing Vd (or equivalently E field) and the electron density gradient length L along E is significant to cause the growth rate in equation (6) to become positive (destabilizing). In a situation where the electron density profile during the time of a ESb layer is highly jagged with the presence of very sharp positive and negative electron density gradients, together with the large changes in the altitude structure of the associated Vd profile (as observed in many of the ESb cases discussed in this paper), the destabilization and sustenance of type II irregularities become possible. This type of a situation never exists during the time of a normal daytime electrojet where the electron density gradient scale length (L) is positive and is of the order of 10 km or more. Though the electron density gradients are shallower, the presence of fairly large values of positive Vd (because of the substantial magnitude of the eastward electric field) compensates for this and helps in the triggering of gradient instabilities. Essentially, it means that during normal EEJ, when ESq is present, type II irregularities are generated with positive values of Vd (exceeding a threshold level) and large L values. On the other hand, the ESb layers during CEEJ events give rise to an explosive growth of type II irregularities at significantly lower heights with very low values of Vd (both positive and negative) and very low values of L (due to very sharp electron density gradients, upward or downward). It must be remembered that under these circumstances, even though the electric field driven gradient drift instability mechanism is weak, the effect of the neutral wind is to be taken into account in the growth rate of the instability in equation (6), through the drift velocity, Vd = Ve − Vi, where the motion of electrons and ions are controlled by the electric field and neutral wind (through the generation of wind generated electric field).
 As it has been observed during all the ESb events cited in the present study, the very narrow spectra with Doppler shifts close to zero could be generated directly when very sharp electron density gradients in ESb layer with L ≈ 100 m are combined with low Vd values. Woodman et al.  proposed a qualitative mechanism, which postulates the presence of gravity waves with significantly large amplitudes distorting the ES layers and producing a condition of unstable electron density gradient, which would appear and disappear in accordance with the driving gravity wave period. This type of ESb layer distortions indicated by large changes in the altitude structure has indeed been observed during the spectacular events on 4 July 2000 and also on 20 June 2000. The absence of significant distortions in the altitude structure on 6 July 2000 thus explains the lack of proper electron density gradients in the ESb layer, resulting in very weak backscattered signals. The occurrence of ESb being rather rare over the magnetic equator and that too mostly associated with the CEEJ events, the explosive growth of gradient instabilities, under conditions of very low east-west electric field can only be explained in terms of the presence of very sharp and rapidly varying electron density gradient with scale lengths of the order of 100–400 meters. Further, the present study also provides an experimental evidence for the rapid variation in the altitude structure of the mean Doppler velocity of type II irregularities as possibly due to the modulation of the electrojet current by the electric fields generated by height varying winds with large shears [Reddy and Devasia, 1981; Hanuise et al., 1983].
 The authors wish to acknowledge with thanks the active participation of our colleagues in Atmospheric Technology Division of Space Physics Laboratory in the ISTEP campaign on ES by operating the Ionosonde and the VHF radar at Trivandrum. The campaign was supported by the Multiagency Indian Solar Terrestrial Energy program (WG II) for which the Department of Science and Technology (DST) of the Government of India is the nodal agency. This work is supported by the Department of Space, Government of India. The authors also wish to acknowledge the cooperation of Indian Institute of Geomagnetism by supplying the magnetograms used in this study.