Equatorial Spread F Irregularities Research Articles

Modeling Equatorial F‐Region Ionospheric Instability Using a Regional Ionospheric Irregularity Model and WAM‐IPE

AbstractThis paper uses a regional simulation of plasma convective instability in the postsunset equatorial ionosphere together with a global atmosphere/ionosphere/plasmasphere GCM (WAM‐IPE) to forecast irregularities associated with equatorial spread F (ESF) for 1–2 hr after sunset. First, the regional simulation is initialized and forced using ionosphere state parameters derived from campaign data from the Jicamarca Radio Observatory and from empirical models. The irregularities produced by these simulations are found to be quantitatively similar to those observed. Next, the aforementioned state parameters are replaced with parameters from WAM‐IPE, and the resulting departures between the simulated and observed irregularities are noted. In one of five cases, the forecast failed to accurately predict ESF irregularities due to the late reversal of the zonal thermospheric winds. In four of five cases, significant differences between the observed and predicted prereversal enhancement (PRE) of the background vertical drifts resulted in degraded forecast accuracy. This highlights the need for improved PRE forecasting in the global‐scale model.

The Role of Storm‐Time Electrodynamics in the Dawn and Dusk Sectors Across Equatorial and Low‐Latitude Ionosphere During December 19–21, 2015

AbstractWe present the distinct responses of the nighttime ionosphere due to an intense and long lasting geomagnetic storm of December 19–21, 2015 using a chain of ionosondes from magnetic equator to anomaly crest region over India. The observations exhibited unique features throughout the night including dawn and dusk sectors. The results showed twin episodes of pre‐reversal enhancement (PRE) in dusk sector associated with oscillatory vertical plasma drift due to shielding electric field that produced briefly post‐sunset equatorial spread F (ESF) irregularities. Accordingly, relative dominance of interplanetary magnetic field (IMF) and auroral electrojet (AE) related electric field fluctuations in vertical and zonal drifts at different times are investigated throughout the night. Also the application of wavelet/wavelet coherence analysis on interplanetary and geomagnetic conditions and ionospheric data showed distinct relationships. The results showed that vertical drift is in phase with interplanetary electric fields (IEFy) with period of ∼30 min at ∼18:30 IST (13:00 UT) that drives first PRE whereas ∼60 min period in AE which is in phase with vertical drifts drives second PRE at ∼20:30 IST (15:00 UT). However, other short term periods in vertical drifts are in phase with IEFy and AE at other times suggesting that both are contributing to the drift variations. The simultaneous upliftment of F layer at all stations producing fresh ESF irregularities in the dawn sector is attributed to the unique electrodynamical response of the Indian low latitude stations to the eastward disturbance dynamo electric fields (DDEFs). These results are also in concomitant with satellite observations and thermospheric ionospheric electrodynamics general circulation model (TIE‐GCM) drifts.

The post sunset equatorial F- region zonal drift variability and its linkage with equatorial spread F onset and duration over Indian longitudes

The seasonal and solar activity variation of the post sunset F- region zonal plasma drift, at the magnetic equatorial region over Indian longitudes is analyzed using the Republic of China Satellite-1 data from January 2000 to April 2004. The post sunset F- region zonal drifts are observed to be higher in the years of high solar activity in comparison with low solar activity, while seasonally the drifts are minimum in summer with much higher values in other seasons. The seasonal and solar activity variations of zonal plasma drift are attributed to the corresponding variations in the neutral winds. The dependences of the F region peak vertical drift on the zonal plasma drift at 18.5 IST (Indian Standard Time) and the time difference of the conjugate points sunset times, are quantitatively analyzed. Further an integrated parameter (incorporating the above mentioned two independent factors), which is able to predict the peak vertical drift and growth rate of Rayleigh Taylor instability is proposed. The other major outcome of the study is the successful prediction of the Equatorial Spread F (ESF) onset time and duration using the new integrated parameter at 18.5 IST. ESF irregularities and associated scintillations adversely affect communication and navigation systems. Hence, the present methodology for the prediction of the characteristics of these nocturnal irregularities becomes relevant.

Active (Evolving) Phase of Equatorial Plasma Bubbles Observed Over Tirunelveli

AbstractUsual post‐sunset generation of equatorial spread F (ESF) irregularities on quiet days (Q‐days) and midnight or post‐midnight generation of ESF on magnetically disturbed days (D‐days) is widely known. However, the duration of evolving or active phase of these freshly generated ESF (FESF) irregularities is not clearly understood. Here, the active phase duration refers to the time period during which the perturbation electric field associated with equatorial plasma bubbles drifting over Tirunelveli is alive. Recently, Gurram et al. (2018) quantified the active phase duration of FESF for both Q‐days and D‐days. It was the first exercise to obtain the average active phase duration of FESF from observations. In view of this, we carried out detailed analysis to examine their dependence on solar flux and geomagnetic activity using long‐term amplitude scintillation observations on 251‐MHz signal. It is found that percentage of days with longer active phase (td≥ 50 min) is higher on D‐days, and it is consistent for all levels of solar flux. Average duration of the active phase of FESF, ⟨td⟩, is 57.9 ± 52.3 min on D‐days and 33.8 ± 23.4 min on Q‐days. The active phase of FESF has tendency to sustain for longer time during low solar flux, which is attributed to presence of weaker ambient ionospheric electric field during that period. It is noted that FESF generated on days with stronger geomagnetic activity tend to have longer active phase. Also, days with FESF cause more moderate–strong scintillations as compared to days with drifted‐in ESF.

A Statistical Study on The Local Time Dependence of Equatorial Spread F (ESF) Irregularities and Their Relation to Low‐Latitude Es Layers Under Geomagnetic Storms

AbstractIn this paper, we present a study on the local time dependence of equatorial spread F (ESF) irregularities and their relation to low‐latitude Es layers in response to geomagnetic storms using simultaneous observations of two ionosondes one located at Tirunelveli (8.73°N, 77.70°E), an equatorial station and other located at Hyderabad (17.36°N, 78.47°E), an off‐equatorial station during the years 2007–2015 that covers solar cycles 23 and 24. The Aarons criteria for the ESF irregularities for different seasons under geomagnetic storms are evaluated. In the category I, we noticed partial enhancement in prereversal enhancement (PRE) during postsunset resulting in ~30% occurrence of spread F instead of total inhibition during equinox and winter seasons. Also, occurrence of ESF in summer is suppressed by only ~75% due to partial increase in PRE. In category II, we observed presunrise height enhancement mostly during winter, which caused ESF to occur at ~50% followed by equinox and summer. The results presented here suggest that many are actually not following the Aarons criteria possibly due to not considering other background conditions. Accordingly, we examined the plausible role of low‐latitude Es layers on the generation of ESF irregularities under these categories during geomagnetic storms. We noticed the absence/presence of Es layers in both categories during postsunset and postmidnight hours resulting in increase/decrease of PRE due to modifications in the field‐line‐integrated Pederson conductivity. Accordingly, the results suggest that one of the plausible reasons for ESF irregularities not following Aarons criteria is linked to the variability of the low‐latitude Es layers.

Observations of Evolution‐Type Band‐Like Structures of F Region Irregularities

AbstractGenerally, equatorial spread‐F (ESF) irregularities initiated at the bottom of the ionospheric F region and manifested as plume structures with an altitude extent more than 50 km in the RTI (range‐time‐intensity) plot. By using the Fuke and Sanya VHF radars located at low latitude of China, evolution‐type band‐like structures of ESF irregularities were observed near local sunset. One interesting aspect is that the ESF band‐like structures initially appeared at high altitudes more than 400 km and then uplifted to higher altitude without zonal propagation. The lifetime of these ESF band‐like structures, with a similar pattern in the RTI plot for each beam of radar, is less than 25 min. The band‐like structures of F region irregularities were observed with big Doppler velocities and narrow spectral widths. During the presence of ESF band‐like structures, apparent amplitude scintillations (S4 > 0.2), total electron content (TEC) fast fluctuations (ROTI > 1), and range‐type spread F were detected. Possible mechanisms responsible for the initial generation of band‐like ESF irregularities at high altitude are discussed.

Statistical Study on the Occurrences of Postsunset Ionospheric E, Valley, and F Region Irregularities and Their Correlations Over Low‐Latitude Sanya

AbstractThe coupling of equatorial and low‐latitude ionosphere has been an active area of research for many years. At low latitude, the occurrences of E and valley region irregularities during postsunset hours are obviously different from those around the magnetic equator. In this work, we statistically investigated the occurrences of postsunset ionospheric E, valley, and F region irregularities, the postsunset rise of F layer, and their correlations by using the Sanya (18.4°N, 109.6°E, dip latitude 12.8°N) very high frequency radar and ionosonde observations during equinoctial months of 2012–2016. A statistically significant correlation between the presence of valley region irregularities and that of equatorial spread F (ESF) was found over Sanya. For the days without ESF, valley region irregularity was rarely observed. The Doppler velocities of valley region irregularities were mainly upward which resemble those of simultaneous ESF irregularities. On the other hand, an occurrence peak of E region irregularities was detected around sunset. After sunset, E region irregularity echo was frequently disrupted. The disruption can appear on both ESF and non‐ESF days, when the amplitudes of F layer postsunset rise were significantly different. The effects of polarization electric field associated with ESF and of equatorial ionospheric background condition on the E and valley region irregularities over Sanya are discussed.

On the Genesis of Postmidnight Equatorial Spread F: Results for the American/Peruvian Sector

AbstractPrevious studies of the Earth's low‐latitude ionosphere using in situ measurements made by sensors on the Communication/Navigation Outage Forecasting System (C/NOFS) satellite showed an unexpected predominance of equatorial spread F (ESF) events in the postmidnight sector during June and December solstice months of the 2008–2009 deep solar minimum. It has been suggested that these events might have been driven by the unusual behavior of the equatorial plasma drifts, which showed an abnormal upward peak around midnight during the same period. We use coherent backscatter radar (Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere ‐ JULIA) measurements made at the Jicamarca Radio Observatory (11.95°S, 76.87°W, ∼1∘ dip lat) in Peru to better understand the origin of the ESF irregularities observed by C/NOFS. The radar observations show that ESF events during December solstice start in the postsunset sector. These ESF events and the conditions for their development are shown to continue through midnight hours. The predominance of postmidnight irregularities on C/NOFS observations during December solstice is caused by the slow vertical development of the ESF structures, which only reach the topside near midnight in most cases. On June solstice, however, JULIA observations show that ESF started predominately in the midnight to postmidnight sector. Collocated digisonde observations provide additional insight on F‐region conditions leading to these ESF events.

The Response Time of Equatorial Ionization Anomaly Crest: A Unique Precursor to the Time of Equatorial Spread F Initiation

AbstractThe time delay with which the magnitude and location of the total electron content (TEC) at the equatorial ionization anomaly (EIA) crest responds to the mean height of the F layer (hmF2) is examined seasonally using the ionosonde data at Trivandrum and the Global Positioning System data at different stations in the EIA region for low and high solar activity years. The study brings out the fact that for the low solar activity year, the crest TEC responds fastest in winter solstice compared to other seasons. Further, the response time of the crest is found to decrease with solar activity. The seasonal variation in the EIA response time is attributed to the modulation by meridional neutral winds, whereas the solar activity variation seems to be basically controlled by the diffusion times and the background ionization gradients in the respective epochs. The derived relationship between EIA crest location/magnitude and hmF2 for any season can be used for prediction purposes. Furthermore, this study for the first time establishes the relation of EIA response time to equatorial spread F (ESF) start time, for days when the F layer is in the neutral dynamically controlled domain. The deleterious effects of ESF irregularities on communication and navigation systems are well known, and the above result is significant in this context. The present study has the potential to be extended into a model to predict the ESF start time, from the EIA response time, earlier in the day.

Daytime F-region irregularity triggered by rocket-induced ionospheric hole over low latitude

Unexpected daytime F-region irregularities following the appearance of an ionospheric hole have been observed over low latitude. The irregularities developed initially above the F-region peak height (~ 360 km) with a thickness of about 30 km and an east-west extension of more than 200 km around 1057 LT and then expanded upward to 500 km altitude behaving like the equatorial spread-F (ESF) irregularities of the nighttime ionosphere. These daytime F-region irregularities cannot be explained on basis of an earlier suggestion that the F-region irregularities observed during daytime are the continuation of the irregularities initially generated on the previous night. Based on the coincidence, both in space and time, with the appearance of an ionospheric hole, which was generated after the passage of a rocket, we conclude that the daytime F-region irregularities must have been artificially generated locally through a manifestation of plasma instability triggered by the rocket exhaust-induced ionospheric hole over low latitude.

The characteristics and generation mechanism of small‐amplitude and large‐amplitude ESF irregularities observed by the C/NOFS satellite

AbstractIt is well understood that equatorial plasma bubbles are initiated in the bottomside F region and grow into the topside F region, producing equatorial spread F (ESF) irregularities in both bottomside and topside F regions. However, it is not known whether small‐amplitude and large‐amplitude irregularities have the same occurrence pattern and how the generation of small‐amplitude and large‐amplitude irregularities is related to the postsunset vertical plasma drift and potential seeding processes. In this study, we use ion density and velocity data measured by the Communications/Navigation Outage Forecasting System satellite during May 2008 to July 2014 to derive the longitude‐month distributions of the occurrence probability of ESF irregularities at different amplitudes. It is found that the occurrence probability of large‐amplitude irregularities is well correlated with the postsunset vertical plasma drift and that small‐amplitude irregularities do not show a clear pattern at low solar activity but is anticorrelated with that of large‐amplitude irregularities at moderate solar activity. That is, the months and longitudes with high occurrence probability of large‐amplitude irregularities are exactly those with low occurrence probability of small‐amplitude irregularities, and vice versa. ESF irregularities are mostly limited below 500 km during low solar activity in 2008–2010, but large‐amplitude irregularities can reach 700–800 km in altitude during moderate solar activity in 2011–2014. The generation of large‐amplitude ESF irregularities is controlled by the postsunset vertical plasma drift at both low and moderate solar activities. In contrast, small‐amplitude ESF irregularities at moderate solar activity occur more frequently in the areas where the postsunset vertical plasma drift is small.

Features of 3–7-day planetary-wave-type oscillations in F-layer vertical drift and equatorial spread F observed over two low-latitude stations in China

Abstract. Recent studies on the equatorial atmosphere–ionosphere coupling system have shown that planetary-wave-type oscillations, as an important seeding mechanism for equatorial spread F (ESF), play an important role in ESF irregularity development and its day-to-day variability in the equatorial latitudes. In this study, ionosonde virtual height and ESF measurements over Sanya (18.4° N, 109.6° E; 12.8° N dip latitude) and meteor radar neutral-wind measurements over Fuke (19.5° N, 109.1° E; 14° N dip latitude) during 2013 are used to investigate the features of planetary-wave-type oscillations in both the lower atmosphere and the ionosphere and their possible influences on ESF occurrence under the weak solar maximum year. The ∼ 3-day and ∼ 7-day planetary-wave-type oscillations have been observed in the neutral zonal winds and the time rate of change in F-layer virtual heights. According to the propagation characteristics, the 3-day and 7-day planetary-wave-type oscillations are basically recognized as ultrafast and fast Kelvin waves, respectively. With increasing heights, the 3-day wave oscillations are gradually amplified, while the 7-day wave oscillations are generally constant. By performing a cross-wavelet transform on the onsets of ESF and the vertical drifts of the F layer, we found that there are simultaneously occurring 7-day and 3-day common wave oscillations between them. The 7-day waves are mainly in the inversion phase, while the 3-day waves are mostly in an in-phase state, indicating that the 7-day waves may play a main role in ESF initiation. Approximate delays of 6 days for the 7-day waves and 5 days for the 3-day waves in their propagation upward from the lower atmosphere to the ionosphere are evaluated with wavelet power spectrum analysis. The estimated upward velocities from these time delays provide consistent evidence that the 7-day and 3-day waves propagate vertically upward with typical Kelvin wave characteristics. The results highlight the role of planetary-wave-type oscillations in the initiation and development of ESF in the Chinese low-latitude region.

Gravity wave control on ESF day‐to‐day variability: An empirical approach

AbstractThe gravity wave control on the daily variation in nighttime ionization irregularity occurrence is studied using ionosonde data for the period 2002–2007 at magnetic equatorial location Trivandrum. Recent studies during low solar activity period have revealed that the seed perturbations should have the threshold amplitude required to trigger equatorial spread F (ESF), at a particular altitude and that this threshold amplitude undergoes seasonal and solar cycle changes. In the present study, the altitude variation of the threshold seed perturbations is examined for autumnal equinox of different years. Thereafter, a unique empirical model, incorporating the electrodynamical effects and the gravity wave modulation, is developed. Using the model the threshold curve for autumnal equinox season of any year may be delineated if the solar flux index (F10.7) is known. The empirical model is validated using the data for high, moderate, and low solar epochs in 2001, 2004, and 1995, respectively. This model has the potential to be developed further, to forecast ESF incidence, if the base height of ionosphere is in the altitude region where electrodynamics controls the occurrence of ESF. ESF irregularities are harmful for communication and navigation systems, and therefore, research is ongoing globally to predict them. In this context, this study is crucial for evolving a methodology to predict communication as well as navigation outages.

Structuring of intermediate scale equatorial spread F irregularities during intense geomagnetic storm of solar cycle 24

AbstractHere we examine the structuring of equatorial plasma bubble (EPB) during intense geomagnetic storm of solar cycle (SC) 24 that occurred on 17 March 2015 using spaced receiver scintillation observations on a 251 MHz radio signal, recorded by a network of stations in Indian region. As yet, this is the strongest geomagnetic storm (Dstmin∼−223nT) that occurred in present SC. Present study reveals that the structuring of equatorial spread F (ESF) irregularities was significantly different on 17 March as compared to quiet days of corresponding month. ESF irregularities of intermediate scale (100 m to few kilometers) are observed at unusually higher altitudes (≥ 800 km) covering wider longitudinal‐latitudinal belt over Indian region. A presence of large‐scale irregularity structures with stronger ΔN at raised F peak with small‐scale irregularities at even higher altitudes is observed. It caused strong focusing effect (S4>1) that prevails throughout premidnight hours at dip equatorial station Tirunelveli. Other observational aspect is that zonal irregularity drifts over low‐latitude station Kolhapur exhibited a large deviation of ∼230 m/s from their average quiet time pattern. During this geomagnetic storm, two southward turnings of significant strength (BZ≤−15 nT) occurred at 11.4 IST (Indian standard time) and 17.9 IST. The later southward turning of interplanetary magnetic field (IMF)BZ resulted in a large eastward prompt penetration electric field (PPEF) close to sunset hours in Indian longitude. Estimates of PPEF obtained from real‐time ionospheric model are too low to explain the observed large upliftment of F region in the post sunset hours. Possible reason for observed enhanced PPEF‐linked effects is discussed.

Coherent and incoherent scatter radar study of the climatology and day‐to‐day variability of mean F region vertical drifts and equatorial spread F

AbstractWe conducted a comprehensive analysis of the vertical drifts and equatorial spread F (ESF) measurements made by the Jicamarca incoherent scatter radar (ISR) between 1994 and 2013. The ISR measurements allowed us to construct not only updated climatological curves of quiet‐time vertical plasma drifts but also time‐versus‐height maps of ESF occurrence over the past two solar cycles. These curves and maps allowed us to better relate the observed ESF occurrence patterns to features in the vertical drift curves than previously possible. We identified an excessively high occurrence of post‐midnight F region irregularities during December solstice and low solar flux conditions. More importantly, we also found a high occurrence of ESF events during sudden stratospheric warming (SSW) events. We also proposed and evaluated metrics of evening enhancement of the vertical drifts and ESF occurrence, which allowed us to quantify the relationship between evening drifts and ESF development. Based on a day‐to‐day analysis of these metrics, we offer estimates of the minimum pre‐reversal enhancement (PRE) peak (and mean PRE) values observed prior to ESF development for different solar flux and seasonal conditions. We also found that ESF irregularities can reach the altitudes at least as high as 800 km at the magnetic equator even during low solar flux conditions.

Data‐driven numerical simulations of equatorial spread F in the Peruvian sector 3: Solstice

AbstractWe present results from a continuing effort to simulate equatorial spread F (ESF) using observations from the Jicamarca Radio Observatory near Lima, Peru. Jicamarca measures vertical and zonal plasma drifts along with plasma number density profiles overhead. The number density profiles are used to initialize a three‐dimensional regional model of the ionosphere capable of simulating plasma density irregularities produced during ESF conditions. The vertical drifts measurements are used to drive the numerical simulation continuously. Neutral winds are derived from the new Horizontal Wind Model '14 (HWM‐14) model, and the zonal winds are scaled so as to make the zonal plasma flows at the start of the simulation agree with the ISR profile measurements. Coherent scatter radar imagery from Jicamarca is used to validate the simulation results. Campaign data were collected in April and December 2014, and a few events representative of low and high ESF activity were selected for analysis. The numerical simulations are able to reproduce the level of activity observed along with the gross features of the ESF irregularities and radar plumes. Data from a network of HF beacons are being incorporated into the forecast analysis in order to elucidate radar plumes which sometimes appear even when the simulation fails to predict them.

Radar and satellite investigations of equatorial evening vertical drifts and spread <i>F</i>

Abstract. We analyzed pre-midnight equatorial F region observations made by the 30 MHz coherent backscatter radar of São Luis, Brazil between August 2010 and February 2012. These measurements were processed, and used to create monthly maps of the echo occurrence as a function of local time and height. The maps show the inter-annual variability associated with equatorial spread F (ESF) occurrence in the Brazilian longitude sector. We also constructed monthly curves of the evening vertical drifts, for the Brazilian sector, using measurements by the ion velocity meter (IVM) onboard the C/NOFS satellite. The IVM evening drifts show a good overall agreement with the Scherliess and Fejer (1999) empirical model. Measured and model drifts show the development of the pre-reversal enhancement (PRE) of the vertical plasma drifts during ESF season. Using joint radar and satellite measurements, we found that evening (18:00–18:30 LT) mean non-negative drifts provide a necessary but not sufficient condition for the occurrence of topside ESF echoes. Evening downward (negative) drifts preceded the absence of topside ESF irregularities.

A statistical study of satellite traces and evolution of equatorial spread F

The ionosonde observations made at 5-min intervals at the Indian dip equatorial station Tirunelveli (8.7°N, 77.8°E geographic; 1.1°N dip latitude) from March 2008 to February 2009 during the extended solar minimum period are used to study the interlink between equatorial spread F (ESF) and satellite traces (STs) which are assumed to represent tilts in the bottomside iso-electron density surfaces probably caused by large-scale wave-like structures (LSWS). The data show different patterns of ESF onset in the bottomside F region, which are illustrated through examples. In addition, the statistics of occurrence of ST and its relation to the formation of ESF are studied. The results indicate that (1) the zonally drifting ESF irregularities can be differentiated from those forming over the observing station. (2) Nearly half of the ESF events were preceded by ST. (3) In about 30% of the cases of occurrence of ST, ESF was not formed afterwards implying that LSWS may not always lead to ESF. (4) The percentage of ESF following ST was high in summer and increased with the time of the night. (5) Following the first occurrence of ST, the ESF onset was delayed by about 30 min on the average suggesting that ST may be used as a precursor of ESF. (6) Pre-reversal enhancement (PRE) of upward plasma drift was found insignificant during the period of study. The trapping of high-frequency radio waves between the E and F regions during intense sporadic E is also illustrated.

Implications of the equipotential field line approximation for equatorial spread F analysis

Three different approaches to the evaluation of the electrostatic potential in the ionosphere under equatorial spread F(ESF) conditions are considered. First, we calculate the potential using an analytical approach, applying force balance laws to a simplified ionosphere. Second, we compute the potential around a cylinder‐like plasma depletion in an idealized ionosphere using both the equipotential field line (EFL) approach and the full 3‐D solution to the electrostatic potential problem. Our third approach involves an initial boundary value simulation in a realistic ionosphere using both EFL and 3‐D potential solutions. The results show that the equipotential field line assumption does not fully capture the 3‐D structure of the ionospheric current system and leads to an underestimation of the growth rate of ESF irregularities in numerical simulations.

Simultaneous observations of ESF irregularities over Indian region using radar and GPS

Abstract. In this paper, we present simultaneous observations of temporal and spatial variability of total electron content (TEC) and GPS amplitude scintillations on L1 frequency (1.575 GHz) during the time of equatorial spread F (ESF) while the MST radar (53 MHz) located at Gadanki (13.5° N, 79.2° E, Dip latitude 6.3° N), a low latitude station, made simultaneous observations. In particular, the latitudinal and longitudinal extent of TEC and L-band scintillations was studied in the Indian region for different types of ESF structures observed using the MST radar during the low solar activity period of 2004 and 2005. Simultaneous radar and GPS observations during severe ESF events in the pre-midnight hour reveal that significant GPS L band scintillations, depletions in TEC, and the double derivative of the TEC index (DROTI), which is a measure of fluctuations in TEC, obtained at low latitudes coincide with the appearance of radar echoes at Gadanki. As expected, when the irregularities reach higher altitudes as seen in the radar map during pre-midnight periods, strong scintillations on an L-band signal are observed at higher latitudes. Conversely, when radar echoes are confined to only lower altitudes, weak scintillations are found and their latitudinal extent is small. During magnetically quiet periods, we have recorded plume type radar echoes during a post-midnight period that is devoid of L-band scintillations. Using spectral slopes and cross-correlation index of the VHF scintillation observations, we suggest that these irregularities could be "dead" or "fossil" bubbles which are just drifting in from west. This scenario is consistent with the observations where suppression of pre-reversal enhancement (PRE) in the eastward electric field is indicated by ionosonde observations of the height of equatorial F layer and also occurrence of low spectral width in the radar observations relative to pre-midnight period. However, absence of L-band scintillations during post-midnight event, when radar observed plume like structures and scintillations were recorded on VHF band, raises questions about the process of evolution of the irregularities. A possible explanation is that whereas small scale (~3 m) irregularities are generated through secondary waves that grow on the walls of km scale size irregularities, in this case evolution of the Rayleigh-Taylor instability itself did not extend to irregularities of scale sizes of a few hundred meters that produce scintillation on a L-band signal.