Large-scale Wave Structure Research Articles

Onset Conditions and Features of Equatorial F Region Irregularities: New Insight From Collocated Digisonde and Radar Observations From Gadanki

AbstractIn this paper, we study the onset conditions and features of equatorial F region irregularities linked with equatorial plasma bubbles (EPBs) using observations made simultaneously by using a DPS‐4D digisonde and the Gadanki Ionospheric Radar Interferometer (GIRI), both collocated at Gadanki. Importantly, we have employed specific analysis tools on DPS‐4D observations, providing range‐time displays of radio frequency and signal‐to‐noise ratio (SNR) of the reflected/backscattered echoes and the angle‐of‐arrival of the echoes, to characterize the echoes and to study the ambient and disturbed states of the ionosphere. Observations clearly show noticeably different background conditions, in terms of the height of the F layer base, electron density gradient and vertical drift, for the freshly generated and drifting EPBs. The zonal wavelengths of the pre‐sunset large‐scale wave structures (LSWS) observed using the DPS‐4D, however, are found to show a close connection with EPB spacings for freshly generated and drifting EPBs, consistent with earlier findings. The satellite traces were observed just prior to the equatorial spread F (ESF) and were found to be associated with the bottomside upwellings. Comparison of the range‐time displays of radio frequency and SNR of the DPS‐4D observations and the height‐time variations of SNR of the GIRI observations demonstrates that the relatively weak echoes in the DPS‐4D observations, which represent the ESF echoes, correlate fairly well temporally with the plume structures observed by the GIRI. The GIRI observations also reproduce the bottomside upwellings observed by the DPS‐4D. We propose that the bottomside upwellings are due to the growth of the LSWS and these bottomside upwellings are instrumental for the growth of the Rayleigh‐Taylor instability generating EPBs. Finally, the new aspects of the digisonde observations are discussed with regard to their usefulness in understanding and forecasting EPBs/ESF.

Observation Impact Study of an Arctic Cyclone Associated with a Tropopause Polar Vortex (TPV)-Induced Rossby Wave Initiation Event

AbstractA case study characterized by Arctic cyclogenesis following a tropopause polar vortex (TPV)-induced Rossby wave initiation event is used to better understand how well existing observations constrain analyses of processes influencing Arctic cyclone predictive skill. Complementary techniques of observation system experiments (OSE) and ensemble sensitivity analysis (ESA) are used to investigate the impacts of existing observation networks on predictions for this case. The ESA reveals that the large-scale Rossby wave structure is correlated with both Arctic cyclone track and amplitude errors. The ensemble analyses of midlevel moisture in the warm conveyor belt region were correlated with forecast cyclone amplitude, but this feature was poorly sampled in existing observations. There is also a sensitivity of Arctic cyclone forecast amplitude error to low-level temperature in the air mass of the cyclogenesis region at analysis time and a sensitivity of Arctic cyclone forecast track error to low-level temperature in the region of an Arctic cold front and a coastal front at the analysis time. The OSEs for this case reveal that Arctic cyclone track error is more sensitive to denial of existing observations than amplitude error. While lower-level (below 700 hPa) observations had the greatest impact on the surface cyclone during the early stages, upper-level (above 500 hPa) observations had the dominant impact during its later evolution. Denying temperature from just three well-placed sondes substantially increased track error by degrading analyses of the TPV amplitude and its interaction with the waveguide and developing Rossby wave packet. These results are encouraging for further Arctic cyclone forecast improvements through addition of even a small number of well-placed observations.

On the Assessment of Day‐To‐Day Occurrence of Equatorial Plasma Bubble

AbstractIn this paper, we address the day‐to‐day variation in the development of equatorial plasma bubble (EPB) using simultaneous observations made by the 30 MHz Gadanki Ionospheric Radar Interferometer (GIRI) and ionosonde (DPS‐4D) from Gadanki and C/NOFS. We show that wave‐like variations with horizontal wavelengths of 200–660 km observed prior to sunset have a close connection with EPB spacings. We also show that the locations of EPB development at their origins, which varied on a day‐to‐day basis, were as large as 2600 km from Gadanki. A detailed analysis clearly reveals the cause of such variability and provides clue as to where the background ionospheric conditions led to the growth of the Rayleigh Taylor instability (RTI) generating EPB and where they failed. Results clearly show that while ionosonde observations show great potential for understanding day‐to‐day variation and predicting EPB development overhead (within ±1° longitude), they are inadequate to assess the growth potential of EPB at longitudes away from a longitude zone of about ±1° from overhead. A detailed analysis suggests that the day‐to‐day variability in EPB development is governed by large scale wave structures (LSWS) and it is inferred that the LSWS troughs (low electron density) are the sites for the EPB development. Results further suggest that LSWS with horizontal wavelength of 200–660 km acts as seed for the growth of the RTI resulting in EPBs with same spacing. While the sources of LSWS remain to be identified, it is suggested that observations with longitudinally distributed ground‐based sensor, viz., ionosonde, preferably separated by 250–300 km, would be an immediate step forward in this effort.

Multi-instrument study of longitudinal wave structures for plasma bubble seeding in the equatorial ionosphere

Large Scale Wave Structures (LSWS) in the equatorial ionospheric F-region were observed by measuring spatial and temporal variations within detrended total electron content (dTEC) data obtained by ground-based GNSS receivers over the South American continent. By using dTEC-maps, we have been able to produce, for the first-time, two-dimensional representations of LSWS. During the period from September to December, the LSWS frequently occurred starting a few hours prior to Equatorial Plasma Bubble (EPB) development. From 17 events of LSWS observed in 2014 and 2015, wave characteristics were obtained: the observed wavelengths, periods, and the phase speeds are respectively, ~900 km, ~41 min and ~399 m/s; the waves propagated from the northeast to southeast. In some cases the front of the oscillation was meridionally aligned, extending to more than 1600 km, the first time such large extension of the wavefront has been reported. From F-layer bottom height oscillation data, measured by ionosonde, LSWS exhibit two different vertical phase propagation modes, in-phase and downward phase. The former mode indicates the presence of a polarization electric field in the F-layer bottom side; the latter suggests propagation of atmospheric gravity waves. The presence of LSWS near the solar terminator, followed by the development of EPBs, suggests that the upwelling of the F-layer bottom height produces a condition favorable to the development of Rayleigh–Taylor instability.

Observations of TEC Depletion, Periodic Structure of TEC, ROTI and Scintillation Associated with ESF Irregularities over South China

The observations of Global Positioning System (GPS) scintillation, Total Electron Content (TEC) depletion, the periodic structure of TEC and Rate of TEC Index (ROTI) over south China were presented. Data were collected from GPS observations at stations of Shenzhen and Guangzhou from 2011 to 2012. This study reported that the ratio of simultaneous occurrences of TEC depletions with strong scintillations was higher than that of TEC depletions with weak scintillations in vernal and autumnal equinoxes of 2011 over South China. The number of the periodic structures of TEC with depletion contained was greater than that with no depletion contained corresponding to strong scintillations. The structure of the slab of plasma irregularities could be responsible for the simultaneous occurrences of TEC depletion with strong scintillations and ROTI. Before and during the occurrences of strong scintillation, there was Large-Scale Wave Structure (LSWS) which provided the seed ionization perturbation to trigger ESF irregularities and contributed to the periodic structure of TEC.

Investigation of Spatiotemporal Morphology of Plasma Bubbles Based on EAR Observations

AbstractSpatiotemporal characteristics of equatorial plasma bubble (EPB) have been studied based on Equatorial Atmosphere Radar (EAR) fan sector observations from Kototabang. EPBs were characterized as either locally generated (whose genesis and subsequent evolution was observed in the fan sector maps) or drifted type (which drifted into fan sector scans from west). Investigation of the radar echo spectral width indicated that the drifted EPB could be less evolving type. Height of the F layer in the postsunset period over Kototabang and Chumphon were examined in the two types. Average peak h′F was higher in locally generated EPB cases than in drifted EPB cases (by ~25 km), which is also found to be statistically significant. This scenario has been visualized in terms of a standing large‐scale wave structure (LSWS) whose phase determine whether EPB will be seen evolving over EAR or drifted from a western longitude. Furthermore, low‐latitude sporadic E (Es) variabilities in the two cases were also examined. Es indicated an inverse relation with the peak h′F (which depends on crest/trough of LSWS). These results have been discussed in the light of earlier works on spatial morphology of EPB clusters, particularly in context to the possible generation mechanism of LSWS by the spatial inhomogeneity in the low‐latitude E region conductivity. Lesser occurrence of low‐latitude Es in locally generated EPB cases also signifies the vital role of E region conductivity on Rayleigh‐Taylor instability. These results highlight a need for networked spatial observation of low‐latitude Es.

Relationship between day-to-day variability of equatorial plasma bubble activity from GPS scintillation and atmospheric properties from Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) assimilation

The relationship between day-to-day variability of equatorial plasma bubbles (EPBs) and the neutral atmosphere is studied. This study is based on the previous study in which the GPS scintillation index and the tropospheric cloud-top temperature are used as proxies for EPB activity and atmospheric perturbations, respectively, and a correlation was found between their day-to-day variations. In this paper, we maintained the same GPS scintillation data but substituted the atmospheric data via an assimilation run of the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). Cross-correlation between the EPB activity and the atmospheric temperature is similar to the results in Ogawa et al. (Earth Planets Space 61:397–410, 2009). The new findings from our study include (1) an enhanced correlation between the EPB activity and the neutral atmosphere is found in horizontally and vertically large areas, (2) the longitudinal disturbance of atmospheric temperature and wind velocity during the EPB-active days is enhanced, and (3) the enhancement of atmospheric disturbance during the EPB-active days shows a similarity to the characteristics of large-scale wave structures in the ionosphere. These results more clearly support couplings between EPBs and the neutral atmosphere.

Multi-instrumented observations of the equatorial F-region during June solstice: large-scale wave structures and spread-F

Typical equatorial spread-F events are often said to occur during post-sunset, equinox conditions in most longitude sectors. Recent studies, however, have found an unexpected high occurrence of ionospheric F-region irregularities during June solstice, when conditions are believed to be unfavorable for the development of plasma instabilities responsible for equatorial spread-F (ESF). This study reports new results of a multi-instrumented investigation with the objective to better specify the occurrence of these atypical June solstice ESF in the American sector and better understand the conditions prior to their development. We present the first observations of June solstice ESF events over the Jicamarca Radio Observatory (11.95° S, 76.87° W, ∼ 1° dip latitude) made by a 14-panel version of the Advanced Modular Incoherent Scatter Radar system (AMISR-14). The observations were made between July 11 and August 4, 2016, under low solar flux conditions and in conjunction with dual-frequency GPS, airglow, and digisonde measurements. We found echoes occurring in the pre-, post-, and both pre- and post-midnight sectors. While at least some of these June solstice ESF events could have been attributed to disturbed electric fields, a few events also occurred during geomagnetically quiet conditions. The late appearance (22:00 LT or later) of three of the observed events, during clear-sky nights, provided a unique opportunity to investigate the equatorial bottomside F-region conditions, prior to ESF, using nighttime airglow measurements. We found that the airglow measurements (630 nm) made by a collocated all-sky camera show the occurrence of ionospheric bottomside F-region perturbations prior to the detection of ESF echoes in all three nights. The airglow fluctuations appear as early as 1 hour prior to radar echoes, grow in amplitude, and then coincide with ESF structures observed by AMISR-14 and GPS TEC measurements. They also show some of the features of the so-called large-scale wave structures (LSWS) that have been detected, previously, using other types of observations and have been suggested to be precursors of ESF. The bottomside fluctuations have zonal spacings between 300 and 500 km, are aligned with the magnetic meridian, and extend at least a few degrees in magnetic latitude.

Nonzonal structure of the response of the global field of the Earth’s atmospheric temperature to solar activity

The work describes the results of calculations obtained with the Atmospheric Research Model (ARM) general circulation model. The temperature response of the troposphere and middle atmosphere to variations in UV solar radiation were found to have a large-scale wave structure when planetary waves at the lower model boundary were taken into account. In the present paper, the results from the processing of global temperature fields with three databases (ERA-20C, NOAA-CIRES 20th Century Reanalysis, v2, and NCEP/NCAR Reanalysis I) are provided. Analysis of the differences of the mean monthly temperature global fields (January and July) between the maxima and minima of three solar activity cycles (21, 22, and 23 cycles) also demonstrated their nonzonal structure. It was shown that the amplitude of this difference in January in the stratosphere (10 hPa) can be 7–29 K in the Northern Hemisphere. In July, this effect is prominent in Southern Hemisphere. In the troposphere (500 hPa), a nonzonal temperature effect is present in both the Northern and Southern Hemispheres; the amplitude of the effects amounts to approximately 5–12 K. In conclusion, we discuss that the mechanism of solar energy impact on atmospheric temperature discovered by numerical modeling is supported after reanalysis data processing.

Off‐great‐circle paths in transequatorial propagation: 1. Discrete and diffuse types

AbstractThere is mounting evidence that plasma structure in nighttime equatorial F layer evolves from large‐scale wave structure (LSWS) in the bottomside F layer. This process cannot be ignored because equatorial plasma bubbles (EPBs) arise from large‐amplitude LSWS; and, because intense radiowave scintillations are associated with EPBs, understanding the LSWS‐to‐EPB process is a crucial step toward reliable Space Weather Forecasting. In this regard, the transequatorial propagation (TEP) experiment appears to be the most useful among available research instruments. After a lapse of 30 years, the TEP experiment has been resurrected; a goal of this research is to understand TEP measurements well enough so that they can be used to diagnose the LSWS‐to‐EPB process. Toward this end, new results are presented in two companion papers. Herein (P1), off‐great‐circle (OGC) propagation paths are shown to consist of two types, discrete and diffuse. The new findings include the following: (1) a generalized multireflection model that can explain most of the observed properties; (2) the discrete type is supported by multireflections from an unstructured upwelling, (3) the diffuse type is supported by reflections from plasma structure in EPBs; and (4) the observed east‐west (EW) asymmetry can be explained in terms of a distorted upwelling or plasma structure along the west wall of an upwelling. In Paper 2 (P2), a second form of observed EW asymmetry is explained in terms of plasma structure, which is not aligned with the geomagnetic field. The findings strongly confirm a close relationship between upwellings, ESF patches, and OGC paths.

Off‐great‐circle paths in transequatorial propagation: 2. Nonmagnetic‐field‐aligned reflections

AbstractThere is considerable evidence that plasma structure in nighttime equatorial F layer develops from large‐scale wave structure (LSWS) in bottomside F layer. However, crucial details of how this process proceeds, from LSWS to equatorial plasma bubbles (EPBs), remain to be sorted out. A major obstacle to success is the paucity of measurements that provide a space‐time description of the bottomside F layer over a broad geographical region. The transequatorial propagation (TEP) experiment is one of few methods that can do so. New findings using a TEP experiment, between Shepparton (SHP), Australia, and Oarai (ORI), Japan, are presented in two companion papers. In Paper 1 (P1), (1) off‐great‐circle (OGC) paths are described in terms of discrete and diffuse types, (2) descriptions of OGC paths are generalized from a single‐reflection to a multiple‐reflection process, and (3) discrete type is shown to be associated with an unstructured but distorted upwelling, whereas the diffuse type is shown to be associated with EPBs. In Paper 2 (P2), attention is placed on differences in east‐west (EW) asymmetry, found between OGC paths from the SHP‐ORI experiment and those from another near‐identical TEP experiment. Differences are reconciled by allowing three distinct sources for the EW asymmetries: (1) reflection properties within an upwelling (see P1), (2) OGC paths that depend on magnetic declination of geomagnetic field (B), and (3) OGC paths supported by non‐B‐aligned reflectors at latitudes where inclination of B is finite.

LSWS linked with the low‐latitude Es and its implications for the growth of the R‐T instability

AbstractA comprehensive investigation of spread F irregularities over the Indian sector has been carried out using VHF radar and ionosonde observations. Two different categories of spread F observations, one where the onset of the range spread F (RSF) was concurrent with the peak h′F (category 1) and another where the RSF onset happened ~90 min after the peak h′F time (category 2), are presented. RSF in category 2 was preceded by the presence of oblique echoes in ionograms, indicating the irregularity genesis westward of Sriharikota. The average peak h′F in category 1 was ~30 km higher than that in category 2 indicating the presence of standing large‐scale wave structure (LSWS). Occurrence of the blanketing Es during 19:30 to 20:30 Indian Standard Time in category 1 (category 2) was 0% (>50%). Model computation is also carried out to further substantiate the observational results. Model computation indicates that zonal variation of low‐latitude Es can generate zonal modulation in the F layer height rise. It is found that the modulation of the F layer height, linked with the low‐latitude Es, assists the equatorial spread F onset by modifying both the growth rate of the collisional Rayleigh‐Taylor (R‐T) instability and also its efficiency. A predominant presence of low‐latitude Es has been observed, but the increase in the F layer height and the R‐T instability growth in the evening hours will maximize with complete absence of low‐latitude Es. A new mechanism for the generation of LSWS and its implications on R‐T instability is discussed.

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

Investigation onFlayer height rise and equatorial spreadFonset time: Signature of standing large-scale wave

Equatorial spread F observations have been categorized into three categories based on ionograms recorded over Sriharikota. First category comprised cases where the onset of equatorial spread F (ESF) was concurrent with the peak h′F time. Second and third categories comprised cases where the onset of ESF happened with a delay of 30 min and more than 30 min, respectively, with reference to the peak h′F time. Average peak h′F in the first category was more than 35 km higher than that in the second and third categories. Also, the peak vertical (upward) plasma drift was higher in the first category. Assuming the genesis of F region irregularity to have happened at or before the time of F layer attaining the peak height, late onset of ESF indicates the genesis of irregularities to have happened westward of Sriharikota. The fact that the peak h′F values were remarkably different in the three categories indicates a zonal variation of eastward electric field and postsunset height rise of F layer. The relative magnitude of the F layer height rise in the three different categories over Sriharikota has also been found to be significantly different than that over Thumba, an equatorial (magnetic) station located ~360 km westward of Sriharikota longitude. This scenario points toward the existence of a large-scale zonal standing wave in the F layer and its important role in F region instability process. Results presented in the manuscript have been discussed in the light of current understanding on the large-scale wave structure.

Nonlinear growth, bifurcation, and pinching of equatorial plasma bubble simulated by three‐dimensional high‐resolution bubble model

AbstractA new three‐dimensional high‐resolution numerical model to study equatorial plasma bubble (EPB) has been developed. The High‐Resolution Bubble (HIRB) model is developed in a magnetic dipole coordinate system for the equatorial and low‐latitude ionosphere with a spatial resolution of as fine as 1 km. Adopting a higher‐order numerical scheme than those used in the existing models, the HIRB model is capable of reproducing the bifurcation, pinching, and turbulent structures of EPB. From a seeding perturbation resembling large‐scale wave structure (LSWS), EPB grows nonlinearly from the crest of LSWS upwelling, bifurcates at the top of EPB, then becomes turbulent at the topside of the F region. One of the bifurcated EPB is pinched off from the primary EPB and stops growing after pinching. The narrow channel of EPB tends to have a wiggle due to the secondary instability along the wall of EPB. Because of the fringe field effect above and below the EPB, upward drifting low‐density plasma converges toward the F peak altitude, forming a narrow‐depleted channel, and diverges above the peak, forming a flattened top of the EPB. The flattened top which has a steep upward density gradient is so unstable that bifurcation can easily occur even from a very small thermal perturbation. A higher density region between the bifurcated EPB moves downward due to westward polarization electric field. The EPB is pinched off when it reaches the wall of the primary EPB. It is concluded that turbulent plume‐like irregularities can be spontaneously generated only from large‐scale perturbation at the bottomside F region.

Characteristics of large‐scale wave structure observed from African and Southeast Asian longitudinal sectors

AbstractThe spatial large‐scale wave structure (LSWS) at the base of F layer is the earliest manifestation of seed perturbation for Rayleigh‐Taylor instability, hence, found to play a deterministic role in the development of Equatorial Plasma Bubbles (EPBs). Except for a few case studies, a comprehensive investigation has not been conducted on the characteristics of LSWS because of the complexity involved in detecting the LSWS, particularly, in spatial domain. In this scenario, a comprehensive study is carried out, for the first time, on the spatial and temporal characteristics of LSWS observed in spatial domain over African and Southeast Asian longitudinal sectors during the year 2011. The observations indicate that these wave structures can be detected a few degrees west of E region sunset terminator and found to grow significantly at longitudes past the sunset terminator. The phase fronts of these spatial structures are found to align with the geomagnetic field () lines over a latitudinal belt for at least 5−6° (~500–600 km) centered on dip equator. The zonal wavelengths of these structures are found to vary from 100 to 700 km, which is consistent with the earlier reports, and the EPBs were consistently observed when the amplitudes of LSWS were grown to sufficient strengths. These results would provide better insights on the underlying physical processes involved in excitation of LSWS in terms of important roles being played by E region electrical loading and polarization electric fields induced via spatially varying dynamo current due to neutral wind perturbations associated with atmospheric gravity waves.

On validating the relationship of ionogram signatures to large-scale wave structure

The development of plasma structure in the nighttime equatorial F layer, known as equatorial spread F (ESF), appears to be influenced (if not controlled) by the presence of large-scale wave structure (LSWS). To understand this process, knowledge of the properties of LSWS is crucial. A major obstacle, in this regard, is the virtual lack of LSWS data; most instruments in use today are unable to detect spatial structure in the absence of horizontal drift. There appears to be some information about LSWS contained in two ionogram signatures, multi-reflected echoes (MREs) and satellite traces (STs). How useful they can be depends, however, on knowledge of their relationship to LSWS. Results from a case study are presented herein, which show that MREs are likely produced by a tilted, or shallowly modulated, reflecting surface in the bottomside F layer, whereas STs are produced by more steeply sloped surfaces. The accumulated results are shown to lead to a working hypothesis for LSWS development followed by ESF. Differences between LSWS and a traveling ionospheric disturbance are also pointed out.

A simple model to relate ionogram signatures to large‐scale wave structure

The development of plasma structure in the nighttime equatorial F region, known as equatorial spread F(ESF), appears to be controlled by the preceding presence of large‐scale wave structure (LSWS). To understand this process, knowledge of the properties of LSWS is crucial. Information about LSWS appears to reside in two ionogram signatures, multi‐reflected echoes (MREs) and the so‐called “satellite” traces (STs). However, how LSWS is related to MREs and STs is not yet clear. To gain insight, a tilted, linear reflector, modulated by LSWS, is described and shown to be capable of explaining even the most puzzling forms of MREs and STs. With this kind of model, ionogram signatures can be used to infer the nature of LSWS.

Precursor signatures and evolution of post‐sunset equatorial spread‐F observed over Sanya

Recent case studies on the precursor signatures of equatorial spread‐F (ESF) have shown a one‐to‐one correspondence between the large‐scale wave structures (LSWS) and ESF development at equatorial latitude. In this study, the LSWS and the onset and development of the ESF are investigated over Sanya (18°N, 109°E), a station located at 13° north of the magnetic equator, during both geomagnetic quiet and disturbed conditions in September–October 2011. High‐time‐resolution ionograms from Digisonde Portable Sounder (DPS‐4D) provided the satellite trace measurements that were used to indicate the occurrence of the LSWS. The development of local ESF activity was identified using GPS scintillation and VHF coherent radar echo measurements from the same site, together with the range type spread‐F (RSF) in ionograms. Additionally, the Sanya VHF radar five‐beam scanning measurements in east–west direction were used to characterize the longitudinal difference in establishing the initial conditions for ESF development. Correlative studies between the LSWS and ESF activities during the observational period offer consolidated evidence that the LSWS is a necessary precursor for the ESF development. It is shown that the LSWS and ESF have nearly a one‐to‐one relationship when the F layer undergoes an abrupt post‐sunset rise (PSSR), revealing that the magnitude of the pre‐reversal enhancement in zonal electric field (PRE) that elevates the F layer to a high enough altitude is an important parameter controlling the generation of post‐sunset ESF. However, in the absence of the PSSR, the ESF and GPS scintillation did not always occur following the appearance of LSWS. Sometimes the LSWS events preceded the generation of bottom type spread‐F (BSF) that did not develop vertically into ESF and radar plumes. This result may indicate that under inexpressive, weak, or even moderate PRE conditions, the appearance of the LSWS alone may not be sufficient to produce the post‐sunset F region irregularities responsible for ionospheric scintillations. More factors, other than the LSWS, could play crucial roles favoring the growth of ESF instabilities responsible for ionospheric scintillations.

Ionogram signatures of large‐scale wave structure and their relation to equatorial spread F

Observations of signatures of the large‐scale wave structure (LSWS) and subsequent development of equatorial spread F (ESF) by using data from a newly installed digital ionosonde at Trivandrum (8.5°N; 77°E; 0.5°N magnetic dip latitude) are presented. The LSWS signatures are observed using the ionograms with echo directions. The ‘satellite’ traces are shown as echoes from oblique directions, when an upwelling (associated with LSWS) is not directly over the ionosonde location. When an upwelling is centered overhead, and the radius of curvature of the isodensity contours matches the height of reflection, signals from various directions can add in phase to enhance the ionospheric reflection coefficient, and produce another ionogram signature, referred to as multireflected echoes (MREs). It is also shown that the generation of ESF is related to the presence of LSWS. The skymaps also show the presence of small‐scale wave structures in the bottomsideFregion. The investigation also indicates that the range spread F (RSF) occurs because of the reflections from the embedded smaller‐scale structures within the large‐scale upwelling.