Satellite Traces Research Articles

Occurrence of Postmidnight Bottom‐Type Scattering Layer Over Low Latitude

AbstractThe bottom‐type scattering layer (BSL) that occurs at postsunset or presunset has been reported in previous studies. In this work, two cases of postmidnight BSL occurring on geomagnetic quiet days (12 and 29 April 2015) were investigated by using the observational data obtained by beam 4 and 5 of Sanya VHF radar. These BSLs initiate at 00:06–00:18 LT within an altitude range of 220–240 km. Their thickness are relatively uniform, about 10 km. The altitude and Doppler velocities of the BSLs oscillate with time and the oscillation period of Doppler velocities is about 1 hr. During the presence of postmidnight BSL, spread echoes and satellite traces (STs) appeared in the ionograms recorded by Sanya ionosonde. The oscillation of Doppler velocities and the presence of STs suggest that the postmidnight BSL may be related to the F region bottomside large‐scale wave structure. Amplitude scintillations (S4) larger than 0.2 were also recorded by Sanya GPS scintillation receiver. These observations could imply an important role of gravity waves in the generation of the postmidnight BSL.

Multi‐Instrument Observations of the Ionospheric Response to the 26 December 2019 Solar Eclipse Over Indian and Southeast Asian Longitudes

AbstractHere, the impact of the 26 December 2019 solar eclipse on the equatorial and low latitude ionosphere has been investigated using ground and space‐based observations over Indian and Southeast Asian longitudes. The high‐resolution Ionosonde observations at Tirunelveli, GPS Total Electron Content (TEC) observations from a chain of GPS receivers along and across the eclipse path, TIMED‐SABER, and Ionospheric Connection Explorer (ICON) satellites were utilized to investigate the eclipse‐induced variations in electron density and thermospheric cooling. We noticed a tremendous increase and decrease in the base height of the F‐layer, resembling the nighttime Pre‐Reversal Enhancement (PRE). Near the eclipse maximum, a strong blanketing sporadic E layer was observed at Tirunelveli with a top frequency of ∼18 MHz for 1 hr and 26 min. Satellite traces (STs) and “U” shaped ionograms were noticed for the first time over Tirunelveli during eclipse maximum and end phases. The “STs” and “U” shaped traces indicate the presence of short‐period gravity waves or TID type of wave perturbations over the Indian region. A maximum of ∼5–7 TECU (30%–40%) decrease in TEC is observed on the eclipse day for iisc, hyde, and tiru stations. Periodogram analyses of TEC data showed the presence of wavelike structures with periodicities of 18–24 min for different stations. Simultaneous observations from the ICON satellite showed an increase and decrease in hmF2 and NmF2 which matches well with the ionosonde observations from Tirunelveli. The temperature profiles from TIMED‐SABER and ICON satellites showed a reduction and enhancement in the lower and upper E regions respectively.

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.

Investigation of Satellite Trace (ST) and Multi-reflected Echo (MRE) ionogram signatures and its possible correlation to nighttime spread F development from Cyprus over the solar mini-max (2009–2016)

Multi-reflected echoes (MREs) and satellite traces (STs) are referred in literature as ionogram signatures of Travelling Ionospheric Disturbances (TIDs) which is a phenomenon that apparently drives spread F development mainly at nighttime mid-latitude ionosphere. A long-term statistical study has been undertaken to investigate the morphological aspect of these signatures over the lower midlatitude European station of Nicosia, Cyprus (35.19°N, 33.38°E geographic; magnetic dip. 29.38°N) by inspecting all ionograms recorded by the DPS-4D digisonde in the interval 2009–2016. The results underline the systematic manifestation of these TID signatures over Cyprus with a possible (although not quite clear) solar activity dependence and a distinctive seasonal and diurnal occurrence rate with a seasonal peak of STs during summer and of MREs during January to April. Based on the experimental results of the present study, the seasonal occurrence rate of MREs and STs is found to increase by 75% and 56% during high solar activity periods. Satellite traces are well known ionogram signatures of TIDs and mostly correlated to the nighttime spread F formation. The occurrence of mid-latitude spread Fs over European longitude sector normally increases during summer. The occurrences of TIDs are also prominent at this interval of the year over nighttime mid-latitude ionosphere. The presence of MREs as an ionogram signature of TIDs over mid-latitude ionosphere is unique in nature.

Long-term aspects of nighttime spread F over a low mid-latitude European station

In an effort to explore the morphology of nighttime spread F at the lower mid-latitude European station of Nicosia, Cyprus (35°N, 33°E geographic; magnetic dip. 29.38°N), all ionograms recorded by the DPS-4D digisonde during the interval 2009–2016 have been analyzed. Subsequent detailed investigation was performed to establish the possible effect of various triggering mechanisms on spread F within the framework of Perkins instability on a statistical basis by correlating spread F occurrence particular wave pattern signatures (Satellite Trace/Multiple-reflected Echoes), gravity wave signatures, F layer uplift (h'F) and unstable sporadic E layers. The results verify the systematic manifestation and therefore the significance of TIDs and unstable sporadic E layers as triggering factors responsible for seeding spread F development and underline the frequent appearance of multi-reflected echoes and satellite traces as dominant precursors of mid-latitude spread F over European latitudes for the first time. Furthermore clear seasonal characteristics and inverse solar activity dependence of spread F occurrence in lower European mid-latitudes is established similar to southern hemisphere studies undertaken previously.

Interpretation of deformed ionograms induced by vertical ground motion of seismic Rayleigh waves and infrasound in the thermosphere

Abstract. The vertical ground motion of seismic surface waves launches acoustic waves into the atmosphere and induces ionospheric disturbances. Disturbances due to Rayleigh waves near the short-period Airy phase appear as wavy fluctuations in the virtual height of an ionogram and have a multiple-cusp signature (MCS) when the fluctuation amplitude is increased. An extremely developed MCS was observed at Kazan, Russia, after the 2010 M 8.8 Chile earthquake. The ionogram exhibited steep satellite traces for which the virtual heights increased rapidly with frequency starting near the top of cusps and continuing for 0.1–0.2 MHz. This complicated ionogram was analyzed by applying a ray tracing technique to the radio wave propagation in the ionosphere that was perturbed by acoustic waves. Acoustic wavefronts were inclined by the effects of finite Rayleigh wave velocity and sound speed in the thermosphere. The satellite echo traces were reproduced by oblique returns from the inclined wavefronts, in addition to the nearly vertical returns that are responsible for the main trace.

Satellite traces ancient riverbeds buried in the Sahara

Satellite traces ancient riverbeds buried in the Sahara

Statistical characteristics of ionogram spread-F and satellite traces over a Chinese low-latitude station Sanya

Ionosonde ionogram measurements over Sanya (18°N, 109°E; 13°N dip latitude), China during 2012–2013 are used to investigate the occurrence characteristics of spread-F (SF) and satellite traces (STs), and the possible correlation between them under the weak solar maximum of solar cycle 24. The SF and STs were manually identified from ionograms. The results show that the diurnal pattern of SF peaks at post-sunset during equinox, and at post-midnight during summer months, respectively. By classifying the SF into range spread-F (RSF) and frequency spread-F (FSF), it is found that the SF during equinox are mostly RSF associated with the equatorial F-region irregularities and can be explained by the generalized Rayleigh–Taylor instability. A statistics on the RSF and STs shows that not all RSF events were preceded by STs, and not all STs led to RSF development. The monthly mean value of the sunset ionospheric F2 layer peak height (hmF2) on RSF days is apparently higher than that on non RSF days. This result provides statistically consistent evidence that both the sunset rapid rising of the F-layer and the presence of F-region bottomside density perturbations (as indicated by STs) are important factors for the equinoctial RSF onset and development. However in summer months, the occurrences of RSF and FSF are comparable. Most FSF initiated at midnight. And there was no close relationship between the summer time FSF, F-layer height increase and STs. We suggest that the midnight FSF during summer months observed over Sanya might not be resulting from the decaying of post-sunset RSF initiated at equatorial latitude, but due to the quite localized generation of F-region 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.

Equatorial spread F initiation and growth from satellite traces as revealed from conjugate point observations in Brazil

AbstractA better understanding of the precursor conditions for the instability growth is very important for identifying the causes of day‐to‐day variability in the equatorial spread F (ESF)/plasma bubble irregularity development. We investigate here the satellite trace (S‐trace) in the ionograms, a precursor to the postsunset ESF occurrence, as observed by Digisondes operated at an equatorial and two magnetic conjugate sites in Brazil during a 66 day observational campaign (Conjugate Point Equatorial Experiment 2002). The satellite traces first occur at the equatorial site, and sequentially, after a variable delay of approximately 20 to 50 min, they are observed nearly simultaneously over the two conjugate sites. The evening prereversal enhancement in the zonal electric field/vertical drift is found to control its development. Using a three‐dimensional simulation code based on collisional interchange instability mechanism, it is shown that the observed S‐trace occurrence sequence is fully consistent with the instability initiation over the equator with the field‐aligned plasma depletion vertical growth marked by latitudinal expansion of its extremities to conjugate locations. The delay in the S‐trace occurrence at the conjugate sites (a measure of the nonlinear growth of the instability for plasma depletion) is controlled also by field line parallel (meridional) neutral wind. The relationship between the S‐trace and the large‐scale wave structure in the F layer, another widely known characterization of the precursor condition for the ESF development, is also clarified.

Longitudinal characteristics of spread F backscatter plumes observed with the EAR and Sanya VHF radar in Southeast Asia

AbstractThe development of equatorial plasma irregularity plumes can be well recorded by steerable backscatter radars operated at and off the magnetic equator due to the fact that the vertically extended plume structures are tracers of magnetically north‐south aligned larger scale structures. From observations during March 2012, using two low latitude steerable backscatter radars in Southeast Asia, the Equatorial Atmosphere Radar (EAR) (0.2°S, 100.3°E; dip lat 10.4°S) and the Sanya VHF radar (18.4°N, 109.6°E; dip lat 12.8°N), the characteristics of backscatter plumes over the two sites separated in longitude by ~1000 km were simultaneously investigated. The beam steering measurements reveal frequent occurrences of multiple plumes over both radar sites, of which two cases are analyzed here. The observations on 30 March 2012 show plume structures initiated within the radar scanned area, followed by others drifting from the west of the radar beam over both stations. A tracing analysis on the onset locations of plasma plumes reveals spatially well‐separated backscatter plumes, with a maximum east‐west wavelength of about 1000 km, periodically generated in longitudes between 85°E and 110°E. The postsunset backscatter plumes seen by the Sanya VHF radar are found to be due to the passage of sunset plumes initiated around the longitude of EAR. Most interestingly, the EAR measurements on the night of 21 March 2012 show multiple plume structures that developed successively in the radar scanned area with east‐west separation of ~50 km, with however no sunset plasma plume over Sanya. Colocated ionogram measurements show that spread F irregularities occurred mainly in the bottomside F region at Sanya, whereas satellite traces in ionograms that are indications of large‐scale wave structures were observed on that night at both stations. Possible causes for the longitudinal difference in the characteristics of radar backscatter plumes are discussed.

Ionogram-based range-time displays for observing relationships between ionosonde satellite traces, spread F and drifting optical plasma depletions

A form of range-time plots derived from ionograms taken from a standard digital ionosondes, situated at the low latitude sites of Vanimo, Port Moresby and Darwin, exhibit bursts of spread F at the center of descending and ascending off-angle reflectors. This particular type of event has since been identified with the passage of optically imaged ionospheric plasma depletions (bubbles) over a Darwin ionosonde. This paper describes the process for producing this form of range-time display and its relationship to ionospheric height, satellite traces and range spread F as seen on individual ionograms. First hop satellite traces are proposed to be via direct reflection from the steep electron density gradients at the base of bubbles while second hop satellite traces then involve a single additional ground reflection. Measurements of night equatorial drift velocity were made from the range-time displays and found to be in the range 20–220m/s peaking at approximately 90–100m/s in good agreement with values derived from drift measurements made by a variety of other types of equipment.

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.

On the importance of wave‐like structures in the occurrence of equatorial plasma bubbles: A case study

Coordinated observations of equatorial plasma bubbles (EPBs) have been made with an all‐sky airglow imager, narrow bandwidth photometer, VHF radar, and ionosonde over the Indian sector on the night of 23 March 2009. The prereversal enhancement (PRE) in the vertical plasma drift during the postsunset hours on this day was moderate. Range type spread F was found to occur immediately after the satellite traces were noted in the ionograms. This was well recorded in measurements made by all‐sky imager, narrow band photometer, and VHF radar. The airglow emission intensities also revealed the presence of a large‐scale wave‐like structure (LSWS) together with the plasma bubbles that coincided with plume structures observed in the VHF radar echoes. The periodicity of the occurrence of bubbles (and plumes) and their interdepletion distances suggest the presence of small‐scale wave‐like structures (SSWS) on this night. The results are compared with the ionosonde observations made on the night of 21 February 2008. The PRE and the maximum height attained by the F layer were very similar to that of 23 March 2009. In addition, the ionograms showed the presence of satellite traces. However, no subsequent evolution of spread F was noticed. Considering the satellite traces to have their origin in LSWS, these observations imply that though the presence of LSWS is important for the triggering of EPBs, they alone are not sufficient. However, the coexistence of both LSWS and SSWS may have the potential to trigger EPBs.

Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging

[1] Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions.

Satellite traces, range spread-F occurrence, and gravity wave propagation at the southern anomaly crest

Abstract. Range spread-F (RSF) and occurrence of "satellite" traces prior to RSF onset were studied at the southern peak of the ionospheric equatorial anomaly (EA). Ionograms recorded in September 2007 at the new ionospheric station of Tucumán, Argentina (26.9° S, 294.6° E, dip latitude 15.5° S), by the Advanced Ionospheric Sounder (AIS) developed at the Istituto Nazionale di Geofisica e Vulcanologia (INGV), were considered. Satellite traces (STs) are confirmed to be a necessary precursor to the appearance of an RSF trace on the ionograms. Moreover, an analysis of isoheight contours of electron density seems to suggest a relationship between RSF occurrence and gravity wave (GW) propagation.

Multi‐reflected echoes: Another ionogram signature of large‐scale wave structure

One or more steeply sloped traces have been found in evening ionograms taken from the Kwajalein Atoll (4.3°N dip latitude) during July 1979. Their resemblance to the normal F trace suggests that they are echoes that have undergone a large number of reflections from the F layer. These multi‐reflected echoes (MREs) are interpreted in terms of focusing produced by curved isodensity contours in the bottomside F layer, which appear to be associated with large‐scale wave structure (LSWS) that develops in the bottomside F layer. MREs appear to be another signature for LSWS, together with satellite traces that appear later in time, closer to the onset of plasma structure referred to as equatorial spread F. MREs are interesting because they display, for the data set examined, a strong preference to occur during the post‐sunset rise of the F layer, which includes E‐region sunset. How this finding affects our understanding of LSWS is discussed.

Equatorial ionospheric disturbance observed through a transequatorial HF propagation experiment

Abstract. A transequatorial radio-wave propagation experiment at shortwave frequencies (HF-TEP) was done between Shepparton, Australia, and Oarai, Japan, using the radio broadcasting signals of Radio Australia. The receiving facility at Oarai was capable of direction finding based on the MUSIC (Multiple Signal Classification) algorithm. The results were plotted in azimuth-time diagrams (AT plots). During the daytime, the propagation path was close to the great circle connecting Shepparton and Oarai, thus forming a single line in the AT plots. After sunset, off-great-circle paths, or satellite traces in the AT plot, often appeared abruptly to the west and gradually returned to the great circle direction. However, there were very few signals across the great circle to the east. The off-great-circle propagation was very similar to that previously reported and was attributed to reflection by an ionospheric structure near the equator. From the rate of change in the direction, we estimated the drift velocity of the structure to range mostly from 100 to 300 m/s eastward. Multiple instances of off-great-circle propagation with a quasi-periodicity were often observed and their spatial distance in the east-west direction was within the range of large-scale traveling ionospheric disturbances (LS-TIDs). Off-great-circle propagation events were frequently observed in the equinox seasons. Because there were many morphological similarities, the events were attributed to the onset of equatorial plasma bubbles.

Mid-latitude range spread and travelling ionospheric disturbances

This paper first discusses some early results (most hitherto unpublished) on off-vertical reflections which result from tilted isoionic contours associated with the passage of travelling ionospheric disturbances (TIDs). Azimuth of arrival and zenith angle information on these returning signals is discussed together with the role of these signals in producing both resolved and unresolved range spread on ionograms. Some ray tracing through a model ionosphere which incorporates wavelike structures is shown to predict fixed-frequency patterns (on recordings of virtual height vs. time) of converging and diverging satellite traces similar to those observed in practice. New experimental evidence is then presented to suggest (from the N( h) analyses performed) that TID wavetrains of several cycles are effective in producing spread traces on ionograms by specular reflections from the tilted surfaces of each cycle of these wavetrains. Ionograms from a modern ionosonde show that range spread consists primarily of discrete satellite traces even though at times these traces overlap and vary in intensity as a function of frequency thus creating a diffuse range spread ionogram.