Total Electron Content Depletions Research Articles

Morphological and spectral characteristics of L-band and VHF scintillations and their impact on trans-ionospheric communications

Abstract Amplitude scintillations recorded at 1.5 GHz frequency during the high (1998–1999) and low (2004–2005) sunspot activity periods over a low latitude station, Waltair (17.7°N, 83.3°E) revealed that the L-band scintillations mostly occur during the post-sunset to midnight hours peaking around 21:00 hr local time with maximum occurrence during equinoxes, moderate during winter and minimum during the summer months. The occurrence, as well as the intensity of scintillations, is found to be strongly dependant on both the season of the year and the sunspot number. Strong (S4-index >0.45) and fast fading scintillations (fading rates >40 fads/min) observed during the post-sunset hours of equinoxes and winter months manifest as several short duration patches at both VHF (244 MHz) and L-band (1.5 GHz) frequencies and are found to be always associated with the range or total Spread-F on ionograms and bubbles/depletions in the Total Electron Content (TEC) measured from a colocated dual frequency GPS receiver, suggesting that these scintillations are of the Plasma Bubble Induced (PBI) type. On the other hand, relatively weak and slow fading scintillations (fading rates <8 fads/min) observed around the post-midnight hours of the summer months which appear as long-duration patches (>3 hr) at 244 MHz signal (with practically no scintillation activity at the L-band frequencies) are often found to be associated with frequency Spread-F on ionograms with no depletions in TEC. Further, the presence of Fresnel oscillations observed in the spectrum of 244 MHz suggests that the long-duration scintillations observed are due to the presence of a thin layer of irregularities in the bottom side F-region which are generally known as Bottom Side Sinusoidal (BSS) irregularities. Further, the PBI-type scintillations at L-band frequencies are often found to exceed 10 dB power levels (S4 > 0.45) even during the low sunspot activity period of 2004–2005, and cause Loss of Lock in the GPS receivers resulting in a total interruption in the received signals.

Equatorial bubbles as observed with GPS measurements over Pune, India

Ionospheric total electron content (TEC) and scintillations have been recorded continuously since April 2003 using a dual‐frequency GPS receiver at Pune, India (geographic latitude 19.1°N, longitude 74.05°E; 24°N dip), situated in between the magnetic equator and the northern crest of the equatorial anomaly. The TEC often shows bite‐outs when severe amplitude scintillations are observed on the GPS L1 carrier level. The apparent duration of the bite‐outs may be different from the true east‐west duration, as observed with geostationary links, because of the presence of a relative velocity between the irregularity cloud and the satellite. The trajectory of a GPS satellite plays an important role in observing the bubble characteristics. The distributions of amplitude and duration of the bubbles have been obtained during the equinoctial months February through April of 2004. The median values are found to be 9 TEC units (1 TECU = 1016 el/m2) and 3.3 min, respectively. The range error at GPS L1 frequency corresponding to the median TEC depletion is 1.4 m, while that corresponding to the 95th percentile value is 4.5 m. An asymmetry in the east‐west walls of the bubble and sharp edges of the depletions resulting in high range error rates ∼30 cm/min has been noted.

Observations in equatorial anomaly region of total electron content enhancements and depletions

Abstract. A GSV 4004A GPS receiver has been operational near the crest of the equatorial anomaly at Udaipur, India for some time now. The receiver provides the line-of-sight total electron content (TEC), the phase and amplitude scintillation index, σφ and S4, respectively. This paper presents the first results on the nighttime TEC depletions associated with the equatorial spread F in the Indian zone. The TEC depletions are found to be very well correlated with the increased S4 index. A new feature of low-latitude TEC is also reported, concerning the observation of isolated and localized TEC enhancements in the nighttime low-latitude ionosphere. The TEC enhancements are not correlated with the S4 index. The TEC enhancements have also been observed along with the TEC depletions. The TEC enhancements have been interpreted as the manifestation of the plasma density enhancements reported by Le et al. (2003). Keywords. Ionosphere (Equatorial ionosphere; Ionospheric irregularities)

Night−day imprints of ionospheric slab thickness during geomagnetic storm

Spatial maps of the ionosphere–plasmasphere slab thickness ( τ) were generated as a ratio of the total electron content (TEC) to the F-region peak electron density (NmF2) at 1° spaced grid points from the instantaneous maps of TEC and foF2 at latitudes 35° to 70°N, and longitudes −10° to 40°E. Data of 23 observatories are used for the construction of TEC and foF2 maps with Kriging technique from independent networks of GPS–TEC and ionosonde observations at solar minimum (1995–1996) and maximum (2002) under quiet and disturbed magnetic conditions. The net-weight factor ( w) is introduced as a ratio of disturbance to quietness representing area mean TEC, foF2, and τ for a particular day and time normalized by relevant monthly median value. Analysis of w evolution for TEC, foF2 and τ maps have revealed that TEC and foF2 depletion is accompanied by positive increment of slab thickness for more than 48 hrs during the magnetic storm at solar maximum but τ enhancement is shorter and delayed by 12 to 24 hrs regarding the storm onset at solar minimum. The slab thickness positive increment at the main phase of geomagnetic storm has been associated with relevant increase of the real thickness of the topside ionosphere. To estimate the upper boundary of the ionosphere the International Reference Ionosphere expanded towards the plasmasphere (IRI*) is modified to assimilate the ionosonde F2 layer peak and the GPS–TEC observations. Slab thickness is decomposed in three parts (the bottomside and topside ionosphere, and the plasmasphere). Eliminating the plasmasphere part from the total slab thickness, we obtain the ratio of bottomside slab thickness to the real thickness below the F2 layer peak. Assuming that this ratio is also valid above the F2 layer peak, we obtain the topside boundary of the ionosphere varying from 500 km by day to 2300 km by night.

Latitudinal extension of low-latitude scintillations measured with a network of GPS receivers

Abstract. A latitudinal-distributed network of GPS receivers has been operating within Colombia, Peru and Chile with sufficient latitudinal span to measure the absolute total electron content (TEC) at both crests of the equatorial anomaly. The network also provides the latitudinal extension of GPS scintillations and TEC depletions. The GPS-based information has been supplemented with density profiles collected with the Jicamarca digisonde and JULIA power maps to investigate the background conditions of the nighttime ionosphere that prevail during the formation and the persistence of plasma depletions. This paper presents case-study events in which the latitudinal extension of GPS scintillations, the maximum latitude of TEC depletion detections, and the altitude extension of radar plumes are correlated with the location and extension of the equatorial anomaly. Then it shows the combined statistics of GPS scintillations, TEC depletions, TEC latitudinal profiles, and bottomside density profiles collected between September 2001 and June 2002. It is demonstrated that multiple sights of TEC depletions from different stations can be used to estimate the drift of the background plasma, the tilt of the plasma plumes, and in some cases even the approximate time and location of the depletion onset. This study corroborates the fact that TEC depletions and radar plumes coincide with intense levels of GPS scintillations. Bottomside radar traces do not seem to be associated with GPS scintillations. It is demonstrated that scintillations/depletions can occur when the TEC latitude profiles are symmetric, asymmetric or highly asymmetric; this is during the absence of one crest. Comparison of the location of the northern crest of the equatorial anomaly and the maximum latitude of scintillations reveals that for 90% of the days, scintillations are confined within the boundaries of the 50% decay limit of the anomaly crests. The crests of the anomaly are the regions where the most intense GPS scintillations and the deepest TEC depletions are encountered. In accord with early results, we observe that GPS scintillations/TEC depletions mainly occur when the altitude of the magnetic equator F-region is above 500km. Nevertheless, in many instances GPS scintillations and TEC depletions are observed to exist when the F-layer is well below 500km or to persist when the F-layer undergoes its typical nighttime descent. Close inspection of the TEC profiles during scintillations/depletions events that occur when the equatorial F-layer peak is below 500km altitude reveals that on these occasions the ratio of the crest-to-equator TEC is above 2, and the crests are displaced 10° or more from the magnetic equator. When the equatorial F-layer is above 500km, neither of the two requirements is needed, as the flux tube seems to be inherently unstable. We discuss these findings in terms of the Rayleigh-Taylor instability (RTI) mechanism for flux-tube integrated quantities. We advance the idea that the seeming control that the reverse fountain effect exerts on inhibiting or suppressing GPS scintillations may be related to the redistribution of the density and plasma transport from the crests of the anomaly toward the equatorial region and then to much lower altitudes, and the simultaneous decrease of the F-region altitude. These two effects originate a decrease in the crest/trough ratio and a reduction of the crests separation, making the whole flux tube more stable to the RTI. The correspondence between crest separation, altitude of the equatorial F-region, the onset of depletions, and the altitude (latitude) extension of plumes (GPS scintillations) can be used to track the fate of the density structures.

The Weddell sea anomaly observed with the Topex satellite data

This paper introduces the complete image of the Weddell Sea Anomaly, observed with the over-the-ocean ionospheric total electron content (TEC) values obtained from the TOPEX satellite data with an almost unlimited coverage over the oceans, the first time according to the literature; and investigates its development. With a series of TOPEX TEC maps, this paper demonstrates the diurnal variations of both the night-time and the day-time Weddell Sea Anomaly, which appeared as a night-time TEC enhancement and as a day-time TEC depletion, during the near sunspot maximum period of 1998 and 1999 investigated. Several TOPEX passes, plotted in geomagnetic latitudes, are also presented to demonstrate the longitudinal variations of the Weddell Sea Anomaly, and also to show other ionospheric features appearing such as the southern-hemisphere mid-latitude day-time and night-time trough, the northern-hemisphere mid-latitude night-time trough and the equatorial anomaly. This paper demonstrates how large the anomaly is in reality situated west of the Faraday ionosonde station over the Bellinghausen Sea and not over the Weddell Sea that is east of Faraday. Thus the correct name should be Bellinghausen Sea Anomaly. Based upon the review paper of Dudeney and Piggott (1978), the development of the Weddell Sea Anomaly is explained with the combined effects of solar ultraviolet radiation and thermospheric neutral winds.

Irregularities of the topside ionosphere observed by GPS/MET radio occultation

Large‐scale fluctuations (vertical scales ∼40–60 km) of total electron content (TEC) are retrieved from phase path fluctuations of GPS radio occultation links of the GPS/MET experiment. The large‐scale TEC fluctuations at observation heights 400–600 km are maximal at local times from 20:00 to 24:00 h. The fluctuations mainly occur on the summer hemisphere at low magnetic latitudes 0–30°N and seem to be concentrated over Africa and Arabia. The derived global distribution of TEC fluctuations around solar minimum (June/July 1995) is assumed to be closely related to the occurrence of spread F and radio scintillations. The meridional distribution of the background electron density (averaged for 19:00–23:00 LT) has a maximum in the topside ionosphere at around 10°N, beyond an anomaly structure of the F2 peak layer. The meridional location of the anomaly structure agrees with the location of enhanced TEC fluctuations. Both phenomena are possibly related to the enhancement of upward ion velocity in the postsunset ionosphere at equatorial latitudes. Large TEC depletions of 1–20 TEC‐units are present at observation heights 200–400 km in many TEC profiles of the equatorial ionosphere after sunset. The TEC depletions possibly indicate plasma bubbles on their way from the lower ionosphere to the topside ionosphere. GPS navigation signal loss due to spread F or high‐latitude irregularities is not found in the GPS/MET data of June/July 1995.

The Characteristic Variations of the Eastward Drift Velocities of the Equatorial Plasma Bubbles in East Asia during High Solar Activity Period.

From January 1988 to December 1990, ionospheric total electron content depletions associated with amplitude scintillations observed simultaneously at two equatorial anomaly stations (Kaohsiung and Chihpen) have been used to investigate the eastward drift velocities of the equatorial plasma bubbles in East Asian region. It is found that there are considerable large variability of eastward drift velocities. The nocturnal variation pattern shows a tendency for the eastward drift velocities to decrease from early evening to presunrise hours. Seasonal variations show large eastward drift velocities in summer than in equinox. During midnight and postmidnight periods, eastward drift velocities decrease slightly with the increasing of magnetic activity.

Drift motion of ionospheric bubbles at the equatorial anomaly crest region

By simultaneous observations of ionospheric total electron content depletions associated with amplitude scintillations at three spaced locations on Taiwan Island, the poleward and eastward zonal drift motions of the total electron content depletions have been investigated. Using the data observed during January 1986 to December 1988, it has been found that the most probable poleward drift velocity ranges from 26 m/s to 71 m/s with a mean velocity of 45 m/s, and the most probable eastward zonal drift velocity ranges from 64 m/s to 130 m/s with a mean velocity of 97 m/s. Both poleward and zonal drift velocities decrease from early evening to presunrise hours.

Equatorial ionospheric irregularities produced by the Brazilian Ionospheric Modification Experiment (BIME)

On two separate evenings in September 1982, rockets were launched into the bottomside equatorial F2 region off the coast of Natal, Brazil, to inject chemicals, consisting of mainly H2O and CO2, to create a hole in ionization. The chemicals were injected near the height where the density gradient was steepest, and at a time when the F2 region was rising rapidly, to see whether plasma bubble irregularities could be generated from instabilities triggered by the ionization hole. On both occasions, hole‐induced depletions in total electron content (TEC) of more than 1016 el/m² were observed over horizontal distances of at least 60 km from the chemical injection point. The eastward drifts of these artificial depletions were observed by the time difference in the TEC features observed at various TEC monitoring stations, and from the changing range of oblique ionosonde echoes observed by an ionosonde located 300 km magnetically east of the chemical release point. Their subsequent evolution into plasma bubble irregularities was demonstrated from the observations of spread F echoes, strong‐amplitude scintillation, and TEC depletion at distances of from 300 to 500 km eastward of the release points. The fact that similar behavior of the ionosphere was observed during the evenings of both rocket chemical releases, and on no other nights of the campaign, is strong evidence of successful artificial generation of bubble irregularities by chemical injection into the bottomside F2 region.

Deep depletions of total electron content associated with severe mid‐latitude gigahertz scintillations during geomagnetic storms

Using 136‐MHz Faraday rotation data obtained at three closely spaced stations, we present evidence that severe nighttime gigahertz scintillations, which appear rarely at mid‐latitudes around Japan only during geomagnetic storm conditions, are closely associated with deep depletions of total electron content (TEC). The TEC depletions amount to 2–8×1016 el/m² (10–30% of the background TEC), and their durations range from 10 min to 1 hour. These depletions move northeastward or eastward with velocities between 60 and 260 m/s. The depletions are probably not counterparts of the equatorial bubbles but seem to be formed in localized regions around Japan under complicated and peculiar ionospheric conditions. There is an indication that the oscillation of the F region caused by large‐scale TID's propagating from north to south (∼600 m/s) may initiate the generation of the depletion.

Ionospheric electron content depletion associated with amplitude scintillation at the equatorial anomaly crest region

The diurnal and seasonal behaviors of the ionospheric total electron content (TEC) depletion associated with VHF amplitude scintillation observed at Lunping, Taiwan (25.00°N, 121.17°E geographic), during the period March 1977 to February 1980 were presented. Diurnally, the maximum occurrence of TEC depletion appears during the late evening hours for all seasons except winter. Seasonally, the maximum occurrence of TEC depletion is not fixed at a certain season but varies with sunspot number because of the different sunspot number dependence of the occurrence of TEC depletion in different seasons. The most probable maximum TEC depletion ranges from 0.8 to 3.6×1016 el/m², and the duration ranges from 10 to 33 min. No correlation was found between the maximum TEC depletion and the duration or between the maximum TEC depletion and the associated ionospheric scintillation index. The association between TEC depletion and range spread F is much better than that between TEC depletion and frequency spread F.

The electron content and its variations at Natal, Brazil

The Faraday rotation and amplitude data for the period September 1978 through August 1979, recorded from the geostationary satellite SMS 1 (90°W) at Natal (5.9°S, 35.2°W), have been utilized to study the behavior of total electron content (TEC). Apart from diurnal and seasonal behavior of TEC, two characteristic features of the nighttime ionosphere are discussed: (1) a postsunset enhancement in TEC which lasts for several hours following the rapid sunset decrease and (2) the sharp, isolated changes in TEC. In particular, relating to phenomenon 2, the depletions in TEC are usually accompanied by a simultaneous increase in fading rate, scintillation index, and amplitude level, while the enhancements are accompanied by a simultaneous decrease in fading rate, scintillation index, and amplitude level. The statistics of their occurrence and nature are described. The average behavior of the signal amplitude after propagating through either a depleted or an enhanced ionosphere is modeled theoretically and shown to be in agreement with the observed experimental behavior.

Optical signature of an ionospheric hole

The recent launch of a NOAA weather satellite provided an “experiment of opportunity” for simultaneous radio and optical diagnostics of a large artificially‐induced ionospheric modification. The rocket plume caused disturbances that were monitored using intensified imaging and photometer observations at 6300 Å along the same raypath as VHF polarimeter measurements of the ionosphere’s total electron content (TEC). A rapid chemical depletion of TEC by −16.8 × 1012 el/cm² caused a burst of 6300 Å radiation that expanded to more than 60° of the sky, with peak intensities near 9 k R. The event was used as an active space plasma experiment to test theoretical estimates for atmospheric diffusion and O(¹D) quenching rates.

On the spatial relationship of 1‐meter equatorial spread‐F irregularities and depletions in total electron content

An experiment was conducted at Kwajalein AtollMarshall Islands to investigate the spatial relationship of 1‐m equatorial spread‐F irregularities to total electron content (TEC) depletions. A high‐power radar was operated (1) in a backscatter scan mode to spatially map the distribution of 1‐m irregularitiesand (2) in a dual‐frequencysatellite‐track mode to obtain the longitudinal TEC variations. We show that radar backscatter “plumes” found in the disturbednighttime equatorial ionosphere are longitudinally coincident with TEC depletions. We suggest that the TEC depletions are probably due to the presence of plasma “bubbles” in the equatorial F layer.

Latitude dependence of ionosphere total electron content: Observations during sudden commencement storms

The latitude dependence of the changes in the ionosphere total electron content (TEC) has been studied during 12 magnetic storms. TEC observations were obtained at Hamilton, Massachusetts, and Arecibo, Puerto Rico. Definite latitude differences are observed in the TEC responses during the magnetic storms: both TEC enhancements and TEC depletions are observed at Hamilton, while only enhancements are measured at Arecibo. A pre-local midnight TEC ‘ledge’ is frequently observed in the Arecibo storm data but is seldom observed in the higher-latitude Hamilton data. These enhanced ledges in the Arecibo TEC, together with the nighttime TEC depletions measured at Hamilton, produce large south-to-north differences in the ionosphere TEC. The ledges at Arecibo may arise from electrodynamic drift effects associated with westward electric fields; the role of the motion term in the continuity equation for the TEC is assessed under these conditions. The local evening enhancements in the TEC during certain large storms are discussed in the context of the latitude dependence of the storm time magnetic field variations measured from Great Whale River to San Juan. It is shown that the large TEC enhancements appear to be associated with large positive geomagnetic bays at middle to high latitudes.

DISCUSSION

Global Positioning System (GPS) is a remote sensing tool of space weather and ionospheric variations. However, the interplanetary space-dependent drifts in the ionospheric irregularities cause predominant ranging errors in the GPS signals. The dynamic variability of the low-latitude ionosphere is an imperative threat to the satellite-based radio communication and navigation ranging systems. The study of temporal and spatial variations in the ionosphere has triggered new investigations in modelling, nowcasting and forecasting the ionospheric variations. Hence, in this paper, the dynamism in the day-to-day, month-to-month and seasonal variability of the ionospheric Total Electron Content (TEC) has been explored during the solar maximum period, January-December 2013, of the 24th solar cycle. The spatial and temporal variations of the ionosphere are analysed using the TEC values derived from three Indian low-latitude GPS stations, namely, Bengaluru, Guntur and Hyderabad, separated by 13–18° in latitude and 77–81° in longitude. The observed regional GPS-TEC variations are compared with the predicted TEC values of the International Reference Ionosphere (IRI-2012 and 2007) models. Ionospheric parameters such as Vertical TEC (VTEC), relative TEC deviation index and monthly variations in the grand-mean of ionosphere TEC and TEC intensity, along with the upper and lower quartiles, are adopted to investigate the ionosphere TEC variability during quiet and disturbed days. The maximum ionospheric TEC variability is found during March and September equinoxes, followed by December solstice while the minimum variability is observed during June solstice. IRI models are in reasonable agreement with GPS TEC but are overestimating during dawn hours (01:00–06:00 LT) as compared to the dusk hours. Higher percentage deviations are observed during equinoctial months than summer over EIA stations, Guntur and Hyderabad. GPS TEC variations are overestimated during dawn hours for all the seasons over Bengaluru. It has also been observed that positive storm effect (enhancement of TEC) is observed during the main phase of the March storm, 2013 (March 16–18, 2013) while both positive and negative storm effects (depletion of TEC) are registered during the main phase of the June storm, 2013 (June 28–30, 2013) at Bengaluru and Guntur, respectively. IRI-2012 model has slightly large discrepancies with the GPS-VTEC compared with the IRI-2007 model during the June storm, 2013 over Guntur station. This analysis highlights the importance of upgrading the IRI models due to their discrepancies during quiet and disturbed states of the ionosphere and developing an early warning forecast system to alert about ionosphere variability.