Ionospheric Electron Content Research Articles

GPS detection of ionospheric perturbations following a space shuttle ascent

The exhaust plume of the Space Shuttle during its ascent is a very powerful source of energy that excites atmospheric acoustic perturbations. Because of the coupling between neutral particles and electrons at ionospheric altitudes, these low frequency acoustic perturbations induce variations of the ionospheric electron density. We computed ionospheric electron content time series using Global Positioning System data collected on Bermuda island during the STS‐58 Space Shuttle launch. The analysis of these time series shows a perturbation of the ionospheric electron content following the launch and lasting for 35 mn, with periods less than 10 mn. The perturbation is complex and shows two sub‐events separated by about 15 mn at 200 km from the source. The phase velocities and waveform characteristics of the two sub‐events lead us to interpret the first impulsive arrival as the direct propagation of the shock wave front, followed by oscillatory guided waves probably excited by the primary shock wave and propagating along horizontal atmospheric interfaces at 120 km altitude and below.

Ionospheric irregularities and storm-induced equatorial and high-latitude effects at the anomaly crest region

IEC information received through VHF RB measurements has been analyzed over the anomaly crest region of Guwahati (92° E, 26° N, 15° N geo.mag) along with a few low/low-mid latitude observations for understanding the roles of influx of plasma to anomaly crest regions, from equatorward and poleward processes during geomagnetically disturbed situations. The conditions leading to inflow of plasma to equatorial anomaly crest region or inhibition of such processes have been described in the paper through systematic analysis of disturbed day (free from sudden commencements) ionospheric electron content (IEC) variations at different temporal situtions. The storm-induced effects in relation to the development of the above conditions have also been examined for moderate and moderately severe isolated storm cases. Finally the paper deals with a few severe storms. The storm-triggered IEC features indicate inhibition or suppression of plasma dumping process at anomaly crests, through equatorial anomaly phenomenon. During winter and many equinoxial storms, the compression effect pushes this station away from the region where effective dumping of ionization from the equator is expected. Diffusion of plasma from the polar region to the crest area has also been observed through penetration of the eastward electric field during many disturbed situations. Depletion of noontime density during winter and equinoxial months and enhancement of the same during summer geomagnetically active situations are examined through anomaly compression (or inhibition) process as well as plasma replenishment through the equatorward wind.

An empirical model for IEC over Lunping

The diurnal, seasonal and solar cycle variations of ionospheric electron content (IEC) are studied. An attempt has been made to develop an empirical model for the IEC at Lunping (25°N,121.2°E) using harmonic analysis. The proposed model predicts the diurnal variation of the monthly mean values of IEC for any month for sun spot numbers up to 150. The amplitudes and phases of all harmonic components show seasonal and solar cycle dependences. The daily mean IEC and the amplitude of the first order harmonic depend seriously on the solar activity. A set of 81 coefficients of the zero and first four orders is determined and found to be sufficient for modelling the IEC. The agreement between observed and modelled IEC values is rather good; maximum deviation is limited to ±15% in winter and 10% in summer and equinox. The agreement between observed and modelled IEC values is remarkably good in May and is generally better in years of moderate solar activity.

The equator anomaly region as seen by the TOPEX/Poseidon satellite

Satellite altimeter observations of ice and sea surface heights are affected by the retardation/refraction of the altimeter signal in the ionospheric plasma. Altimeter data therefore need to be correct for the ionospheric influence using a ionospheric model like the International Reference Ionosphere. Another possibility is the use of a dual-frequency instrument like the TOPEX altimeter on the TOPEX/Poseidon satellite. By measuring the heights above the Earth surface at two frequencies the ionospheric electron content (IEC) between the ground and the satellite can be determined and thus the ionospheric correction (which is proportional to IEC) can be properly accounted for in the altimeter data analysis. Dual-frequency altimeter thus in effect provide a IEC measurement as a byproduct of the sea surface height observation. The ionospheric data base established by TOPEX/Poseidon since its launch in August 1992 provides a unique opportunity to study the distribution of IEC above the oceans with a 10-day repeat cycle, a region covered only by very few prior observations. The highly variable equator anomaly region is of particular interest since it is the region where globally the highest IEC values occur. We compare the TOPEX IEC with IEC values calculated from the IRI model for different satellite passes representing different conditions in terms of local time, latitude, longitude, and season. Our study points to the possible improvements of IRI that could be accomplished with the TOPEX ionospheric data base.

Ionospheric Electron Content over Brazilian low latitude and its comparison with the IRI and SUPIM models

Ionospheric Electron Content (IEC), measured at the Brazilian low latitude station of Cachoeira Paulista (22.5°S, 45°W, dip latitude 14°S) was compared with IEC calculated using the IRI-90 and Sheffield University Plasmaphere-Ionosphere (SUPIM) models for a large range of solar fluxes at 10.7 cm (F10.7). The analysis of the F10.7 influence on IEC showed that, for Cachoeira Paulista (CP) at 05 LT (around the diurnal minimum), there is a good agreement between the IEC measurements and the models. At 16 LT (around the diurnal maximum), substantial discrepancies were observed in the IRI-90 results while SUPIM presents a good agreement up to F10.7 ≈ 200. The measured IEC diurnal variation is better represented by SUPIM, mainly during higher F10.7. At low latitudes two critical parametric inputs for the SUPIM mathematical model are the equatorial ExB plasma drifts and thermospheric neutral winds. Therefore, updated values of these parameters should be introduced in the model calculations everytime they are available. At CP, a station located close to the southern Appleton anomaly peak due to the fountain effect, where large discrepancies between IRI-90 and observations were found mainly with high F10.7, this model should include adequately this effect, which we believe is the main reason for the discrepancies.

Seasonal variation in ionospheric electron content and irregularities over Waltair — A comparison with SLIM model

Beacon satellite measurements made during 1984–1985 and 1989–1990 at a low latitude station Waltair (17.7°N, 83.3°E) are used to study the diurnal and seasonal variations in ionospheric electron content (IEC) and amplitude scintillations. In 1989–1990, under high solar activity, the daytime IEC shows winter anomaly in the peak values and local time differences in their occurrence. By night, enhancements in IEC and the occurrence of scintillations and spread-F were frequent, particularly at equinoxes. The seasonal variations of the meridional wind direction and phase computed from the SLIM model/1/ compare well with the daytime peak density anomaly and with nighttime IEC over Waltair. A probable effect of neutral winds on the occurrence of ionospheric irregularities is discussed.

Modeling studies of equatorial plasma fountain and equatorial anomaly

The importance of diffusion, electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University Plasmasphere-Ionosphere Model (SUPIM) for the ionosphere above Jicamarca (77°W) under magnetically quiet ( Ap = 4) equinoctial conditions (day 264) at medium solar activity ( F10.7 = 145). The study also investigates the effects of the fountain, which include the equatorial anomaly. The F-region vertical E × B drift velocity measured at the equatorial station Jicamarca is used to represent the electrodynamic drift. The neutral wind is obtained from the HWM90 thermospheric wind model. As expected, the F-region electrodynamic drift generates the plasma fountain and the anomaly, which are symmetric with respect to the equator. The neutral wind makes the fountain and the anomaly asymmetric, with larger plasma flow (towards the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. The paper also addresses many important (some new) features which are related to the fountain. The features are: (1) the possibility of existence of an additional layer (called the G-layer) in the equatorial ionosphere, (2) the reverse plasma fountain, (3) the equatorial anomaly in vertical ionospheric electron content (IEC), (4) the presence (in Nmax) and absence (in IEC) of noon bite-out, (5) the occurrence of nighttime increase in ionization, and (6) plasma bubbles and spread- F.

Variations of the ionosphere and related solar fluxes during solar cycles 21 and 22

The ionospheric electron content (IEC) and peak electron density (NmF2) data collected at low- and mid-latitudes during 1980–1991 and values of solar EUV (50–1050 Å) fluxes obtained from the EUV91 solar EUV flux model are analysed to study the variations of the ionosphere during the intense solar cycles 21 and 22. The study shows that while the ionosphere responds linearly to the solar EUV fluxes its variation with the conventional solar activity index F10.7 is non-linear during both solar cycles. The behaviour of the ionosphere during the most intense solar cycle 19 has been shown to be similar to that during solar cycles 21 and 22 /1/. The ionospheric variations confirm that the relationship between the shorter (EUV and UV) and longer (10.7 cm) wavelength solar fluxes is non-linear during all solar cycles.

An overview of IRI-observational data comparison in American (Brazilian) sector low latitude ionosphere

This paper presents an overview of the existing results of comparisons between the IRI-90 predictions and the observational data for the Brazilian longitude sector and attempts to place the major findings in a global perspective. The observational data subjected to comparison with the IRI model include NmF2/foF2 (the peak density/critical frequency of the F2-layer), hmF2/ hpF2 (the height of NmF2/foF2), TEC (total ionospheric electron content) and the electron density-height profiles. The Brazilian stations for which results are presented are: Fortaleza near the magnetic equator and Cachoeira Paulista near the equatorial anomaly crest. The results are compared with those from Asian and Indian longitude sectors. An attempt is made to identify points of agreement/disagreement between the observational data and the IRI. Seasonal and solar activity dependent persistent trends of discrepancies have been found with respect the NmF2, hmF2 and TEC. We have tried to highlight the global nature of such discrepancies aiming at providing an eventual basis for incorporating needed modification for improving the predictive capability of the IRI for the equatorial and low latitude ionosphere.

Short term variabilities of ionospheric electron content (IEC) and peak electron density (NP) during solar cycles 20 and 21 for a low latitude station

Hourly values of IEC and of f 0F 2 (critical frequency) for a low latitude station, Hawaii (21.2°N, 157.7°W), during the solar maxima (1969 and 1981) and minima (1965 and 1985) years of two consecutive solar cycles, 20 and 21, are used to study the day to day variabilities of the ionospheric parameters IEC and NP. It is found that there is good correspondence in the day to day variations of IEC and NP from one solar cycle to the other for both solar maximum and minimum years in the two solar cycles. Depending on solar phase and season, while the mean daytime IEC and NP variations range from about 20% to 35%, the mean night time values vary from about 25% to 60%. The mean daytime variations in NP for the solar minimum phase are remarkably higher in all the three seasons compared to the solar maximum phase. However, no such increase is observed in the mean daytime IEC variations, indicating the highly variable nature of the daytime ionospheric F region compared to the topside during solar minimum for this low latitude station. The winter night time IEC also seems to be a relatively stable parameter during the solar minimum. The short term day to day variabilities of the day time peak values of IEC and NP (ie IEC max and NP max) are not closely associated with the variations in F 10.7 solar flux. Contrary to the common expectation, the variabilities in both the parameters, particularly in NP max, are somewhat reduced during the solar maximum (when the variability in F 10.7 solar flux is much higher compared to the solar minimum) which is more evident in the stronger 21 solar cycle. A larger number of significant components are seen in the spectra of the percentage variation of both IEC max and NP max during both solar phases of the two solar cycles compared to the corresponding F 10.7 solar flux spectra. The number of additional components for both the parameters with periods less than 15 days are more for the low solar activity years than for the solar maximum years.

Equatorial plasma fountain and its effects: Possibility of an additional layer

The importance of diffusion, perpendicular electrodynamic drift, and neutral wind on the generation and modulation of the equatorial plasma fountain of the Earth's ionosphere is studied using the Sheffield University plasmasphere‐ionosphere model for the ionosphere above Jicamarca under magnetically quiet equinoctial conditions at medium solar activity. The effects of the fountain, which include the equatorial anomaly, are also investigated. As expected, the F region electrodynamic (E×B) drift generates the plasma fountain and the anomaly, which are symmetric with respect to the magnetic equator. The neutral wind introduces asymmetries, with larger plasma flow (toward the hemisphere of stronger poleward wind) and stronger anomaly crest occurring in opposite hemispheres. During daytime, when the drift is upward, the fountain rises to about 800 km altitude at the equator and covers about ±30° magnetic latitude; outside the reach of the fountain, plasma flows toward the equator from both hemispheres. This convergence of plasma leads to the formation of an additional layer (called the G layer) within ±10° of the magnetic equator during the prenoon hours when the drift is large. At the magnetic equator, the maximum plasma concentration of the G layer can be greater than that of the F layer for a short period of time just before noon and when the drift starts to decrease. In the evening, soon after the drift turns downward, the fountain becomes a reverse fountain with supply of ionization from both hemispheres from regions outside the fountain. The reverse fountain acts as the main source for the nighttime increase in ionization at equatorial anomaly latitudes, with some contribution from the prereversal strengthening of the forward fountain. The importance of the prereversal strengthening of the forward fountain, which rises to about 1000 km altitude at the equator, and the following reverse fountain on the generation and propagation of plasma bubbles and spread F irregularities is discussed. It is also shown that the equatorial anomaly in the vertical ionospheric electron content need not be as pronounced as the interpretation of the observations suggests.

Equatorial ionospheric electric fields during magnetospheric disturbances: local time/longitude dependences from recent EITS campaigns

Data sets collected during a few coordinated Equatorial Ionosphere-Thermosphere System (EITS) observational campaign periods, mainly from the Brazilian and Asian longitude sectors are analysed in this paper. Ionosonde magnetometer and Ionospheric Electron Content (IEC) data from the EITS-1 and -2 campaigns (during March and December 1991) are complemented by interplanetary magnetic field and some ground based data sets from other campaigns. The analysis focuses on the response of the equatorial ionospheric heights and ionization anomaly to disturbance electric fields, identified as a direct penetration electric field associated with IMF B z changes and development of the ring current (especially the asymmetric component), and that produced by a disturbance zonal neutral wind. New evidence on the local time and longitudinal dependences of these electric fields constitute the main results of this paper. Especially, a large eastward electric field (associated with the asymmetric ring current) in the dusk-dawn sector causes significant expansion of the EIA in this sector, and amplification of the evening prereversal uplift of the F-layer over Brazil. Significant inhibition of the evening prereversal electric field enhancement seems to be produced by the disturbance zonal wind associated with the magnetic disturbances prevailing several hours earlier. Some tentative evidence on the Brazilian dusk sector disturbance field being larger than that of the Asian dusk sector support the existence of a longitude asymmetry in the intensity of the disturbance electric field.

GPS detection of ionospheric perturbations following the January 17, 1994, Northridge Earthquake

Sources such as atmospheric or buried explosions and shallow earthquakes producing strong vertical ground displacements are known to produce pressure waves that propagate at infrasonic speeds in the atmosphere. At ionospheric altitudes low frequency acoustic waves are coupled to ionospheric gravity waves and induce variations in the ionospheric electron density. Using Global Positioning System (GPS) data recorded by the permanent GPS network operating in southern California, we computed ionospheric electron content time series for several days preceding and following the January 17, 1994, Mw=6.7 Northridge earthquake. We observe an anomalous signal beginning several minutes after the earthquake with time delays that increase with distance from the epicenter. The signal frequency and phase velocity are consistent with results from numerical models of atmospheric‐ionospheric acoustic‐gravity waves excited by seismic sources as well as previous electromagnetic sounding results. We believe that these perturbations are caused by the ionospheric response to the strong ground displacement associated with the Northridge earthquake.

Nighttime enhancements in ionospheric electron content: seasonal and solar cycle variation

Abstract. Various characteristics of anomalous nighttime enhancement in ionospheric electron content (IEC) at Lunping (14.08°N geomagnetic), a station near the crest of the equatorial anomaly, have been presented by considering the IEC data for the 21st solar cycle. Out of a total of 1053 enhancements, 354 occur in pre-midnight and 699 occur in post-midnight hours, which indicates an overall dominance of post-midnight events at Lunping. The occurrence is more frequent during summer, less during the equinox and least during winter months. All the characteristics of the enhancements have seasonal dependencies and they reach their maximum values during summer months. The occurrence of the pre-midnight events show positive and post-midnight events show negative correlation with solar activity. The results have been discussed and compared with those at low-latitude stations in India and Hawaii and at the mid-latitude station, Tokyo.

Nighttime enhancements in ionospheric electron content: seasonal and solar cycle variation

Various characteristics of anomalous nighttime enhancement in ionospheric electron content (IEC) at Lunping (14.08°N geomagnetic), a station near the crest of the equatorial anomaly, have been presented by considering the IEC data for the 21st solar cycle. Out of a total of 1053 enhancements, 354 occur in pre-midnight and 699 occur in post-midnight hours, which indicates an overall dominance of post-midnight events at Lunping. The occurrence is more frequent during summer, less during the equinox and least during winter months. All the characteristics of the enhancements have seasonal dependencies and they reach their maximum values during summer months. The occurrence of the pre-midnight events show positive and post-midnight events show negative correlation with solar activity. The results have been discussed and compared with those at low-latitude stations in India and Hawaii and at the mid-latitude station, Tokyo.

On the mathematical modelling of the spectral structure of geomagneticPC1 pulsations via the response of Alfvén's ionospheric resonator

A method of numerical simulation of the coefficient of reflection of the ionospheric transition layer as a function of frequency is applied to the experimental data related to several series of “pearl-type” pulsations Pc1 (f = 0.2 – 2 Hz) recorded at the observatories of Kerguelen, Sogra and Nurmijarvi. The inverse problem of modelling, i.e. determining the vertical profiles of ionospheric electron concentration corresponding to the actual experimental situations, was solved approximately. The initial assumption for interpreting the specific nature of the series of Pc1 micropulsations parallel in time was their resonance origin under reflection of the signal at magnetically conjugate ionospheres, Alfven's resonators, in both of the Earth's hemispheres.

Correlated environmental corrections in TOPEX/POSEIDON, with a note on ionospheric accuracy

Estimates of the effectiveness of an altimetric correction, and interpretation of sea level variability as a response to atmospheric forcing, both depend upon assuming that residual errors in altimetric corrections are uncorrelated among themselves and with residual sea level, or knowing the correlations. Not surprisingly, many corrections are highly correlated since they involve atmospheric properties and the ocean surface's response to them. The full corrections (including their geographically varying time mean values), show correlations between electromagnetic bias (mostly the height of wind waves) and either atmospheric pressure or water vapor of −40%, and between atmospheric pressure and water vapor of 28%. In the more commonly used collinear differences (after removal of the geographically varying time mean), atmospheric pressure and wave height show a −30% correlation, atmospheric pressure and water vapor a −10% correlation, both pressure and water vapor a 7% correlation with residual sea level, and a bit surprisingly, ionospheric electron content and wave height a 15% correlation. Only the ocean tide is totally uncorrelated with other corrections or residual sea level. The effectiveness of three ionospheric corrections (TOPEX dual‐frequency, a smoothed version of the TOPEX dual‐frequency, and Doppler orbitography and radiopositioning integrated by satellite (DORIS) is also evaluated in terms of their reduction in variance of residual sea level. Smooth (90–200 km along‐track) versions of the dual‐frequency altimeter ionosphere perform best both globally and within 20° in latitude from the equator. The noise variance in the 1/s TOPEX ionospheric samples is ∼(11 mm)2, about the same as noise in the DORIS‐based correction; however, the latter has its error over scales of order 103 km. Within 20° of the equator, the DORIS‐based correction adds (14 mm)2 to the residual sea level variance.

Ionospheric calibration of single frequency VLBI and GPS observations using dual GPS data

The ionospheric effect is one of the main sources of error in Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) high precision geodesy. Although the use of two frequencies allows the estimation of this effect, in some cases dual observations are not possible due to the available equipment or the type of observation. This paper presents the ionospheric calibration of single frequency VLBI and GPS observations based on the ionospheric electron content estimated from dual frequency GPS data. The ionospheric delays obtained with this procedure and the VLBI baseline length results have been compared with those obtained with dual frequency data. For the European geodetic VLBI baselines, both solutions agree at the 3–5 parts in 10−9 level. The noise introduced by the GPS-based calibration is in the order of 3 cm for the VLBI observables and of 10 cm for the GPS observables.

Modeling studies of ionospheric variations during an intense solar cycle

Modeling studies are carried out using the Sheffield University plasmasphere‐ionosphere model to investigate the relative importance of neutral winds, neutral densities, and solar EUV fluxes in leading to the saturation of ionospheric ionization observed during the intense solar cycle 21. Values of mean daytime (1100‐1700 LT) ionospheric electron content, peak electron density (Nmax), and peak height (hmax) computed for four midlatitude stations for the month of October 1980‐1985, when the 10.7‐cm solar activity index (F10.7) varied from 66 to 303, increase nonlinearly with F10.7 with saturation for high values of F10.7. Neutral winds, neutral densities, and solar EUV fluxes (obtained from empirical models based on observed data) used in the model computations also undergo similar nonlinear increase with F10.7. The study reveals that (1) the nonlinear increase of neutral winds and neutral densities has no net effect on the saturation of ionospheric ionization, and (2) the saturation of ionization is caused by the saturated production of ionization due to the nonlinear increase of the solar EUV fluxes. The model values of ionospheric height saturate mainly due to the nonlinear increase of the neutral densities determined by solar EUV and UV radiations. The study concludes that the ionosphere (and atmosphere) responds linearly to the solar EUV (and UV) inputs and nonlinearly to F10.7 because the expected linear relationship between the EUV (and UV) fluxes and F10.7 breaks down during solar maximum. Thus it is recommended that the F10.7 proxy, which has conventionally been used as an index of solar activity, must be replaced or be used carefully, particularly during solar maximum.

F-region ionospheric irregularities over King George Island and Argentine Islands – a comparative study

Spread-F is caused by the presence of ionospheric electron concentration irregularities of scale-size of order 5 km at F-region altitudes. Estimates of spread-F in the vicinity of the maximum plasma frequency of the Flayer (foF2) have been determined at 15 min intervals from ionograms recorded over a ten day period (1–10 May 1986) both at Marsh (62.2°S, 58.9°W), King George Island, and Faraday (65.2°S, 64.3°W), Argentine Islands. The interval, at low solar activity, includes periods of quiet and disturbed geomagnetic activity. Spread-F is observed on every night at both stations. It is more frequent, slightly more intense and starts earlier at Argentine Islands than at King George Island. On most nights, spread-F ceases about local sunrise at 120 km altitude at both stations. On the days of highest geomagnetic activity, the onset of spread-F is delayed compared with days of lower activity. Spread-F is usually most intense on the night(s) following largest geomagnetic activity level, as measured by the geomagnetic index, Kp. The growth rate of the plasma instability processes causing the ionospheric irregularities is inversely related to electron concentration (foF22), amongst other parameters. Thus the lower foF2 values over Argentine Islands are consistent with more spread-F being observed by the higher latitude observatory. However, no firm relationship between the absolute value of foF2, the horizontal gradient of foF2 between the two observatories, and the onset of spread-F, is found. Thus it has not been possible to determine uniquely the instability process responsible for the formation of the plasma irregularities.