Ground-based Ionosonde Research Articles

Auroral ionospheric E region parameters obtained from satellite-based far ultraviolet and ground-based ionosonde observations: 1. Data, methods, and comparisons.

A large number (~1000) of coincident auroral far ultraviolet (FUV) and ground-based ionosonde observations are compared. This is the largest study to date of coincident satellite-based FUV and ground-based observations of the auroral E region. FUV radiance values from the NASA Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) Global Ultraviolet Imager (GUVI) and the Defense Meteorological Satellite Program (DMSP) F16 and F18 Special Sensor Ultraviolet Spectrographic Imager (SSUSI) are included in the study. A method is described for deriving auroral ionospheric E region maximum electron density (NmE) and height of maximum electron density (hmE) from N2 Lyman-Birge-Hopfield (LBH) radiances given in two channels using lookup tables generated with the Boltzmann 3-Constituent (B3C) auroral particle transport and optical emission model. Our rules for scaling (i.e., extracting ionospheric parameters from) ionograms to obtain auroral NmE and hmE are also described. Statistical and visual comparison methods establish statistical consistency and agreement between the two methods for observing auroral NmE, but not auroral hmE. It is expected that auroral non-uniformity will cause the two NmE methods to give inconsistent results, but we have not attempted to quantify this effect in terms of more basic principles, and our results show that the two types of NmE observations are well correlated and statistically symmetrical, meaning that there is no overall bias and no scale-dependent bias.

Multi-precursory Analysis of Phalla Earthquake (July 2015; Mw 5.1) Near Islamabad, Pakistan

The recent scientific progress in understanding the hierarchical nature of the lithosphere and its dynamics based on systematic earthquake monitoring helps in designing an improved earthquake forecasting strategy. This study presents a detailed multi-precursory analysis of a shallow, moderate Phalla earthquake (July 24, 2015; Mw 5.1; Depth = 10 km) near Islamabad, Pakistan, 24 km away from the monitoring site within the context of earthquake forecasting. The instrumental setup (soil radon monitor and space–ground-based ionosonde) for precursory monitoring of earthquake activity is installed at the foothills of Margalla along the Main Boundary Thrust. For identification of abnormal behavior of ionospheric parameters and soil radon flux, a statistical criterion consistent with other studies is used. Our analyses reveal the effect of pre- and co-seismic changes in the form of anomalous behavior of local ionospheric perturbations (2–15 days prior), soil radon flux (3 days prior), and seismic quiescence related to Phalla earthquake. The solar/geomagnetic conditions and meteorological parameters remained normal during the anomalous periods. Moreover, multi-precursory approach adopted in this study for earthquake prediction/forecasting can serve as a potential indicator of seismic activity.

Winter anomaly in NmF2 and TEC: when and where it can occur

For the first time, by using a regression procedure, we analyzed the solar activity dependence of the winter anomaly intensity in the ionospheric F2-layer peak electron density (Nm F2) and in the Total Electron Content (TEC) on a global scale. We used the data from global ionospheric maps for 1998–2015, from GPS radio occultation observations with COSMIC, CHAMP, and GRACE satellites for 2001–2015, and ground-based ionosonde data. The fundamental features of the winter anomaly in Nm F2 and in TEC (spatial distribution and solar activity dependence) are similar for these parameters. We determined the regions, where the winter anomaly may be observed in principle, and the solar activity level, at which the winter anomaly may be recorded in different sectors. A growth in geomagnetic disturbance or in the solar activity level is shown to facilitate the winter anomaly intensity increase. Longitudinal variations in the winter anomaly intensity do not conform partly to the generally accepted Rishbeth theory. We consider the obtained results in the context of spatial and solar cycle variations in O/N2 ratio and thermospheric meridional wind. Additionally, we briefly discuss different definitions of the winter anomaly.

Three-dimensional data assimilation for ionospheric reference scenarios

Abstract. The reliable estimation of ionospheric refraction effects is an important topic in the GNSS (Global Navigation Satellite Systems) positioning and navigation domain, especially in safety-of-life applications. This paper describes a three-dimensional ionosphere reconstruction approach that combines three data sources with an ionospheric background model: space- and ground-based total electron content (TEC) measurements and ionosonde observations. First the background model is adjusted by F2 layer characteristics, obtained from space-based ionospheric radio occultation (IRO) profiles and ionosonde data, and secondly the final electron density distribution is estimated by an algebraic reconstruction technique.The method described is validated by TEC measurements of independent ground-based GNSS stations, space-based TEC from the Jason 1 and 2 satellites, and ionosonde observations. A significant improvement is achieved by the data assimilation, with a decrease in the residual errors by up to 98 % compared to the initial guess of the background. Furthermore, the results underpin the capability of space-based measurements to overcome data gaps in reconstruction areas where less GNSS ground-station infrastructure exists.

Comparison of midlatitude ionospheric F region peak parameters and topside Ne profiles from IRI2012 model prediction with ground-based ionosonde and Alouette II observations

The ionospheric F2 peak parameters recorded by a ground-based ionosonde at the midlatitude station Alma-Ata [43.25N, 76.92E] were compared with those obtained using the latest version of the IRI model (http://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html) for quiet geomagnetic conditions of winter (January), equinoxes (March and September), and summer (July) months of 2008–2010.It was found that for the Alma-Ata (Kazakhstan) location, the IRI2012 model describes well the morphology of seasonal and diurnal variations of the ionospheric critical frequency (foF2) and peak density height (hmF2) monthly medians. The model errors in the median foF2 prediction (percentage deviations between the median foF2 values and their model predictions) were found to vary approximately in the range from about −20% to 34% and showed a stable overestimation in the median foF2 values for daytime in January and July and underestimation for day- and nighttime hours in the equinoctial months. The comparison between the ionosonde hmF2 and IRI results clearly showed that the IRI overestimates the nighttime hmF2 values for March and September months, and the difference is up to 30km. The daytime Alma-Ata hmF2 data were found to be close to the IRI predictions (deviations are approximately ±10–15km) in winter and equinoctial months, except in July when the observed hmF2 values were much more (from approximately 50–200km).The comparison between the Alouette foF2 data and IRI predictions showed mixed results. In particular, the Alouette foF2 data showed a tendency to be overestimated for daytime in winter months similar to the ionosonde data; however, the overestimated foF2 values for nighttime in the autumn equinox were in disagreement with the ionosonde observations. There were large deviations between the observed hmF2 values and their model predictions. The largest deviations were found during winter and summer (up to −90km).The comparison of the Alouette II electron density profiles with those predicted by the adapted IRI2012 model in the altitude range hmF2 of the satellite position showed a great difference in the shape of the Alouette-, NeQuick-, IRI02-coorr, and IRI2001-derived Ne profiles, with overestimated Ne values at some altitudes and underestimated Ne values at others.The results obtained in the study showed that the observation–model differences were significant especially for the real observed (not median) data. For practical application, it is clearly important for the IRI2012 model to be adapted to the observed F2-layer peak parameters. However, the model does not offer a simple solution to predict the shape of the vertical electron density profile in the topside ionosphere, because of the problem with the topside shape parameters.

The fast development of solar terrestrial sciences in Taiwan

In Taiwan, research and education of solar terrestrial sciences began with a ground-based ionosonde operated by Ministry of Communications in 1952 and courses of ionospheric physics and space physics offered by National Central University (NCU) in 1959, respectively. Since 1990, to enhance both research and education, the Institute of Space Science at NCU has been setting up and operating ground-based observations of micropulsations, very high-frequency radar, low-latitude ionospheric tomography network, high-frequency Doppler sounder, digital ionosondes, and total electron content (TEC) derived from ground-based GPS receivers to study the morphology of the ionosphere for diurnal, seasonal, geophysical, and solar activity variations, as well as the ionosphere response to solar flares, solar wind, solar eclipses, magnetic storms, earthquakes, tsunami, and so on. Meanwhile, to have better understanding on physics and mechanisms, model simulations for the heliosphere, solar wind, magnetosphere, and ionosphere are also introduced and developed. After the 21 September 1999 Mw7.6 Chi–Chi earthquake, seismo-ionospheric precursors and seismo-traveling ionospheric disturbances induced by earthquakes become the most interesting and challenging research topics of the world. The development of solar terrestrial sciences grows even much faster after National Space Origination has been launching a series of FORMOSAT satellites since 1999. ROCSAT-1 (now renamed FORMOSAT-1) measures the ion composition, density, temperature, and drift velocity at the 600-km altitude in the low-latitude ionosphere; FORMOSAT-2 is to investigate lightning-induced transient luminous events, polar aurora, and upper atmospheric airglow, and FORMOSAT-3 probes ionospheric electron density profiles of the globe. In the near future, FORMOSAT-5 and FORMOSAT-7/COSMIC-2 will be employed for studying solar terrestrial sciences. These satellite missions play an important role on the recent development of solar terrestrial sciences in Taiwan.

Using IRI and GSM TIP model results as environment for HF radio wave propagation model during the geomagnetic storm occurred on September 26–29, 2011

This paper analyses the geomagnetic storm on September 26–29, 2011. We compare the calculation results obtained using the Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) and IRI-2012 (Bilitza et al., 2014) model with ground-based ionosonde data of stations at different latitudes and longitudes. We examined physical mechanisms responsible for the formation of ionospheric effects during the main phase of geomagnetic storm that occurred at the rising phase of the 24th solar cycle. We used numerical results obtained from IRI-2012 and GSM TIP models as propagation environment for HF signals from an equatorial transmitter during quiet and disturbed conditions. We used the model of HF radio wave propagation developed in I. Kant Baltic Federal University (BFU) that is based on the geometrical optics approximation. We compared the obtained radio paths in quiet conditions and during the main and recovery storm phases and evaluated radio wave attenuation in different media models.

Spatial features of Weddell Sea and Yakutsk Anomalies in foF2 diurnal variations during high solar activity periods: Interkosmos-19 satellite and ground-based ionosonde observations, IRI reproduction and GSM TIP model simulation

We studied the main morphology of the formation of summer ionospheric anomalies which includes the Weddell Sea Anomaly (WSA) and the Yakutsk Anomaly (YA) in terms of positive night-to-day foF2 differences. For this purpose, we have for the first time used topside sounding (Interkosmos-19 satellite) and ground-based (ionosonde) data as well as results from an empirical model (International Reference Ionosphere – IRI) and from a first-principles model (Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere – GSM TIP). The main spatial morphological characteristics of the WSA and YA are similar which points to a common cause for their occurrence. Our preliminary GSM TIP model results showed that the foF2 values at nighttime are greater than at daytime due to: (1) the horizontal plasma transport due to electromagnetic drift; (2) the vertical component of plasma transport along the geomagnetic field lines via meridional neutral wind; (3) the distribution of the neutral composition of the thermosphere.

The European Ionosonde Service: nowcasting and forecasting ionospheric conditions over Europe for the ESA Space Situational Awareness services

The Earth’s ionosphere is a magnetoionic medium imbedded in a background neutral atmosphere, exhibiting very interesting refractive properties, including anisotropy, dispersion, and dissipation. As such, it poses a challenge for several radio systems that make use of signal transmission through all or some portion of the medium. It is important therefore to develop prediction systems able to inform the operators of such systems about the current state of the ionosphere, about the expected effects of forthcoming space weather disturbances and about support long-term planning of operations and data post-processing projects for improving modelling and mitigation techniques.The European Space Agency (ESA) in the framework of the Space Situational Awareness (SSA) Programme has supported the development of the European Ionosonde Service (EIS) that releases a set of products to characterise the bottomside and topside ionosphere over Europe. The Service is based on a set of prediction models driven by data from ground-based ionosondes and supportive data from satellites and spacecraft. The service monitors the foF2 and the electron density profile up to the height of the Global Navigation Satellite System (GNSS) at European middle and high latitudes and provides estimates for forthcoming disturbances mainly triggered by geo-effective Coronal Mass Ejections (CMEs). The model’s performance has been validated and based on these results, it was possible to issue together with the products, quality metrics characterizing the product’s reliability. The EIS products meet the requirements of various SSA service domains, especially the transionospheric radio link and the spacecraft operations. Currently, the service is freely available to all interested users, and access is possible upon registration.

Comparative analysis of nocturnal vertical plasma drift velocities inferred from ground-based ionosonde measurements of hmF2 and h′F

Abstract Variations in the evening/nighttime ionosonde vertical plasma drift velocities inferred from the time rate of change of both the base of the F-layer height (Vz (h′F) ) and height of the peak electron density (Vz (hmF2) ) from an equatorial station were compared for better description of the E×B drifts. For better interpretation, both results were compared with the Incoherent Scatter (IS) radar observations (Vz (ISR) ) which is taken to be the most accurate method of measuring drift, and therefore the data of reference level. An equinoctial maximum and June solstice minimum in post-sunset pre-reversal enhancement (PRE) was observed for Vz (hmF2) , Vz (ISR) , and Vz (h′F) . The percentage correlation between Vz hmF2 and Vz h′F ranges within 55–70%. While PRE for Vz (hmF2) peaked at 19 LT for all seasons, Vz (h′F) peaked at 18 LT for September equinox and December solstice, and start earlier. The nighttime downward reversal peak magnitudes for Vz (hmF2) and Vz (h′F) are respectively within the range of −4 to −14 and −2 to −14 m/s; whereas Vz (ISR) ranges within −12 and −34 m/s; and the peak time was reached earlier with the ionosonde observations than for the ISR. The PRE peak magnitude for Vz (hmF2) , Vz (h′F) and Vz (ISR) varies between 3–14, 2–14, and 4–14 m/s for the entire seasons. Our results revealed higher drift correlation coefficients in both Vz (hmF2) vs. Vz (ISR) (0.983) and Vz (h′F) vs. Vz (ISR) (0.833) relationships during the equinoxes between 16–20 LT, at which time the F-layer altitude is higher than the 300 km threshold value; and lower for solstice period (0.326 and 0.410 in similar order). A better linear relationship between Vz (hmF2) and Vz (h′F2) was observed during the reversal (19–21 LT) phase period. PRE velocity was shown to be seasonal and solar activity dependent. Both Vz hmF2 and Vz h′F compares almost equally with the ISR measurement. However, the PRE peak magnitude for the drift inferred using h′F2 is closer to the corresponding ISR magnitude during the equinoxes; whereas the drift inferred from hmF2 best represent the ISR magnitude for solstices. We established that both Vz hmF2 and Vz h′F are governed by the same mechanism at nighttime, and as such any of them can be used to infer vertical drift as long as the 300 km threshold value condition is considered, otherwise chemical correction may be required for the F-layer uplift.

Quantifying the EEJ current with ground-based ionosonde inferred vertical E × B drifts in the morning hours over Ilorin, West Africa

The relationship between the ground-based inferred vertical E × B drifts, Vz, and the magnetic equatorial electrojet current during the year of solar minima was presented. Both the diurnal and seasonal Vz variations are positively directed during the daytime and negative at nighttime. The evening time pre-reversal enhancement occurs around 19:00 LT. The fairly strong linear relationship between the electrojet current strength and Vz exhibited higher correlations during the daytime (06:00–16:00 LT). The maximum morning time proxy parameter described by E = [d (ΔH ILR)/dt]max in the morning hours, indicating the east-west electric field in the EEJ, corresponds reasonably well with the E × B drift and, hence, can be used as a proxy parameter for representing Vz in the morning hours. The daytime EEJ magnitude seasonal changes are connected with a change in conductivity emerging from the action of turbulence and divergence of momentum flux. These waves above the dynamo region are suggested to lead to partial counter electrojet during the equinoctial months.

Global model of the F2 layer peak height for low solar activity based on GPS radio-occultation data

We propose a global median model SMF2 (Satellite Model of the F2 layer) of the ionospheric F2-layer height maximum (hmF2), based on GPS radio-occultation data for low solar activity periods (F10.7A<80). The model utilizes data provided by GPS receivers onboard satellites CHAMP (~100,000 hmF2 values), GRACE (~70,000) and COSMIC (~2,000,000). The data were preprocessed to remove cases where the absolute maximum of the electron density lies outside the F2 region. Ground-based ionospheric sounding data were used for comparison and validation. Spatial dependence of hmF2 is modeled by a Legendre-function expansion. Temporal dependence, as a function of Universal Time (UT), is described by a Fourier expansion. Inputs of the model are: geographical coordinates, month and F10.7A solar activity index. The model is designed for quiet geomagnetic conditions (Kр=1–2), typical for low solar activity.SMF2 agrees well with the International Reference Ionosphere model (IRI) in those regions, where the ground-based ionosonde network is dense. Maximal difference between the models is found in the equatorial belt, over the oceans and the polar caps. Standard deviations of the radio-occultation and Digisonde data from the predicted SMF2 median are 10–16km for all seasons, against 13–29km for IRI-2012. Average relative deviations are 3–4 times less than for IRI, 3–4% against 9–12%. Therefore, the proposed hmF2 model is more accurate than IRI-2012.

Equatorial vertical plasma drift velocities and electron densities inferred from ground-based ionosonde measurements during low solar activity

Average values of ionosonde hmF2 data acquired from an African equatorial station have been used to determine vertical plasma drift (Vz) measurements during period of low solar activity. Pre-noon peak was around 1000h LT for all seasons. The peak daytime F2 drift is higher during the equinoctial months with an average of 18.1m/s than the solsticial months (14.7m/s). At nighttime, Vz is characterized first by upward enhancement around 1900h LT with a range of 0.3–8.0m/s, then by a downward reversal. The highest enhancement was recorded in December solstice and start earliest during the March equinox. The peak reversal values are 13.3, 10.7, 9.0 and 4.2m/s for December Solstice, September Equinox, March Equinox and June Solstice respectively. The observed simultaneous post-sunset rise in hmF2 and in vertical E×B drift together with a sharp drop in NmF2 at all season infer that electrons moving away from the equator are at a region of low recombination loss rate. The abrupt faster drift of the plasma away from the equator as indicated by the pre-reversal enhancement (PRE) in upward plasma drift is responsible for the sharp drop in NmF2 immediately after sunset. Some past results were also confirmed in this work.

Effects observed in the equatorial and low latitude ionospheric F-region in the Brazilian sector during low solar activity geomagnetic storms and comparison with the COSMIC measurements

The main objective of the present investigation has been to compare the ionospheric parameters (NmF2 and hmF2) observed by two ground-based ionospheric sounders (one at PALMAS- located near the magnetic equator and the other at Sao Jose dos Campos-located in the low-latitude region) in the Brazilian sector with that by the satellite FORMOSAT-3/COSMIC radio occultation (RO) measurements during two geomagnetic storms which occurred in December 2006 and July 2009. It should be pointed out that in spite of increasing the latitude (to 10°) and longitude (to 20°) around the stations; we had very few common observations. It has been observed that both the peak electron density (NmF2) and peak height (hmF2) observed by two different techniques (space-borne COSMIC and ground-based ionosondes) during both the geomagnetic storm events compares fairly well (with high correlation coefficients) at the two stations in the Brazilian sector. It should be pointed out that due to equatorial spread F (ESF) in the first storm (December 2006) and no-reflections from the ionosphere during nighttime in the second storm (July 2009), we had virtually daytime data from the two ionosondes.

Ionospheric disturbances on 8 September, 2010: Was it connected with the incoming moderate Chongqing earthquake?

Earthquake prediction stimulates the searches for a correlation between seismic activity and ionospheric anomalies. Contrary to common focuses on strong earthquakes, we report the ionospheric disturbances, 2days before a moderate Ms=4.7 Chongqing earthquake (29.4°N, 105.5°E, depth=7.0km, occurred at 21:21LT, 10 September, 2010) with the data of ground-based ionosondes and IGS receivers. The data covering the period under the quiet geomagnetic conditions and a geomagnetic storm was analyzed with upper and lower bounds. It is found that there were significant enhancements of foF2 and total electron content (TEC) on the afternoon of 8 September, 2010, with a limited area close to the epicentre, which was different from the feature of ionospheric perturbations triggered by the geomagnetic storm on 15 September. Taking into account the heliogeomagnetical condition, we conclude that the observed ionospheric enhancements were very likely associated with the forthcoming moderate Chongqing earthquake, which implies that the relationship between the amplitudes of ionospheric disturbances and earthquakes is very complicated.

Синхронизация бортовых и наземных ионозондов при системном зондировании ионосферы

Синхронизация бортовых и наземных ионозондов при системном зондировании ионосферы

Atmosphere-ionosphere response to the M9 Tohoku earthquake revealed by multi-instrument space-borne and ground observations: Preliminary results

We retrospectively analyzed the temporal and spatial variations of four different physical parameters characterizing the state of the atmosphere and ionosphere several days before the M9 Tohoku, Japan earthquake of March 11, 2011. The data include outgoing long wave radiation (OLR), GPS/TEC, lower Earth orbit ionospheric tomography and critical frequency foF2. Our first results show that on March 7th a rapid increase of emitted infrared radiation was observed from the satellite data and an anomaly developed near the epicenter. The GPS/TEC data indicate an increase and variation in electron density reaching a maximum value on March 8. Starting from this day in the lower ionosphere also there was confirmed an abnormal TEC variation over the epicenter. From March 3 to 11 a large increase in electron concentration was recorded at all four Japanese ground-based ionosondes, which returned to normal after the main earthquake. The joint preliminary analysis of atmospheric and ionospheric parameters during the M9 Tohoku, Japan earthquake has revealed the presence of related variations of these parameters implying their connection with the earthquake process. This study may lead to a better understanding of the response of the atmosphere/ionosphere to the great Tohoku earthquake.

Modeling ionospheric &amp;lt;I&amp;gt;fo&amp;lt;/I&amp;gt;F2 by using empirical orthogonal function analysis

Abstract. A similar-parameters interpolation method and an empirical orthogonal function analysis are used to construct empirical models for the ionospheric foF2 by using the observational data from three ground-based ionosonde stations in Japan which are Wakkanai (Geographic 45.4° N, 141.7° E), Kokubunji (Geographic 35.7° N, 140.1° E) and Yamagawa (Geographic 31.2° N, 130.6° E) during the years of 1971–1987. The impact of different drivers towards ionospheric foF2 can be well indicated by choosing appropriate proxies. It is shown that the missing data of original foF2 can be optimal refilled using similar-parameters method. The characteristics of base functions and associated coefficients of EOF model are analyzed. The diurnal variation of base functions can reflect the essential nature of ionospheric foF2 while the coefficients represent the long-term alteration tendency. The 1st order EOF coefficient A1 can reflect the feature of the components with solar cycle variation. A1 also contains an evident semi-annual variation component as well as a relatively weak annual fluctuation component. Both of which are not so obvious as the solar cycle variation. The 2nd order coefficient A2 contains mainly annual variation components. The 3rd order coefficient A3 and 4th order coefficient A4 contain both annual and semi-annual variation components. The seasonal variation, solar rotation oscillation and the small-scale irregularities are also included in the 4th order coefficient A4. The amplitude range and developing tendency of all these coefficients depend on the level of solar activity and geomagnetic activity. The reliability and validity of EOF model are verified by comparison with observational data and with International Reference Ionosphere (IRI). The agreement between observations and EOF model is quite well, indicating that the EOF model can reflect the major changes and the temporal distribution characteristics of the mid-latitude ionosphere of the Sea of Japan region. The error analysis processes imply that there are seasonal anomaly and semi-annual asymmetry phenomena which are consistent with pre-existing ionosphere theory.

Geomagnetic storm effects in the ionospheric E- and F-regions

Ground-based ionosonde data obtained at Alma-Ata station [ φ=43.25°N, λ=76.92°E, Φ=33.47°N, L=1.44) were analysed to study the ionospheric responses of nine intense (Kp≥8, Dst<−100 nT) geomagnetic storms with storm sudden commencement ( ssc). The collected data show that the ionospheric responses to the geomagnetic storms are highly complex and variable; however, negative ionospheric disturbances are a common feature of the responses. The occurrence of normal night E2-, E-, F1- and auroral type r ( retardation) sporadic Es-layers, which are unusual for Alma-Ata, was observed during most active phase in the Dst index. Employing the International Reference Ionosphere (IRI), the night-time E region electron density Ne was estimated for “quiet” conditions on the epochs of the storm time periods. A direct comparison of the “quiet” and “storm” electron density in the 110–200 km altitude range shows a significant storm-induced increase in Ne that reaches a factor of approximately 10 at the 110 km altitude. The interaction of precipitating energetic neutralised ring current particles with the upper atmosphere during geomagnetic disturbances is assumed to be a possible explanation for the observed night events at this latitude sector.

Response of low-latitude ionospheric total electron content to the geomagnetic storm of 24 August 2005

[1] Response of low-latitude ionosphere to the geomagnetic storm of 24 August 2005 has been studied using total electron content (TEC) data obtained from the Global Positioning System (GPS) receivers. These studies were carried out using the receivers that were located (1) near the northern crest (∼15°N mag. Lat.) of the equatorial ionization anomaly around 56°E, 74°E, and 102°E longitude and (2) from the northern crest of the ionization anomaly down to the magnetic equator in the longitude belt 75°E ± 3°E. These studies have been substantiated with the ground-based magnetometer data at Tirunelveli and Alibag, an equatorial and off equatorial station, respectively. The ground-based ionosonde data at New Delhi, a low-latitude station, have also been used to substantiate the TEC observations. The storm day TEC shows two well-defined humps at all stations wherein enhancements of the order of 80%–100% have been observed. While the first of the enhancements has been attributed to the prompt penetration electric field associated with an interplanetary electric field (IEF Ey) of about 35 mV/m, the other one has been attributed to the second episode of the prompt penetration electric field (IEF Ey ∼ 20 mV/m) and abnormal equatorial plasma fountain in late evening hours, respectively. During the unsteady ring current conditions when the IMF Bz was still southward, penetration of a westward electric field has been inferred. Two peaks in foF2 have been observed whose time of occurrence coincides with those of the humps in the low-latitude TEC. Results from stations having nearly the same magnetic latitude show that the ionospheric response, in terms of GPS TEC, to the prompt penetration electric fields is longitudinally independent. Formation of the first hump in TEC is progressively delayed in time from low to near-equatorial latitudes for stations in different magnetic latitudes along nearly the same longitude. However, its time of appearance at Diego Garcia, a station magnetically conjugate to Udaipur, is the same as that at Udaipur. The results also reveal the poleward expansion of the ionization anomaly due to the storm. Disturbance dynamoelectric fields have been inferred to be responsible for the suppressed plasma fountain, resulting in suppressed TEC values and equatorward contraction of the ionization anomaly on 25 August, compared to a reference quiet day.