Electron Density Height Research Articles

On the variations of ionospheric parameters made at a near equatorial station in the African longitude sector: IRI validation with the experimental observations

We examine the main morphological patterns and climatological behaviour of equatorial F2 region over African sector using hourly observational values of F2 peak height of maximum electron density (h m F2), F2 layer peak electron density (N m F2/ foF2), and propagation factor (M3000F2) hitherto made by the Ibadan ionosonde at 7.4°N, 3.9°E, dip latitude 2.3°S, in Nigeria; between January to December 1958, during a period of high solar activity (yearly averaged Rz12 = 190 units) and magnetically quiet conditions (Kp ≤ 3). A direct comparison between these measurements and the International Reference Ionosphere 2007 (IRI-2007) model-predictions are also made. The results of comparisons illustrate that good advancement has been made but reveal some important discrepancies. The trends in the experimental data are found to be in excellent agreement with the trends in the simulation results for maximum electron density and propagation factor, but fair-to-good for F2 layer peak altitude. The model is unable to capture the sharp postsunset and predawn enhancements in h m F2 and M3000F2, respectively. The model results have errors ranging from approximately 8–15%, 9–17%, and 3–5%, respectively, for h m F2, N m F2, and M3000F2. On average, the percent absolute relative difference of the model from the experimental observations varies from about 0–20%, 0–30%, and 0–10% for h m F2, N m F2, and M3000F2, in that order. Our results are essentially consistent with other equatorial and low-latitude ground-based measurements over South America, India, and Southeast Asia.

Positive ionospheric storm effects at Latin America longitude during the superstorm of 20–22 November 2003: revisit

Abstract. Positive ionospheric storm effects that occurred during the superstorm on 20 November 2003 are investigated using a combination of ground-based Global Positioning System (GPS) total electron content (TEC), and the meridian chain of ionosondes distributed along the Latin America longitude of ~280° E. Both the ground-based GPS TEC and ionosonde electron density profile data reveal significant enhancements at mid-low latitudes over the 280° E region during the main phase of the November 2003 superstorm. The maximum enhancement of the topside ionospheric electron content is 3.2–7.7 times of the bottomside ionosphere at the locations of the ionosondes distributed around the mid- and low latitudes. Moreover, the height of maximum electron density exceeds 400 km and increases by 100 km compared with the quiet day over the South American area from middle to low latitudes, which might have resulted from a continuous eastward penetration electric field and storm-generated equatorward winds. Our results do not support the conclusions of Yizengaw et al. (2006), who suggested that the observed positive storm over the South American sector was mainly the consequence of the changes of the bottomside ionosphere. The so-called "unusual" responses of the topside ionosphere for the November 2003 storm in Yizengaw et al. (2006) are likely associated with the erroneous usage of magnetometer and incomplete data.

A new global model for the ionospheric F2 peak height for radio wave propagation

Abstract. The F2-layer peak density height hmF2 is one of the most important ionospheric parameters characterizing HF propagation conditions. Therefore, the ability to model and predict the spatial and temporal variations of the peak electron density height is of great use for both ionospheric research and radio frequency planning and operation. For global hmF2 modelling we present a nonlinear model approach with 13 model coefficients and a few empirically fixed parameters. The model approach describes the temporal and spatial dependencies of hmF2 on global scale. For determining the 13 model coefficients, we apply this model approach to a large quantity of global hmF2 observational data obtained from GNSS radio occultation measurements onboard CHAMP, GRACE and COSMIC satellites and data from 69 worldwide ionosonde stations. We have found that the model fits to these input data with the same root mean squared (RMS) and standard deviations of 10%. In comparison with the electron density NeQuick model, the proposed Neustrelitz global hmF2 model (Neustrelitz Peak Height Model – NPHM) shows percentage RMS deviations of about 13% and 12% from the observational data during high and low solar activity conditions, respectively, whereas the corresponding deviations for the NeQuick model are found 18% and 16%, respectively.

A new global empirical NmF2 model for operational use in radio systems

The ionospheric F2 region around the peak electron density height hmF2 of about 250–500 km causes the most pronounced impact on transionospheric radio wave propagation. Therefore, the peak electron density of the F2 layer NmF2 is a key parameter for characterizing the ionosphere. We present an empirical model approach that allows determining global NmF2 with a limited number of model coefficients. The nonlinear approach needs 13 coefficients and a few empirically fixed parameters for describing the NmF2 dependencies on local time, geographic/geomagnetic location and solar irradiance and activity. The model approach is applied to a vast quantity of global NmF2 data derived from GNSS radio occultation measurements by CHAMP, GRACE and COSMIC satellite missions and about 60 years of processed NmF2 data from 177 worldwide ionosonde stations. The model fits to these input data with the same standard and root mean squared (RMS) deviations of 2 × 1011 m−3. The proposed Neustrelitz global NmF2 model (Neustrelitz Peak Density Model ‐ NPDM) is climatological, i.e., the model describes the average behavior under quiet geomagnetic conditions. A preliminary comparison with the electron density NeQuick model reveals similar results for NmF2 with RMS deviations in the order of 2 × 1011 m−3 and 5 × 1011 m−3 for low and high solar activity conditions, respectively.

Diagnostics of the Solar X-Flare Impact on Lower Ionosphere through the VLF-NAA Signal Recordings

Abstract An analysis of four solar flare X-ray irradiance effects on VLF signal amplitude and phase delay variations on the NAA/24.0 kHz signal trace during the period from 2005 September to 2006 December was carried out. Solar flare data were taken from the GOES12 satellite one-minute listings. For the VLF data, recordings at the Institute of Physics, Belgrade were used. It was found that solar flare events affect VLF wave propagation in the Earth-ionosphere waveguide lowering the changes of the ionosphere electron density height profiles. This follows from the variation during the solar flare events of the following propagation parameters: the sharpness of the lower edge of the ionosphere and the reflection height.

Solar zenith angle dependence of plasma density and temperature in the polar cap ionosphere and low-altitude magnetosphere during geomagnetically quiet periods at solar maximum

[1] We constructed an empirical model of the electron density profile with solar zenith angle (SZA) dependence in the polar cap during geomagnetically quiet periods using 63 months of Akebono satellite observations at solar maximum. The electron density profile exhibits a transition at ∼2000 km altitude only under dark conditions. The electron density and scale height at low altitudes change drastically, by factors of 25 (at 2300 km altitude) and 2.0, respectively, as the SZA increases from 90° to 120°. The SZA dependence of the ion and electron temperatures is also investigated statistically on the basis of data obtained by the Intercosmos satellites and European Incoherent Scatter (EISCAT) Svalbard radar (ESR). A drastic change in the electron temperature occurs near the terminator, similarly to that in the electron density profile obtained by the Akebono satellite. The sum of the ion and electron temperatures obtained by the ESR (∼6500 K at ∼1050 km altitude under sunlit conditions and ∼3000 K at ∼750 km altitude under dark conditions) agrees well with the scale height at low altitudes obtained from the Akebono observations, assuming that the temperature is constant and that O+ ions are dominant. Comparisons between the present statistical results (SZA dependence of the electron density and ion and electron temperatures) and modeling studies of the polar wind indicate that the plasma density profile (especially of the O+ ion density) in the polar cap is strongly controlled by solar radiation onto the ionosphere by changing ion and electron temperatures in the ionosphere during geomagnetically quiet periods.

Latitudinal dependence of the ionospheric response to solar eclipse of 15 January 2010

[1] The ionospheric responses to the solar eclipse of 15 January 2010 in the equatorial anomaly region have been investigated by three vertical-incidence and seven oblique-incidence ionosondes arranged along the meridian from geomagnetic latitudes 18°N to 30°N in eastern China. Though the solar eclipse occurred later in the evening, the eclipse effect on electron density and reflection height of ionospheric F2 layer was clearly observed. The study of the eclipse lag (the time lag between the occurrence of the eclipse maximum obscuration and the occurrence of the maximum depletion of foF2) with latitude indicates it increased with F2 layer altitude. Results suggest also that this eclipse enhanced the prereversal enhancement. An unusual peak occurred after the maximum reduction in foF2 and this was observed by all our ionosondes. The following F2 layer plasma density increase was considered to be caused by the increased westward electric field.

Comparison between IRI-2001 predictions and observed measurements of hmF2 over three high latitude stations during different solar activity periods

The monthly median values of the height of peak electron density of the F2-layer (hmF2) derived from ionosonde measurements at three high latitude stations, namely Narssarssuaq (NAR) (61.2 °N, 314.6 °E), Sondrestrom (SON) (67°N, 309.1°E) and College (COL) (69.9°N, 212.2°E) were analyzed and compared with the International Reference Ionosphere (IRI-2001) model, using Comité Consultatif International des Radio communications) (CCIR and Union Radio-Scientifique Internationale (URSI) options. The analysis covers hmF2 values for March Equinox (February, March, April), June Solstice (May, June, July), September Equinox (August, September, October), and December Solstice (November, December, January), during periods of high (2000–2001), medium (2004–2005) and low (2007–2008) solar activity. Generally, the IRI-2001 prediction follow fairly well the diurnal and seasonal variation patterns of the observed values of hmF2 at all the stations. However, IRI-2001 overestimates and underestimates hmF2 at different times of the day for all solar activity periods and in all the seasons considered. The percentage deviation never exceeded 20%, except during DEC SOLS at COL and SON and during MARCH EQUI at SON during low solar activity period. For all solar activity periods considered, both the URSI and CCIR options of the IRI-2001 model give hmF2 values close to the ones measured, but the URSI option performed better than the CCIR option.

Comparison of observed hmF2 and IRI 2007 model with M(3000)F2 estimation of hmF2 at low solar activity for an equatorial station

In this paper, the ways of improving the equatorial F2 layer peak heights estimated from M(3000)F2 ionosonde data measured using the Ionospheric Prediction Service (IPS-42) sounder at Ouagadougou, Burkina Faso (geog lat +12.4°N, geog long +1.5°W, magnetic dip +5.9°N) during high solar activity year (1991) was investigated. For this purpose, the observed height of maximum electron density of F2 region (hmF2obs), deduced using an algorithm from scaled virtual heights of quiet day ionograms; and the predicted hmF2 values, which is given by the IRI 2007 model (hmF2IRI2007) have been compared with the ionosonde measured M(3000)F2 estimation of the hmF2 values (hmF2est), respectively. A strong correlation with its coefficients R 2 for all the seasons ranging 0.562 - 0.857 for hmF2obs values, and ranging 0.876 - 0.968 for the hmF2IRI2007 values was observed in the linear regressions of hmF2obs and hmF2IRI2007 with M(3000)F2 inverse. During the nighttime, estimated hmF2 (hmF2est) was found to be positively correlated with the hmF2obs values by the post-sunset peak representation, which is not represented by the hmF2IRI2007 values. Also, the validity of the hmF2est values has been investigated by finding the percentage deviations when compared with the hmF2obs and hmF2IRI2007.

Geographical distribution of long-term changes in the height of the maximum electron density of theFregion: A nonmigrating-tide effect?

[1] The geographical distribution of trends in the height of the maximum electron density of the F2 region (hmF2) seemed to indicate that ionospheric stations showing decreasing (i.e., negative) hmF2 trends are mostly located on or in the vicinity of seashores. In contrast, ionospheric stations in continental areas exhibit generally increasing (i.e., positive) hmF2 trends. On the basis of the present study, it is shown that considering the whole world, the geographical distribution of hmF2 trends is affected essentially by the nonmigrating tidal wind. Moreover, besides the field-aligned drift, the elecrodynamical drift must also be considered in the development of hmF2 trends. Taking into account both the field-aligned and the electrodynamical drift, northward (eastward) tidal winds and southward (westward) tidal winds would create downward and upward drifts, respectively, in the Northern Hemisphere but southward (eastward) and northward (westward) tidal winds in the Southern Hemisphere. An upward drift is accompanied by an increase and a downward drift is accompanied by a decrease of hmF2. It has been suggested that because of the thermal driven origin of the nonmigrating tides, the distribution may also be connected with ocean currents warming or cooling the neighborhood of the coastline. The effect of ocean currents would create downward drift due to cooling at the western coastlines of the continents but upward drift at the eastern coastlines as a result of the warming. This has been shown by comparing the distribution of hmF2 trends with the map of ocean currents. The corresponding hmF2 trends may develop because of changes in the land/sea-warming ratio, where the land-surface air temperature increases faster than the sea-surface air temperature.

Ionosphere F2 layer stratification caused by shear excited atmospheric vortical perturbation

We found that the height distribution of electron density of the mid-latitude ionosphere F2 region during nighttime being under the influence of a vortical perturbation excited in the horizontal shear flow (horizontal wind with horizontal linear shear) is characterized with a multiple structure. The analytical description for the ionosphere F2 region electron density height distribution was obtained by solving the ambipolar diffusion equation taking into account the presence of atmospheric shear waves (vortical perturbations excited in the horizontal shear flow). The peak heights and corresponding multi-layer electron densities in the F2 region depend on the values of the meridional wind zonal shear and the shear wave vertical wavenumber. The atmospheric shear wave is transformed into short-period atmospheric gravity wave (AGW) which is reflected in the oscillations of the F2 layer peak height ( hmF2) and peak density ( NmF2).

GPS/ГЛОНАСС-томографии ионосферы

The article tackles the problem of the reconstruction of ionospheric electron density height structure by means of GPS/GLONASS systems.

Plasma temperatures in Saturn's ionosphere

We have calculated self‐consistent electron and ion temperatures in Saturn's ionosphere using a series of coupled fluid and kinetic models developed to help interpret Cassini observations and to examine the energy budget of Saturn's upper atmosphere. Electron temperatures in the midlatitude topside ionosphere during solar maximum are calculated to range between 500 and 560 K during the Saturn day, approximately 80–140 K above the neutral temperature. Ion temperatures, calculated for only the major ions H+ and H3+, are nearly equal to the neutral temperature at altitudes near and below the height of peak electron density, while they can reach 500 K during the day at the topside. Plasma scale heights of the dusk electron density profile from radio occultation measurements of the Voyager 2 flyby of Saturn have been used to estimate plasma temperature as a comparison. Such an estimate agrees well with the temperatures calculated here, although there is a topside enhancement in electron density that remains unexplained by ionospheric calculations that include photochemistry and plasma diffusion. Finally, parameterizations of the heating rate from photoelectrons and secondary electrons to thermal, ambient electrons have been developed. They may apply for other conditions at Saturn and possibly at other giant planets and exoplanets as well.

Classification of X-ray solar flares regarding their effects on the lower ionosphere electron density profile

Abstract. The classification of X-ray solar flares is performed regarding their effects on the Very Low Frequency (VLF) wave propagation along the Earth-ionosphere waveguide. The changes in propagation are detected from an observed VLF signal phase and amplitude perturbations, taking place during X-ray solar flares. All flare effects chosen for the analysis are recorded by the Absolute Phase and Amplitude Logger (AbsPal), during the summer months of 2004–2007, on the single trace, Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E) with a distance along the Great Circle Path (GCP) D≈2000 km in length. The observed VLF amplitude and phase perturbations are simulated by the computer program Long-Wavelength Propagation Capability (LWPC), using Wait's model of the lower ionosphere, as determined by two parameters: the sharpness (β in 1/km) and reflection height (H' in km). By varying the values of β and H' so as to match the observed amplitude and phase perturbations, the variation of the D-region electron density height profile Ne(z) was reconstructed, throughout flare duration. The procedure is illustrated as applied to a series of flares, from class C to M5 (5×10−5 W/m2 at 0.1–0.8 nm), each giving rise to a different time development of signal perturbation. The corresponding change in electron density from the unperturbed value at the unperturbed reflection height, i.e. Ne(74 km)=2.16×108 m−3 to the value induced by an M5 class flare, up to Ne(74 km)=4×1010 m−3 is obtained. The β parameter is found to range from 0.30–0.49 1/km and the reflection height H' to vary from 74–63 km. The changes in Ne(z) during the flares, within height range z=60 to 90 km are determined, as well.

Crystal Structures of F420-dependent Glucose-6-phosphate Dehydrogenase FGD1 Involved in the Activation of the Anti-tuberculosis Drug Candidate PA-824 Reveal the Basis of Coenzyme and Substrate Binding

The modified flavin coenzyme F(420) is found in a restricted number of microorganisms. It is widely distributed in mycobacteria, however, where it is important in energy metabolism, and in Mycobacterium tuberculosis (Mtb) is implicated in redox processes related to non-replicating persistence. In Mtb, the F(420)-dependent glucose-6-phosphate dehydrogenase FGD1 provides reduced F(420) for the in vivo activation of the nitroimidazopyran prodrug PA-824, currently being developed for anti-tuberculosis therapy against both replicating and persistent bacteria. The structure of M. tuberculosis FGD1 has been determined by x-ray crystallography both in its apo state and in complex with F(420) and citrate at resolutions of 1.90 and 1.95 A(,) respectively. The structure reveals a highly specific F(420) binding mode, which is shared with several other F(420)-dependent enzymes. Citrate occupies the substrate binding pocket adjacent to F(420) and is shown to be a competitive inhibitor (IC(50) 43 microm). Modeling of the binding of the glucose 6-phosphate (G6P) substrate identifies a positively charged phosphate binding pocket and shows that G6P, like citrate, packs against the isoalloxazine moiety of F(420) and helps promote a butterfly bend conformation that facilitates F(420) reduction and catalysis.

Comparison between IRI predictions and digital ionosonde measurements of hmF2 at New Delhi during low and moderate solar activity

This paper deals with the diurnal and seasonal variations of height of the peak electron density of the F2-layer (hmF2) derived from digital ionosonde measurements at a low–middle-latitude station, New Delhi (28.6°N, 77.2°E, dip 42.4°N). Diurnal and seasonal variations of hmF2 are examined and comparisons of the observations are made with the predictions of the International Reference Ionosphere (IRI-2001) model. Our study shows that during both the moderate and low solar activity periods, the diurnal pattern of median hmF2 reveals a more or less similar trend during all the seasons with pre-sunrise and daytime peaks during winter and equinox except during summer, where the pre-sunrise peak is absent. Comparison of observed median hmF2 values with the IRI during moderate and low solar activity periods, in general, reveals an IRI overestimation in hmF2 during all the seasons for local times from about 06 LT till midnight hours except during summer for low solar activity, while outside this time period, the observed hmF2 values are close to the IRI predictions. The hmF2 representation in the IRI model does not reproduce pre-sunrise peaks occurring at about 05 LT during winter and equinox as seen in the observations during both the solar activity periods. The noontime observed median hmF2 values increase by about 10–25% from low (2004–2005) to high solar activity (2001–2002) during winter and equinox, while the IRI in the same time period and seasons shows an increase of about 10–20%. During summer, however, the observed noontime median hmF2 values show a little increase with the solar activity, as compared to the IRI with an increase of about 12%.

Motions of the equatorial ionization anomaly crests imaged by FORMOSAT‐3/COSMIC

The equatorial ionization anomaly (EIA) structures and evolutions are imaged using radio occultation observation of the newly launched FORMOSAT‐3/COSMIC (F3/C) satellite constellation. Three‐dimensional ionospheric images provide unprecedented detail of the EIA structure globally. This paper presents images of the EIA structures during July–August 2006 and discusses the development and subsidence of the EIA. Clear seasonal asymmetries in both ionospheric electron density and layer height are observed. Two‐dimensional (cross section) maps at a meridian provide dynamic variations and motions of the northern and southern EIA crests. Results suggest that in addition to the asymmetric neutral composition effect, interactions between the summer‐to‐winter (transequatorial) neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the EIA crests as imaged by the F3/C.

Evidence of gravity waves into the atmosphere during the March 2006 total solar eclipse

Abstract. This study aims at providing experimental evidence, to support the hypothesis according to which the movement of the moon's shadow sweeping the ozone layer at supersonic speed, during a solar eclipse, creates gravity waves in the atmosphere. An experiment was conducted to study eclipse induced thermal fluctuations in the ozone layer (via measurements of total ozone column, ozone photolysis rates and UV irradiance), the ionosphere (Ionosonde Total Electron Content – ITEC, peak electron density height – hmF2), and the troposphere (temperature, relative humidity), before, during and after the total solar eclipse of 29 March 2006. We found the existence of eclipse induced dominant oscillations in the parameters related to the ozone layer and the ionosphere, with periods ranging between 30–40 min. Cross-spectrum analyses resulted to statistically significant square coherences between the observed oscillations, strengthening thermal stratospheric ozone forcing as the main mechanism for GWs. Additional support for a source below the ionosphere was provided by the amplitude of the oscillations in the ionospheric electron density, which increased upwards from 160 to 220 km height. Even though similar oscillations were shown in surface temperature and relative humidity data, no clear evidence for tropospheric influence could be derived from this study, due to the modest amplitude of these waves and the manifold rationale inside the boundary layer.

Edge profiles of electron temperature and density during ELMy H-mode in ohmically heated TCV plasmas

Improvement of the spatial resolution of the TCV Thomson scattering system has permitted measurements of the pedestal height and gradient of electron density and temperature profiles near the plasma boundary during ELMy H-mode. Measured profiles were fitted by a modified TANH function and characterized by pedestal height, width and gradient scale length. Taking advantage of an extended quasi-stationary phase with reproducible edge localized modes (ELMs), random sampling was used to reconstruct the time evolution during a typical ELM cycle. The measurements clearly reveal the influence of the ELM event on the edge profiles and have permitted quantification of the associated energy and particle losses. The experimental data, collected from a series of reproducible shots have provided the basis for an MHD stability analysis using the KINX code. The results confirm the type-III identification of the ELMs observed in ohmically heated H-mode plasmas in TCV.

Signature of the 29 March 2006 eclipse on the ionosphere over an equatorial station

This study documents some results of the effect of the 29 March 2006 eclipse on the ionosphere over Ilorin, Nigeria (longitude 4.57°E, latitude 8.53°N, dip 4.1°S), an equatorial station in the West African region. The maximum obscuration of the eclipse at this station was 99 percent and it occurred before midday. True height electron density profile analysis below the F2 peak was employed in the study. The effect on the E and F1 layers was a drastic decrease in electron density, with maximum decrease percentages of 60 and 68 for the E and F1 layers, respectively. A decrease in foF2 began at about 1 hour 20 min after those in the lower layers had started. Variation of electron density with height showed that the decrease in the electron density occurred through out the E and F1 heights at about the same time while that of the F2 region began at lower heights and extended progressively toward the peak of electron density height of the layer. The recovery in the E and F1 layers has already reached an advanced stage before the effect of the eclipse got to the maximum in the F2 region. A major departure of hmF2 from the normal variation was observed and discussed.