Vertical Electron Density Profiles Research Articles

Vertical electron content from ionograms in real time

A new technique for calculating the vertical total electron content (TEC) from ground‐based ionosonde measurements is introduced. The ionogram provides the information to directly calculate the vertical electron density profile up to the peak of the F2 layer. The profile above the peak is approximated by an α‐Chapman function with a scale height that is derived from the profile shape around the F2 peak. The ionosonde TEC, or ITEC, is then calculated as the integral from 0 to ∞ over the entire profile. ITEC values from Digisonde observations at Millstone Hill, Wallops Island, and Jicamarca are compared with incoherent scatter radar and with Faraday and TOPEX satellite TEC measurements, showing very good agreement at middle latitudes and the magnetic equator.

Observation of wave structures in the upper atmosphere by means of radio holographic analysis of the radio occultation data

A radio holographic approach, developed by Pavelyev (1998), Hocke et al. (1999), Igarashi et al. (2000), is applied to observation of wave phenomena in the upper atmosphere using Global Positioning System - Microlab-l satellite (GPS/MET) radio occultation data. In the current state the radio holography approach uses the radar focused synthetic aperture principle to obtain high spatial resolution, and to remove the interference part corresponding to scattering from the upper ionosphere. High spatial resolution and accuracy of the radio holographic method is validated by means of revealing the weak signal reflected from the sea in the GPS/MET radio occultation data. The radio holographic method gives a new possibility to measure directly the vertical gradient of the electron density altitude profile in the D-layer using the radio occultation signal. The results of the application of radio holographic analysis to two GPS/MET occultation events (07 February 1997, No. 0447, 0158), in the D-region of the ionosphere, are discussed. Wave structures in the electron density concentration with a vertical spatial period of 1.4-6 km, and variations in the electron density gradient from +5.10 9 to +8' 109 [l/(m3km)], have been retrieved from the D-layer data. The features observed in the vertical electron density profiles may be connected with breaking of gravity waves in the D-layer of the ionosphere. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

COSMIC GPS Ionospheric Sensing and Space Weather

As our civilization becomes more dependent on space based technologies, we become more vulnerable to conditions in space weather. Accurate space weather specification and forecasting require proper modeling which account for the coupling between the sun, the magnetosphere, the thermo sphere, the ionosphere and the mesosphere. In spite of the tremendous advances that have been made in understanding the physics behind different space weather phenomena, the ability to specify or predict space weather is limited due to the lack of continuous and extensive observations in these regions. Placing a constellation of GPS receivers in low-Earth orbit, such as the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), provides an extremely powerful system for continuously and extensively measuring one of these regions, the ionosphere. COSMIC, by use of GPS occultations, will make it possible to obtain continuous and global3-dimensional images of electron density, irregularities and TIDs in the ionosphere and plasmasphere. COSMIC would provide nearly 5600 globally distributed occultations per day suitable for ionospheric sensing. Occultations can be processed individually to obtain vertical profiles of electron density, with vertical resolution of ~lkm, or collectively by means of tomography or data assimilation to obtain 3-D images of electron density or irregularity structure. In this paper we describe the GPS observables for ionospheric sensing and the occultation geometry. Our presentation evolves from discussing simple to more complicated inversion techniques starting with the Abel inversion, gradient-constrained Abel inversion, tomography, and finally data assimilation. In each of these techniques, the accuracy is assessed either via examination of real data from GPSIMET or via simulation. Brief discussions of measuring ionospheric irregularities and TIDs are given.

Assessment of global TEC mapping using a three-dimensional electron density model

Dual-frequency transmissions from the Global Positioning System satellites can be used to measure and map ionospheric total electron content (TEC) on global scales. Using data exclusively from ground-based GPS networks, global ionosphere mapping has been successfully applied using either two- or three-dimensional techniques. Two-dimensional TEC maps retrieve a horizontally-varying distribution of total electron content, assuming a fixed vertical electron density profile. In three-dimensional mapping, both the horizontal and vertical distribution density are adjusted to fit the data. We describe a three-dimensional TEC mapping algorithm that uses three independent constant-density slabs stacked vertically to model the electron density, and compare with a more conventional two-dimensional approach using a single slab. One apparent benefit of the new method is reduction in a level error of the TEC maps, which decreased by 1.7 TECU using the three-dimensional retrieval on simulated data (1 TEC Unit corresponds to 10 16 electrons/m 2). Another benefit of the multilayer approach is improved slant TEC modeling. Using actual data from an equatorial site at Cocos Islands (96.8E, 12.2 S), three slab modeling improved estimates of slant TEC by a factor of 2 for elevation angles between 10 and 20° (9 versus 4.4 TECU, root-mean-square). However, the global structure of the vertical TEC retrievals we analyzed did not improve using three-dimensional modeling. This may be due to a critical approximation shared by both techniques that TEC persists unchanged at a given local time. This assumption is required to produce global maps from observations acquired from widely scattered ground receivers. Further improving the retrieval of global TEC structure with ground-based data probably requires improved dynamical models of TEC behavior. New data available from GPS receivers in low Earth orbit is also promising.

Investigations of pH-dependent domain structure of lead arachidate Langmuir-Blodgett films by means of x-ray specular and diffuse scattering and atomic force microscopy

Langmuir-Blodgett (LB) multilayer systems are found to be inhomogeneous in morphology and structure. The most characteristic feature of these multilayer films is the appearance of three-dimensional domains in which the molecules arrange themselves as a result of the film transfer onto the substrate. A series of lead arachidate (PbA2) LB multilayers have been prepared as model systems to study the dependence of film morphology on the pH value in the subphase. Atomic force microscopy (AFM) and two different x-ray scattering methods [specular (XSR) and diffuse (XDS) x-ray reflectometry] have been used to investigate the intrinsic interface properties, such as the vertical electron density profile and lateral distribution of head groups and chains, as well as the microscopic description of the interface structure, thus providing an overall picture of the investigated multilayers. With AFM, discrete height variations of domains with minimum step widths of one double layer independent of the salt concentration in the films were observed. The lateral domain size shows a dependence on pH. It was found to be maximum at pH=4.2 (pure acid) but minimum at pH 7.0 (maximum salt content). The AFM pictures were treated by a statistical analysis to extract quantities that can be compared with the x-ray results. A considerable number of Bragg maxima were observed in XSR. The vertical correlation length LZ was calculated from the angular width of Bragg maxima along 2θ and was found to vary with pH value. It follows, in general, the tendency of the domain sizes, being maximum at pH=4.8 and minimum at pH=7.0, respectively. The lateral correlation length LX has been evaluated via XDS from the half widths Δω measured by rocking the sample across a fixed 2θ. It decreases for increasing pH. LX was compared with the respective quantity of the AFM analysis. Estimated by XDS, the correlation lengths for inner interfaces and domains complement one another with the lateral length scales resulting from AFM analysis of the surface.

Improvements in target ranging by HF radar using the European ionospheric model PRIME

Target ranging by HF radar can be done by correcting the group path of target echoes for the delay due to the propagation through the ionosphere. One possibility is to simulate the propagation by ray-tracing but in this case the final accuracy obtained in target registration depends mainly on the quality of the ionospheric model used to raytrace. In this study, the PRIME ionospheric model has been used to perform ray-tracing on a 950 km path, between Toulon and Lannion, for which oblique soundings data from the SCIPION sounder are available for December 1991. The ray-tracings were performed by using the vertical electron density profiles at five points along the path calculated by PRIME for monthly median and/or instantaneous conditions. Vertical electron density profiles from the IRI model have also been considered for comparison. The results of this investigation are presented and the accuracy in target ranging is discussed as obtained for the conditions of the simulations.

Galileo radio occultation measurements of Io's ionosphere and plasma wake

Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Io's surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake was measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Io's equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s−1 at a distance of 3 Io radii from the center of Io to 57 km s−1 at 7 Io radii. The latter corresponds to corotation with Jupiter's magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Io's terminator. The ionosphere is substantial, with the peak density exceeding 50,000 cm−3 at 9 out of 10 locations and reaching a maximum of 277,000 cm−3. The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0°W) and away from (180°W) Jupiter and minima near the center of the downstream (90°W) and upstream (270°W) hemispheres. This pattern may be related to the Alfvénic current system induced by Io's motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from ∼200 km near the center of the upstream hemisphere to ∼400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202±14 K if Na+ is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location.

HF radars: Multifrequency study of refraction effects and localization of scattering

Monostatic HF radars take advantage of the refraction of radio waves in the ionosphere to achieve perpendicularity between the wave vector and the Earth's magnetic field at high latitude. The location of the backscattering region therefore depends on the horizontal and vertical ionospheric electron density profiles. Multifrequency radar data are used to infer estimates of differences in propagation path introduced by refraction without knowledge of the ionospheric plasma distribution. Because the plasma velocity due to the electric field mapped from the magnetosphere is a specific property of the ionosphere, the Doppler velocity measured on the same field line but at different probing frequencies should be identical. Differences in the radial velocity versus group delay profiles obtained at different frequencies can be attributed to differences in the propagation path. These effects are quantified by computing the correlation function between velocity profiles obtained at different frequencies. The shift corresponding to the maximum of the correlation function is a direct measure of the relative delay introduced by differences in ionospheric propagation between the two correlated frequencies. A statistical study has been conducted for several frequencies pairs using data from the Système HF d'Etude Radar Polaires et Aurorales radar. The statistical differences in group path are respectively 20 km for the 11/12‐MHz pair and 60 km for the 11/14‐MHz pair. The observed distribution of propagation path shifts have also been modeled with a ray‐tracing program. It has been shown that the observed experimental values are overestimating the shift. Computed differences, for example, 10 km for the 11/14‐Mhz pair, are lower than experimental ones. The discrepancy is shown to be a consequence of experimental conditions. Finally, errors on the location of scatters made by assuming straight‐line propagation and fixed altitude are quantified. With a judicious choice of the scattering altitude, the maximum error is of the order of 20 km.

Determination of the vertical electron-density profile in ionospheric tomography: experimental results

Determination of the vertical electron-density profile in ionospheric tomography: experimental results

Relationships between GPS-signal propagation errors and EISCAT observations

Abstract. When travelling through the ionosphere the signals of space-based radio navigation systems such as the Global Positioning System (GPS) are subject to modifications in amplitude, phase and polarization. In particular, phase changes due to refraction lead to propagation errors of up to 50 m for single-frequency GPS users. If both the L1 and the L2 frequencies transmitted by the GPS satellites are measured, first-order range error contributions of the ionosphere can be determined and removed by difference methods. The ionospheric contribution is proportional to the total electron content (TEC) along the ray path between satellite and receiver. Using about ten European GPS receiving stations of the International GPS Service for Geodynamics (IGS), the TEC over Europe is estimated within the geographic ranges -20°≤ λ ≤40°E and 32.5°≤ Φ ≤70°N in longitude and latitude, respectively. The derived TEC maps over Europe contribute to the study of horizontal coupling and transport proces- ses during significant ionospheric events. Due to their comprehensive information about the high-latitude ionosphere, EISCAT observations may help to study the influence of ionospheric phenomena upon propagation errors in GPS navigation systems. Since there are still some accuracy limiting problems to be solved in TEC determination using GPS, data comparison of TEC with vertical electron density profiles derived from EISCAT observations is valuable to enhance the accuracy of propagation-error estimations. This is evident both for absolute TEC calibration as well as for the conversion of ray-path-related observations to vertical TEC. The combination of EISCAT data and GPS-derived TEC data enables a better understanding of large-scale ionospheric processes.

Toward ionospheric tomography in Antarctica: first steps and comparison with dynasonde observations

Total electron content (TEC) measurements obtained at two Antarctic stations over nine months beginning early in 1994 have been analysed as a first step to performing ionospheric tomography. Two receiving systems were deployed at the Faraday and Halley research stations operated by the British Antarctic Survey to monitor signals from a random selection of passes of satellites in the Navy Navigational Satellite System. The resultant measurements of total electron content have been inverted and combined with ionosonde measurements of true height and foF2 to yield two-dimensional contour maps of ionospheric electron density. In spite of the poor geometry of the observations, some 130 satellite passes were found to be suitable for reconstruction using the techniques developed for ionospheric tomography. The contour maps of plasma density have been compared with independent observations of the vertical electron density profile measured by the dynasonde ionospheric sounder located at Halley. An example is presented of a deep trough investigated by the technique, illustrating the potential of the tomographic method for study of an extended spatial region of the ionosphere over inhospitable terrain.

On possible observational evidence in electron density profiles of a magnetic field in the Martian ionosphere

The purpose of this paper is to examine the effects of a horizontal magnetic field and of horizontal transport of plasma on the vertical electron density profile in the Martian ionosphere. A two‐dimensional magnetohydrodynamic model for the steady state Martian ionosphere is presented. The model takes into account the basic reactions involving O+, O2+, and CO2+ ions and vertical plasma transport as well as horizontal plasma transport along the hypothesized magnetic field lines. The good agreement between model predictions and experimental electron density radio occultation profiles supports the hypothesis of an intrinsic magnetic field of Mars.

Comparison of ionospheric data from Soviet and Chinese stations with the IRI-86 and the faim models

Median values of ionospheric data from Moscow(55.5° N, 37.3° E), Tashkent (41.3° N, 69.6° E), Beijing (40.0° N, 116.3° E) and Guangzhou (23.1° N, 113.3° E) for different years at LT: 00.00, 06.00, 12.00 and 18.00 were used to produce vertical profiles of electron density with the analytic procedure introduced by Giovanni and Radicella. These profiles were compared with the IRI-86 model profiles adjusted to the F2 experimental peak values and with those obtained with the Fully Analytic Ionospheric Model (FAIM). The main differences with the IRI model are related to the shape of the topside profile under certain conditions and to the presence of the F1 layer in the median values used. The differences with the FAIM model appears to be more marked. The results are discussed.

Comparison of plasma drift modeling to DE 2 data

Nighttime zonal plasma drifts in the equatorial region are calculated from a simplified expression involving Pedersen conductivity, geomagnetic field strength, and zonal neutral wind, integrated along flux tubes between apex altitudes of 250–1500 km. The ambient conditions correspond to solar maximum at equinox over the American sector with vertical electron density profiles derived from average drift measured over Jicamarca. Plasma drift outputs from four neutral wind models are evaluated for overall consistency and compared to similar data from the DE 2 satellite collected during solar cycle 21 from 1.5 to 3 years after the solar maximum peak. Drift velocities are displayed to compare model output against the DE 2 measurements. Model inputs of solar maximum with vertical electron drift reduced to one‐half average strength and a heuristically derived wind are found to give very good agreement with the DE 2 results. The implicit conclusion is that this final model is most representative of actual thermospheric winds for the conditions of near‐solar maximum at equinox in the nighttime sector.

A method of determining horizontal structure of the ionosphere from backscatter ionograms

We have developed a technique lor oblique backscatter sounding (OBS) ionogram inversion as a diagnostic tool for the horizontally inhomogeneous structure of the ionosphere. Input data for the method include the leading edge of a backscalter ionogram that is measured through soundings in a given direction, and the vertical electron density profile measured over the sounding station or over some other site lying in the sounding direction. The method may be useful for reconstructing the two-dimensional electron density distribution in a vertical plane aligned with the direction of sounding. The inverse problem has been solved using the Newton Konlorovich method and the Tikhonov regularization method. The algorithm we have developed was tested against model data, that is, OBS ionograms synthesized using geometrical optics calculations for different models of the inhomogeneous ionosphere. Test results demonstrate that our method converges reliably, is stable to measurement errors and provides a good accuracy of reconstruction of inhomogeneous structures with scales of 100 2000 km. This indicates that this method shows promise as an operative remote diagnostic tool for ionospheric irregularities of natural and artificial origin.

Exact ray path calculations using realistic ionospheres

The paper discusses a technique for fitting quasiparabolic layers to measured vertical electron density profiles. The method adopted is to represent individual ionospheres by the appropriate combination of E, E-F valley, F1 and F2 layers connected by joining layers. The method produces quite accurate representations of measured ionospheric profiles. The advantage of using quasiparabolic layers is that analytical solutions of various ray parameters can be obtained, including the effect on signal strength of ray tube divergence. Furthermore, tilted ionospheres can also be considered. The methods are illustrated by application to two vertical electron density profiles obtained from the same ionogram. The primary difference in the profiles is that one contains an E-F valley. Results are presented showing not only the effects of the profile differences, but also the effects of tilts, antenna patterns and ionospheric absorption.

Solar-cycle changes in the thermal structure of the Venus dayside ionosphere

Vertical profiles of electron density in the dayside ionosphere of Venus, measured by the Pioneer Venus radio occultation experiment, averaged in bins of solar zenith angle, have been used to examine the behavior of the ionospheric thermal structure with the solar cycle. It was found that the plasma scale height decreased by a factor ranging from 1.5 at 190 km to about 3.0 at 300 km altitude as the solar input went from maximum to minimum ( F 10.7 at 1 a.u. changed from 226 to 76). If the assumption of diffusive equilibrium holds above h = 180 km for both the times of solar maximum and solar minimum, similar decreases in the plasma temperature and, consequently, the electron and ion temperatures, are inferred. If, however, diffusive equilibrium conditions do not apply at the time of solar activity minimum, perhaps due to the effects of enhanced solar wind interaction, no information on the plasma temperatures is contained in the data on the topside electron density structure.

Ionospheric characteristics for IRI in real time

Within a year's time, approximately forty Lowell Digisondes will provide ionospheric characteristics in real time. Since the echo traces of the E, Es, Fl, and F2 layers are automatically identified and scaled, all standard characteristics (critical frequencies, minimum heights, MUFF2) are available. In addition, the Digisonde calculates the vertical electron density profiles, providing for the first time an extensive data set of bottomside profiles that can be used for the IRI modeling. Efforts to express the Digisonde profile in terms of LAY functions are underway.

The long‐term behavior of the main peak of the dayside ionosphere of Venus during solar cycle 21 and its implications on the effect of the solar cycle upon the electron temperature in the main peak region

One hundred four measurements of the peak electron density in the dayside ionosphere of Venus (solar zenith angle (SZA)<80°), along with 11 Venera 9–10 measurements, were analyzed in order to establish the response of the dayside peak to the varying solar EUV flux over the maximum to minimum phase of solar cycle 21 (December 1979 to December 1986). The relationship of the peak electron density normalized to SZA = 0° and the EUV flux index was found to be Nm = N150(FEUV/150)0.376, which is in agreement with theoretical predictions based on Chapman theory modified for photochemical processes. The corresponding dependence on solar zenith angle at any given value of FEUV was found to be Nm(SZA) = N0 cos0.511 (SZA). The altitude of the peak was found to be independent of either SZA or FEUV. When the CO2+ production rate PCO2+ was computed using the Venus International Reference Atmosphere (VIRA) upper atmosphere model and the EUV flux computed from Hinteregger's (1981) formulae, it was found that the theoretically expected value of 0.5 for Δ log (Nm/Te0.275)/Δ log (PCO2+) was not obtained. Instead, when the assumption was used that Te = Tn at the altitude of the peak, and the solar cycle corrections given in the VIRA model were applied, a log‐log slope of 0.454±0.010 was obtained, and a value of 0.485±0.009 resulted when the VTS3 Venus thermosphere model (Hedin et al., 1983) was used. This suggested that the assumption Te = Tn is not satisfactory. When the electron temperature Te* was extrapolated from the electron temperature probe model (Theis et al., 1980) to h = 140 km and a solar cycle variation of was assumed, the theoretically expected value of 0.500±0.010 was obtained for Δ log (Nm/Te0.275)/Δ log (PCO2+) for K = 0.30 when the VIRA model was used and K = 0.35 when the VTS3 model was employed. This result implies that the electron temperature at the altitude of the ionosphere main peak on Venus decreases by only about 25% from solar maximum to minimum, in contrast to a decrease of about 50‐75% at altitudes of above 200 km.

EDIA-EISCAT comparison between small scale F-region irregularities and large scale electron density structures at sub-auroral latitudes

Small scale sub-auroral F-region irregularities were observed on 6–7 February 1984 by the two HF radars of the EDIA experiment while the EISCAT UHF system was scanning the ionosphere between 57° and 66° invariant latitude at a slightly different longitude. The bistatic EDIA system was mainly designed to detect the F-region irregularities at sub-auroral latitudes and to measure their perpendicular velocities. This paper is devoted to an examination of the morphology of the irregularity regions detected by the HF radars and of their production mechanisms, by comparison with the horizontal and vertical electron density profiles measured by EISCAT. It is shown that decametric irregularities observed at about 360–430 km height are not associated with any large scale horizontal density gradients in the F-region (350km). However, a strong north-south gradient observed at lower altitudes (150–200km), which is likely to indicate the southern boundary of the high energy particle precipitation zone, is well correlated with the strong scattering regions observed by the HF radars. The EISCAT electron temperature measurements at 350km height also show horizontal gradients which are well correlated with the small scale F-region irregularities. We discuss implications of these observations on the mechanisms of production of irregularities in the sub-auroral F-region.