Ionospheric Electron Content Research Articles

Equatorial anomaly in ionospheric electron content and its relation to dynamo currents

Using the ATS 6 satellite radio beacon data collected over a chain of stations in the Indian zone, a study has been made of the equatorial anomaly in total electron content (TEC) and its relation to the E region currents. Two parameters, ΔHd, the difference in the magnetic field perturbation between the equator and a low‐latitude station, and Vd, the backscatter radar measurement of the draft velocity of the electrojet irregularities, are used to represent the equatorial current strength. The results, presented in the form of regression plots relating the strength of the anomaly and that of the current, indicate that the anomaly dependence on the current strength is about the same for all the three seasons. It is also noted that, regardless of the season, the anomaly crest moves toward a farther latitude from the equator with increasing current strength. The total ionization inventory, Σ TEC, of the anomaly belt is found to increase linearly with the strength of the equatorial current and this result seems to bear some significance for the correlation observed between TECmax and ΔHdmax at the equator.

The effects of neutral air winds on the electron content of the mid-latitude ionosphere and protonosphere in summer

The effects of neutral air winds on the electron content ( N T ) and other parameters of the mid-latitude ionosphere have been modelled by means of mathematical solutions of the time-dependent continuity and momentum equations for oxygen and hydrogen ions. The geometry is chosen to represent a propagation path between a geosynchronous satellite and a ground station, and the computations are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.). It is demonstrated that the electron content responds markedly to the magnitude and phase of the neutral air winds and that the effect induced by the wind on the electron content shows a consistent quantitative relationship with the wind velocity, especially during daytime. Reasonable variations in the phase and magnitude of the wind produce a range of daily electron content patterns which encompass the range of daily variations observed. The computations show that the wind gives rise to enhanced filling of the protonosphere. This shows as a depressed value of the shape factor ( F), which by definition means that a greater fraction of the ionization is at higher altitudes. The depression of F is enhanced by a poleward wind and is suppressed or even superseded by an equatorward wind through changes of the electron density distribution with altitude.

On plasma transport in the vicinity of the rings of Saturn: A Siphon Flow Mechanism

The unique combination of a rapidly rotating ionosphere and meteoroid impact ionization at the rings of Saturn provides the elements of many interesting plasma transport phenomena. One of the most significant processes might be the upward field‐aligned flow of the impact plasma at the equatorial region inside 1.6252 Rs. Such a siphoning mechanism limits the efficiency of ring plane matter recycling to a minimum and could lead to appreciable loss of ring mass in this region. At the same time, channeling of the heavy ions into the mid‐altitude ionosphere could also cause a large reduction in the ionospheric electron content as observed by the Pioneer 11 and the Voyager radio science experiments. The resulting electrodynamical coupling of the Saturnian rings with the ionosphere thus represents a completely new kind of ionospheric process than studied before.

VHF amplitude scintillations and associated electron content depletions as observed at Arequipa, Peru

Results of observations on occurrences and behavior of 137 MHz VHF amplitude scintillations and associated ionospheric electron content depletions obtained at Arequipa, Peru (16.4°S, 71.5°W, geogr.; 9°S dip) during the recent solar maximum period 1979–1980 are presented. The seasonal variation of scintillations shows the usual deep minimum during May–July and a prominent maximum in December, when scintillations also persist for a long period of time, sometimes even beyond sunrise. Ionospheric irregularities have been found to occur in distinct patches whose extent varies with season. During the December solstice the average VHF scintillation patch duration is about 6 h, while numerous patches of much shorter duration are observed in equinoctial months. The depletions in the height integrated ionospheric electron content associated with scintillations have a typical duration of 10–15 min and amplitude less than 5 × 10 16 el m −2, although depletions with an amplitude as large as 20 × 10 16 elm −2 and with a duration of more than 30 min are sometimes encountered. The pre-midnight amplitude scintillations and associated electron content depletions could be identified with range type spread- F, as established earlier, while the association of post-midnight VHF scintillations with frequency type spread- F is found to be a special feature observed during years of very high solar activity. The seasonal dependence of irregularity patch dimensions is examined in terms of variations of drifts in the post-sunset equatorial ionosphere.

The electron content of the ionosphere and the Southern boundary of diffuse Aurora

Measurements of latitudinal dependence of ionospheric electron content over Europe exhibit strong enhancements around 60° geomagnetic latitude, occuring during some winter nights. One of these events — on December 4th, 1977 — could be related to the precipitation of soft electrons (<400 eV) at the equatorward boundary of a bright diffuse Aurora.

Ionospheric electron content depletions associated with amplitude scintillations in the equatorial region

The diurnal and seasonal behavior of the depletions in the ionospheric electron content, associated with amplitude scintillations measured at Arequipa, Peru for about one year around the recent solar maximum period, are presented. The seasonal variation shows asymmetrical equinoctial maxima, the February‐April period having markedly higher occurrences. Amplitude scintillation measurements at 1.54 GHz and 257 MHz at the nearby magnetic equatorial station Huancayo, Peru also show a similar pattern. A comparison of the three sets of data indicates the simultaneous development of irregularities of different scale sizes and earlier erosion of the smaller scale irregularities.

The influence of aurora on ionospheric electron content

The ionospheric electron content was measured at La Ronge, Sask. for a variety of auroral conditions during the Pulsating Aurora Campaign in February of 1980. The two-frequency differential phase technique was used with the NNSS satellite beacons. Comparisons of optical data and the radio results indicate that for quite strong pulsations the electron content is modulated by less than 2%. Even this small change is somewhat larger than the purely temporal variations to be expected on the basis of currently accepted relaxation times in the ionosphere. If the observed fluctuations are interpreted as representing both temporal and spatial variations, good agreement is obtained with model calculations. For irregularity sizes and strengths to which the experiment is sensitive, structure was present in diffuse or patchy aurora but absent from at least some well defined forms. This suggests that the technique can be used to explore the mechanism of formation of the irregularities.

Ionospheric electron content measurements during "Waterhole"

The "Waterhole" experiment is described elsewhere by Whalen et al. This paper describes the somewhat surprising results obtained from the differential phase measurements made during the experiment. While there is evidence that the electron concentration in the immediate neighborhood of the explosion dropped as expected, the more dramatic outcome was the sudden cessation of particle precipitation. The radio measurements show that the electron concentration in the E-region below the "hole" began to decay at the time of the explosion with a rate which is consistent with recombination. It continued to decay over the time that it could be observed, about 2.5 min. It must be concluded that the particle precipitation along magnetic field lines through the hole was cut off for at least that length of time.

Night‐time enhancements of ionospheric electron content at L = 4 during substorms

Observations of the ATS‐6 radio beacon from Fairbanks show structured enhancements of the night‐time electron content during substorms. The probability of their occurrence increases with the level of magnetic disturbance, and they are most likely to be observed when the daily sum of K indices exceeds 20 in winter or 30 in summer. There is a remarkably close coincidence between these electron‐content enhancements and auroral absorption events detected by a riometer, though the relative magnitudes vary with the time of day. Electron density profiles measured with the Chatanika incoherent‐scatter radar confirm that the electron density at 90 km is enhanced during auroral absorption events. However, D and E‐region ionization is insufficient to account for the observed enhancement of the electron content, and an F‐region effect is a more likely cause. During one such event the F‐region critical frequency was 8 MHz.

Ionospheric electron content observations during the total solar eclipse of February 16, 1980

Observations on the Faraday rotation of a transionospheric VHF signal obtained from a network of four stations near the path of totality during the total solar eclipse of 16 February 1980 are reported. A small decrease of 3–4% in the total ionization has been obtained around the time of totality. Absence of any periodic structure following the eclipse indicates that the TIDs are not of significant amplitude in the present case to be detected by the Faraday rotation technique.

Ellipticity variations in Pi2 pulsations at low latitudes

A study of the temporal variations in the horizontal plane ellipticity of Pi2 pulsations recorded at a low latitude station reveals both nocturnal and annual variations. Ellipticity decreases from positive to negative through the night; the greatest decrease occurs during November and the smallest during May. The variations exhibit trends similar to those in plasmaspheric electron density and total ionospheric electron content.

Seasonal variations of the columnar electron content of the ionosphere observed in Havana from July 1974 to April 1975

Measurements of the Faraday rotation effect on ATS-6 radio beacon signals have been carried out in Havana from July 1974 to April 1975. These observations are compared with data from the ionosonde stations San José de las Lajas near Havana and Port Stanley, representing the conjugate latitude region. The day-time ionospheric electron content N F shows maxima around the equinoxes with the autumn maximum in 1974 larger than the maximum in spring 1975. Furthermore the night-time level of N F is higher in winter than in summer. The seasonal variation shows remarkable similarities to the peak electron density variation at the magnetically conjugate latitude region represented by data from the ionosonde station Port Stanley. It is suggested that an interhemispheric coupling mechanism is responsible for this similarity.

Limits on the accuracy of correction of trans‐ionospheric propagation errors by using ionospheric models based on solar and magnetic indices and local measurements

Satellite propagation data have been used to estimate the limits on the accuracy of models of the ionospheric electron content used in calculating refractive corrections. In the first series of comparisons, electron content measurements have been compared with the Penn State Mark I Ionospheric Model. The probability of making errors in estimating electron content has been derived for stations at different latitudes and different time periods. It is shown that with this model there is a 10% probability of making errors, ranging from 2.2 × 1017 m2 at an equatorial station in the time period 1600–2200 hours to less than 7 × 1016 m2 at a midlatitude station. In the second series of comparisons, the same satellite data base was used, but the comparisons were made to an ideal model for each station, obtained by fitting each data set, using a least squares analysis, to functions of the geophysical indices, local time, and day number. These models typically produced only a factor of 2 improvement in the accuracy for given error probabilities. In a third series of comparisons, horizontal gradient measurements were used to estimate the errors that would result if electron content measurements were available along a ray path separated from the desired ray path by 500, 1000, and 1500 km. Error probabilities are given for an equatorial station and a midlatitude station for conditions when the mean horizontal gradients are assumed to be unknown and for the limiting case, when the mean horizontal gradient can be assumed to be known exactly. It is concluded that major advances in ionospheric modeling for predictive purposes will require a much better understanding of the factors controlling the day‐to‐day variability.

Comparison of total electron content measurements made with the ATS-6 radio beacon over the U.S. and Europe

We compare and contrast ATS-6 radio beacon measurements made at three stations in the U.S.A. with similar measurements made in Europe the following year at two locations. It is shown that over the U.S.A. the winter plasmaspheric content reaches its maximum near 0300 LT whereas in Europe the maximum occurs near noon. The plasmaspheric content decreases with increase of magnetic activity in both continents. Winter night maxima are observed in ionospheric content in both hemispheres. Marked differences occured in magnetic storm effects and in the day-to-night ratio of ionospheric electron content.

Digital processing of ionospheric electron content data

Ionospheric electron content data contain periodicities that are produced by a diversity of sources including hydromagnetic waves, gravity waves, and lunar tides. Often these periodicities are masked by the strong daily variation in the data. Digital filtering can be used to isolate the weaker components. The filtered data can then be further processed to provide estimates of the source properties. In addition, homomorphic filtering may be used to identify nonlinear interactions in the ionosphere.

Identification of mixing frequency components in ionospheric electron content data by multiplicative homomorphic filtering

Ionospheric electron content data contain frequency components which are produced by nonlinear processes in the solar‐terrestrial environment. Nonlinear analysis, known as multiplicative homomorphic filtering, is used to identify these components in the data. The homomorphic analysis is used on electron content data taken at Honolulu and Stanford. Mixing frequencies produced by solar rotation, by tidal winds, and by unknown sources have been isolated.

The east-west asymmetry of the ionospheric electron content in south Atlantic geomagnetic anomaly region

The Ionospheric Electron Content (IEC) in the South Atlantic Geomagnetic Anomaly (SAGA) has been investigated by means of the Faraday rotation technique with the use of the VHF signal emitted from the ATS-6 radio beacon satellite during the east-west movement. The measurements were carried out at Atibaia, (46°33′W, 23°11′S) and show an increase of ~50% on the west side of the receiver in relation to the east side, principally during the night period. We suggest that this asymmetry is caused by low energy electrons ( E < 40 keV) precipitated during their longitudinal drift.

Lowest-order average effect of turbulence on atmospheric profiles derived from radio occultation

Turbulence in planetary atmospheres and ionospheres causes changes in angles of refraction of radio waves used in occultation experiments. Atmospheric temperature and pressure profiles, and ionospheric electron concentration profiles, derived from radio occultation measurements of Doppler frequency contain errors due to such angular offsets. The lowest-order average errors are derived from a geometrical-optics treatment of the radio-wave phase advance caused by the addition of uniform turbulence to an initially homogeneous medium. It is concluded that the average profile errors are small and that precise Doppler frequency measurements at two or more wavelengths could be used to help determine characteristics of the turbulence, as well as accuracy limits and possible correction terms for the profiles. However, a more detailed study of both frequency and intensity characteristics in radio and optical occultation measurements of turbulent planetary atmospheres and ionospheres is required to realize the full potential of such measurements.

A comparison of several methods of estimating the columnar electron content of the plasmasphere

Data from several different observation techniques are used to estimate the columnar electron content of the plasmasphere. The estimate is based upon the subtraction of the ionospheric electron content from the total content along the radio path from ground to the geostationary satellite ATS-6. The Radio Beacon Experiment on this satellite allows a very accurate determination of the total electron content up to the height of the satellite. For the ionospheric electron content (ground to appr. 2000 km) the following techniques are used 1. (a) Faraday effect on the ATS-6 signals, 2. (b) Differential Doppler effect on the signals of the low orbiting NNSS satellites, 3. (c) combination of bottomside and topside ionograms, and 4. (d) extrapolation of bottomside ionograms. It is shown that the combination of Group Delay and Faraday effect data on the ATS-6 RBE signals provide the most reliable estimates of the columnar electron content of the plasmasphere. Data from low orbiting satellites support this conclusion.

Determination of thermospheric quantities from simple ionospheric observations using numerical simulation

Measured ionospheric electron content and peak electron concentration data are introduced into a numerical simulation of the ionosphere to yield values of induced plasma drifts and exospheric neutral temperatures consistent with the observations. Data collected on 23–24 March 1970 on the East Coast of the U.S.A. are analyzed and the results are in agreement with incoherent radar measurements at Millstone Hill, Massachusetts. Neutral winds and meridional exospheric temperature gradients that give rise to the computed plasma drifts are calculated through the use of a dynamic model of the thermosphere.