Plasmaspheric Electron Content Research Articles

A new technique for mapping of total electron content using GPS network in Japan

The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

Ingesting GPS-derived TEC data into the International Reference Ionosphere for single frequency radar altimeter ionospheric delay corrections

At the University of New Brunswick (UNB), we have developed the capability to independently produce hourly global total electron content (TEC) maps from Global Positioning System (GPS) data. UNB's hourly GPS-derived global TEC maps can be ingested directly into a modified version of the International Reference Ionosphere 1995 (IRI-95) model and augmented with a plasmaspheric electron content model to update its coefficient sets. We present a technique to provide improved IRI-95 predictions by using the modified model as a sophisticated interpolator between hourly GPS-derived TEC updates. The updated IRI-95 coefficient sets will make it possible to provide ionospheric delay corrections for various applications including single frequency radar altimeter missions such as the upcoming Geosat Follow-On mission. Since the spacecraft will be orbiting “inside” the ionosphere, our GPS-updated IRI-95 electron density profile will allow us to integrate the electron densities up to the spacecraft altitude to remove the bias imposed by the ionosphere. This would not be possible using GPS-derived TEC alone since it provides integrated electron content up to the altitude of the GPS satellites (20,200 km). In this paper, we present results based on 3 days' worth of global GPS data (33 International GPS Service for Geodynamics (IGS) stations) at a medium solar activity time (year 1993) and 3 days' worth of global GPS data (74 IGS stations) at a low solar activity time (year 1995). We also compare our updated IRI-95 predictions using UNB's global TEC maps and the original IRI-95 predictions, against TOPEX/Poseidon (T/P) dual frequency altimeter-derived TEC data. Based on 3 days' worth of global GPS data during the medium solar activity time in 1993, the results show that there was better than a 9 TECU level (1 sigma) agreement in the total electron content on a global scale with the T/P-derived TEC data using the UNB technique. For the 1995 low solar activity time, our results agreed with the T/P data at better than the 5 TECU level (1 sigma). These results suggest than our method may be viable for providing ionospheric delay corrections for future single frequency altimeter missions.

Proposal for an ionosphere/plasmasphere monitoring system

Abstract. A space-based satellite system suited for long-term monitoring of the Earth's ionosphere/plasmasphere systems is proposed. The monitoring system consists of a network of radio beacon satellites capable of measuring the ionospheric and plasmaspheric electron content on a continuous base with high time resolution. It takes advantage of the geometrical relationship between the orbit of geostationary satellites and the position of the plasmapause region characterized by a steep electron density gradient. A combination of geostationary and nongeostationary satellites may explore the three-dimensional structure of the plasmasphere. Taking into account plasmaspheric characteristics some criteria for an effective arrangement of the satellites are derived and discussed. Since the plasmapause position is very sensitive to changes or distortions in the solar wind and the related geomagnetic activity, a continuous monitoring of the position of the plasmapause would be helpful in understanding solar-terrestrial relationships.

Proposal for an ionosphere/plasmasphere monitoring system

A space-based satellite system suited for long-term monitoring of the Earth's ionosphere/plasmasphere systems is proposed. The monitoring system consists of a network of radio beacon satellites capable of measuring the ionospheric and plasmaspheric electron content on a continuous base with high time resolution. It takes advantage of the geometrical relationship between the orbit of geostationary satellites and the position of the plasmapause region characterized by a steep electron density gradient. A combination of geostationary and nongeostationary satellites may explore the three-dimensional structure of the plasmasphere. Taking into account plasmaspheric characteristics some criteria for an effective arrangement of the satellites are derived and discussed. Since the plasmapause position is very sensitive to changes or distortions in the solar wind and the related geomagnetic activity, a continuous monitoring of the position of the plasmapause would be helpful in understanding solar-terrestrial relationships.

Study of ionospheric models for satellite orbit determination

A study of empirical ionospheric models was undertaken to improve the accuracy of the ionospheric correction that is needed in satellite orbit determination. Our evaluation concerns the two most widely used empirical ionospheric models (Bent and international reference ionosphere (IRI)) and suggests improvements based on comparisons with satellite and ground‐based data. In both models, the topside electron density profiles decrease too sharply with increasing altitude. Our improved IRI topside formula compares well with AEROS satellite data and incoherent scatter measurements of the Jicamarca, Peru, radar facility. For the bottomside, a model for the effective parabolic thickness is introduced, based on the detailed IRI predictions. Recent progress in modeling the F2 peak altitude and electron density is reviewed. The increase in prediction accuracy compared to older F2 models is evaluated with Ionosphere Sounding Satellite b results (for density) and incoherent scatter measurements from Millstone Hill, Massachusetts (for altitude). The plasmaspheric electron content is also taken into account. Finally, results of a METEOSAT tracking exercise are used to check the ionospheric delays calculated with the different models. The changes indicated above were found to be helpful and resulted in an overall improvement.

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.

Severe disturbances of VHF and GHz waves from geostationary satellites during a magnetic storm

Severe night-time scintillations of VHF and GHz waves (0.136, 1.7, 4 and 11.5 GHz) emitted from the three Japanese geostationary satellites were observed during a geomagnetic storm on February 15, 1978. The GHz scintillations were strongly enhanced when irregular fluctuation of total electron content (TEC) was superposed on the background TEC which was varying sharply with time. This indicates a close relationship between the scintillations and irregularities. Detailed analysis shows that the one-dimensional wavenumber power spectrum of irregularities obeys a form of k −2 and that the scintillation spectra vary as ƒ s −3 . The scintillation index, S 4, seems to have a frequency dependence of ƒ −0.5 for 0.136 ⪅ ƒ ⪅ 1.7 GHz, f −1 for 1.7 ⪅ ƒ ⪅ 4 GHz and ƒ −2 for 4 ⪅ ƒ ⪅ 11.5 GHz. It is found that this scintillation event was accompanied with large decrease in the plasmaspheric electron content, which continued for one and a half days after the storm onset, and with appearance of localized electron density trough in the F-region ionosphere.

ATS‐6 satellite radio beacon measurements at Ootacamund, India

ATS‐6 radio beacon measurements of modulation phase and Faraday rotation made at Ootacamund, India, in 1975–1976 are discussed with emphasis on the measurement and analysis errors. The modulation‐phase measurements are insensitive to the geomagnetic field and provide an accurate determination of the total columnar electron content. Comparison of modulation‐phase measurements at different frequencies shows a minor sensitivity to the ATS‐6 pitch angle. For the low geomagnetic latitude and nearly transverse propagation conditions of Ootacamund, the use of a fixed conversion coefficient gives an unreliable Faraday content. However, the Faraday rotation measurements may be used to determine the shape factor F, which provides information about the electron density altitude profile. Corrections to the measured Faraday rotation for the ATS‐6 pitch and yaw are important, particularly at night when the rotation on 140 MHz can be of the order of 10° to 20°. Thus the reduction of data to plasmaspheric electron content is not recommended. Sample data are presented showing that, in the daytime, total content is about twice that in middle latitudes. The shape factor shows a low predawn minimum indicating the nearly complete erosion of the F2 layer peak.

The solar wind influences plasmasphere electron content

MAGNETOSPHERIC plasma convection has been reviewed in detail by Axford1. A viscous-like interaction between plasma and magnetic field and magnetic-field-line merging have been suggested as mechanisms producing magnetospheric convection. In either model, convection is sustained by a dawn–dusk electric field (Econv) which has been shown2–4 to depend on solar-wind speed Vsw. Vasyliunas2 suggested a linear dependence, but Mendillo and Papagiannis3 estimated that the convection electric field varies nearly quadratically with solar wind speed—this conclusion was supported by Kivelson4. The convection field Econv and hence solar wind speed have important roles in determining the location of plasmaspause5,6. Model calculations assuming enhancement of the solar wind showed the plasmapause everywhere to be located at smaller L coordinates7 (L is the Mcllwain8 magnetic shell number). The location of the duskside bulge of the plasmapause has been shown to be inversely related to the solar wind speed3. The total plasmasphere electron content (NP) is supposed to increase with increase in L, the rate of increase being dependent on the density distribution assumed and the L values under consideration9. Thus one expects an increase in NP to be associated with a decrease in Vsw and vice versa. In this letter we give an experimental verification of this for two magnetic storms associated with large changes in Vsw, and for which NP and Vsw data are available. The results will prove useful in interpreting the disturbed time behaviour of NP.

Ionospheric and plasmaspheric effects in satellite navigation systems

A satellite navigation concept requires determination of time (or phase) delays that satellite-emitted signals experience when traversing the distance between satellite and observer. A pulse propagating this distance is slowed down somewhat from its free space velocity-by an amount directly proportional to the total number of free electrons (TEC) along its path. Ranging accuracy requirements mandate compensation for this additional signal delay. The total electron content for high-orbit satellites includes the ionospheric as well as the plasmaspheric electron content. The Radio Beacon Experiment (RBE) aboard the geostationary ATS-6 Satellite provides the opportunity to determine the ionospheric content (up to heights of \sim 1500 km), the total electron content (from observer to satellite), and the plasmaspheric content (the difference of the two). Observations of ionospheric contents were carried out at Fort Monmouth from the launch of the ATS-6 in May 1974 to its planned drift eastward in June 1975. Diurnal, day-to-day, and seasonal variations of the contents were observed. Diurnal plasmaspheric changes were smaller in percent and in absolute terms than ionospheric changes. While, in general, ionospheric contents were larger during the fall than during the spring, the plasmaspheric contents exhibited the opposite behavior. The diurnal behavior of the ratio of plasmaspheric-to-total contents exhibited variations from \sim 45 percent at night to \gsim 10 percent during the day, with rapid changes after local sunrise and after local sunset.

Radio beacon studies of the plasmasphere

The radio beacon experiments on ATS 6 are capable of measuring the electron content in the ionosphere by the Faraday rotation technique and the group delay technique (also known as the modulation phase technique). The former is weighted by the earth's magnetic field and hence is not sensitive to electrons in the plasmasphere, while the latter is equally sensitive to all electrons along the radio path. By combining these two techniques the behavior of the plasmasphere can be continuously monitored. The nature and the limitations of these two techniques are simulated in a model study which is used to interpret experimental data. In this connection it has been found that the relative plasmaspheric electron content abundance ratio is a useful quantity for the purpose of diagnosing plasmaspheric conditions. Experimentally, two periods have been picked to show the behavior of the plasmasphere: one period in August 1974, during which it was magnetically quiet, and one period in October 1974, during which three successive magnetic storms occurred. The implications of the quiet day measurements on plasmaspheric parameters are discussed. Evidence is given that supports a large diurnal variation in transition height from below 1000 km at night to approximately 2000 km in the daytime at a midlatitude station. During storms in October the plasmasphere was observed to be depleted, a condition followed by slow replenishment. The data are consistent with the inward movement of plasmapause to magnetic L shell 3.5.

Comparative ionospheric and plasmaspheric electron contents from three world regions

THE radio beacon experiment1–3 aboard the geostationary Applied Technology Satellite ATS-6 enables the determination of total electron content (TEC) to be made by two independent methods. The first—the Faraday polarisation rotation technique—has been widely used in obtaining TEC data. Faraday rotation is dependent on the Earth's magnetic field, and, since its magnitude is heavily weighted near the Earth, it is considered to provide integrated electron content values for altitudes below ∼1,500 km. The second—the dispersive-group-delay technique, in which the phase of the modulation envelope between a carrier and its sideband is compared at two frequencies—is independent of the Earth's magnetic field and thus yields the integrated electron content between the observer and the satellite signal source. The Faraday content, NF, and the dispersive-group-delay content, NT, therefore, yield the TEC up to ∼1,500 km and geostationary altitudes, respectively. The difference between NT and NF yields the content above ∼1,500 km, which is referred to as the plasmaspheric content, Np.