Journal of Atmospheric and Solar-Terrestrial Physics
Global plasmaspheric TEC and its relative contribution to GPS TEC
Introduction
Global satellite navigation system's radio signals traverse through the ionized upper atmosphere of the Earth called the ionosphere and plasmasphere. These signals are refracted by the plasma, which leads to position errors due to the frequency-dependent slowing of the wave. Although correction of this effect can, in principle, be made with dual frequency receivers, many systems use single-frequency receivers, which require the use of combined ionosphere–plasmasphere electron density models to correct for these delays. Usually, such models mainly represent the plasma density in the ionospheric F2 layer. Near-real-time updates to monthly climatological models for various military and civilian users have been made available since the early 1990s. The total electron content (TEC) along the propagation path between the satellite and receivers can be estimated by monitoring the dual frequency signals transmitted from global positioning system (GPS) satellites using GPS receivers on the ground or onboard low Earth orbiting (LEO) satellites (e.g., Soicher, 1977; Klobuchar, 1997; Jakowski et al., 2005; Yizengaw et al., 2004, Yizengaw et al., 2005a, Yizengaw et al., 2005b, Yizengaw et al., 2007). Since the GPS satellites orbit at an altitude of 20,200 km (4.2 L), the signals emitted from these satellites have long paths through the tenuous hydrogen plasma of the plasmasphere. The plasmasphere is the torus of cold, dense, co-rotating plasma, surrounding the Earth out to 3–7 Re, and is populated by ionospheric outflow. The question addressed here is how much does the plasmasphere contribute to ground-based GPS TEC measurements?
Although a number of studies have addressed this issue in the last decade (e.g. Lunt et al., 1999a, Lunt et al., 1999b; Coster et al., 2003; Belehaki et al., 2004), most of these studies of the plasmaspheric contribution to the GPS TEC were not observational comparisons. They were mostly model data comparisons or observational studies from very limited geographic locations. The plasmaspheric electron content was estimated by Lunt et al. (1999a) using the combination of Sheffield University plasmasphere ionosphere model and GPS TEC recorded at a mid-latitude station. In a separate study Lunt et al. (1999b) presented the first experimental plasmaspheric electron density measurements, but only at high latitudes. They performed two independent observations, namely GPS TEC and Navy Ionospheric Monitoring System (NIMS) TEC. The plasmaspheric electron contents were then estimated by differencing the GPS TEC from the corresponding estimates of NIMS TEC. They reported that the electron content attributable to the plasmasphere decreases with increasing latitude, giving an estimate of 2 TECU for the plasmaspheric contribution at Aberystwyth (50.4°N). Breed and Goodwin (1997) determined the median plasmaspheric TEC over Salisbury (34.8°S) from the difference between GPS TEC and Faraday rotation TEC. For December 1992 they obtain a value of 7.2 TECU. Another technique of estimating the plasmaspheric electron content was by using the difference between GPS TEC and TEC calculated from a combination of a Chapman profile above the F2 peak height and electron density profiles observed by ground-based ionosonde at Athens (38°N, 23.5°E) (e.g., Belehaki et al., 2004). They found a clear diurnal variation of the plasmaspheric electron content and concluded that the plasmasphere can contribute to the GPS TEC up to 50% during nighttime and up to 10% during daytime.
According to the basic physics of plasmaspheric processes, the ionosphere in conjugate hemispheres act as sources of plasma along the interconnecting flux tube forming a plasmaspheric reservoir. There is diurnal interchange between ionosphere and plasmasphere, with downward diffusion from the latter helping maintain the nighttime F2-layer. However, the normal trend could be interrupted during geomagnetically active periods. Geomagnetic storm activity can cause rapid contraction of the plasmapause and depletion of the flux tubes inside the plasmasphere (e.g., Park, 1971). A gradual replenishment of the plasmaspheric flux tubes then takes place from the underlying ionosphere over a period of many days. The underlying physics indicates that the diurnal interchange between ionosphere and plasmasphere may be significant, particularly on the lower L-shell flux tubes.
The plasmaspheric contribution to the ground-based GPS TEC varies as a function of latitude. The contribution increases as latitude decreases due to the GPS raypath length decreasing with increasing latitude as shown in Fig. 1. The top panel in the figure shows a schematic showing the vertical raypaths between the orbital altitude of Jason-1 and GPS satellites as a function of latitude. The plot is generated using the dipole field line equation, and the blue circles represent surface of the Earth, JASON and GPS orbital altitudes. The oval-shaped blue curve depicts the plasmapause location, roughly at L=4. The red color represents the raypath length inside the pasmasphere, showing the longest distance at low latitudes compared to high latitudes. The raypath length as a function of latitude is shown in the bottom panel of Fig. 1. As can be seen in the figure, the maximum GPS raypath length occurs at the equator and decreases toward higher latitudes. Therefore, estimation of the plasmaspheric electron content contribution to GPS TEC using purely experimental measurements at a global scale, which has not previously been reported, is essential. The objective of this paper is to present the value of plasmaspheric electron content and its percentage contribution to ground-based GPS TEC and thus to the degradation of GPS navigation and communication systems using experimentally and independently obtained measurements. Ground-based GPS receivers are used to estimate vertical GPS TEC. Similarly, the GPS receiver on board the JASON satellite is used to estimate the electron content above the satellite orbital altitude (1335 km), which is purely a plasmaspheric electron content. Using the combination of these two independent measurements, the plasmaspheric electron content contribution is estimated globally during night and daytime.
Section snippets
Ground-based GPS TEC and JASON GPS TEC
The dual frequency GPS signals transmitted by GPS satellites and recorded by receivers on the ground or on board LEO satellites provide vertical TEC between the receiver and the GPS satellites (e.g., Mannucci et al., 1998; Yizengaw et al., 2004), which are orbiting at 20,200 km altitude (∼4.2 L) at ∼55° inclination. About 1000 GPS receivers around the world are used to obtain the ground-based GPS TEC for this study. Using a similar estimation technique, the plasmaspheric GPS TEC is calculated from
Results and discussion
Using the same technique as the CHAMP GPS TEC technique (Mannucci et al., 2005), JASON GPS TEC has been estimated with high accuracy. Ground-based GPS TEC values must also be calibrated for receiver and satellite differential group delay biases (Mannucci et al., 1998, Mannucci et al., 1999). Ground-based receiver biases and satellite biases are available from an ionospheric mapping system that produces daily estimates (Iijima et al., 1999). The bias of the JASON receiver is estimated by
Summary and conclusion
The estimation of percentage contribution of the plasmaspheric electron content obtained by combining the simultaneous and co-located JASON and ground-based GPS TEC data has been analyzed globally at different observational sites. The result shows a broad agreement with earlier observational and modeling studies of plasmaspheric contribution to the GPS TEC. The general conclusion to be reached from the current global investigation is that the plasmasphere contributes significantly to total TEC
Acknowledgments
This research work has been financially supported by NSF grants ATM-0524711 and ATM-0348398, and NASA grant NNX07AM22G. The authors thank the IGS for providing GPS data provision. Portions of this research were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA).
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