René Warnant

A new method for reconstruction of the vertical electron density distribution in the upper ionosphere and plasmasphere

Ground-based ionosphere sounding measurements alone are incapable of reliably modeling the topside electron density distribution above the F layer peak density height. Such information can be derived from Global Positioning System (GPS)-based total electron content (TEC) measurements. A novel technique is presented for retrieving the electron density height profile from three types of measurements: ionosonde (foF2, foE, M3000F2, hmf2), TEC (GPS-based), and O+-H+ ion transition level. The method employs new formulae based on Chapman, sech-squared, and exponential ionosphere profilers to construct a system of equations, the solution of which system provides the unknown ion scale heights, sufficient to construct a unique electron density profile at the site of measurements. All formulae are based on the assumption of diffusive equilibrium with constant scale height for each ion species. The presented technique is most suitable for middle- and high-geomagnetic latitudes and possible applications include: development, evaluation, and improvement of theoretical and empirical ionospheric models, development of similar reconstruction methods utilizing low-earth-orbiting satellite measurements of TEC, operational reconstruction of the electron density on a real-time basis, etc.

The increase of the ionospheric activity as measured by GPS

The paper outlines a method allowing to compute the TEC with a precision of about 2–3 TECU and to detect Travelling Ionospheric Disturbances using GPS measurements. We describe the solar cycle dependance of the TEC and TIDs. Since the beginning of 1998, we have observed a stronger ionospheric activity due to the increasing solar activity. This ionospheric activity is characterized by larger TEC values which are regularly reaching the level of 60 TECU and by a larger number of Travelling Ionospheric Disturbances. During the winter 1999–2000, the mean daily TEC was above 45 TECU; at solar minimum the mean daily TEC is ranging from 4 TECU to 12 TECU. In January 2000 (close to solar maximum) more than 1300 events due to TID’s were detected: it is 6.5 more than in January 1996 (at solar minimum).

The Netlander Ionosphere and Geodesy Experiment

Abstract The NEtlander Ionosphere and Geodesy Experiment (NEIGE) of the Netlander Mission to Mars has two series of scientific objectives: (1) to determine Mars orientation parameters in order to obtain information about the interior of Mars and about the seasonal mass exchange between atmosphere and ice caps; and (2) to determine the total electron content (TEC) and the scintillation of radio signals in order to study the large- and small-scale structure of the ionosphere of Mars. These two sets of information will be derived from measurements of amplitudes and Doppler shifts of radio links at UHF and X-band between the Netlander microstations on the Mars surface and an orbiter and between this orbiter and the Earth (at X-band).

Reliability of the TEC Computed Using GPS Measurements — The Problem of Hardware Biases

In this paper, we outline the procedure used at the Royal Observatory of Belgium in order to compute the TEC using GPS measurements with a precision of 2–3 TECU. This procedure requires the determination of the so-called receiver and satellite differential group delays. The combined biases (receiver + satellite) are determined on a daily basis (i.e. one solution a day) using the geometry-free combination of code observations. The method is applied to a network of 7 permanently operating Turbo Rogue receivers; the reliability of these computed biases is discussed.

A comparison between the TEC computed using GPS and ionosonde measurements

At Dourbes (in Belgium), an ionosonde operated by the Royal Meteorological Institute of Belgium produces an electron concentration profile up to the maximum of the F2 layer. The Royal Observatory of Belgium has installed a Turbo Rogue GPS receiver on the same site. The ionosonde measurements are used to compute the ionospheric electron content (IEC) above Dourbes: in a first step, numerical integration of the measured bottomside electron concentration profile gives the bottomside part and in a second step, analytical integration of a Chapman function modelling the topside electron concentration profile gives the topside part; the parameters of the Chapman function are evaluated using the information contained in the measured bottomside profile. This IEC is compared with the TEC obtained by GPS on a period of 2 years (1995 and 1996). We show that the results of both methods are in very good agreement: the mean and the standard deviation of the differences between the TEC (GPS) and the IEC (ionosonde) computed for this period are respectively 0.46 TECU and 1.72 TECU.

Space Weather influence on satellite based navigation and precise positioning

Global Navigation Satellite Systems (GNSS) are widely used to measure positions with accuracies ranging from a few mm to about 20 m. The effect of the Earth ionosphere on GNSS signal propagation is one of the main error sources which limits the accuracy and the reliability of GNSS applications. In particular, disturbed Space Weather conditions can be the origin of strong variability in the ionosphere Total Electron Content (TEC) which itself degrades the accuracy of GNSS applications. Space Weather effects on GNSS depend very much on the type of application. In this paper, we discuss the effects of Space Weather conditions on differential positioning with the Differential GPS (DGPS) technique and on relative positioning with the Real Time Kinematic (RTK) technique. We show that DGPS is affected by medium to large-scale gradients in TEC mainly observed at solar maximum when RTK will be degraded by smaller-scale ionospheric variability due to scintillations, TEC noise-like behaviour and Travelling Ionospheric Disturbances

Detection of Irregularities in the Total Electron Content Using GPS Measurements — Application to a Mid-Latitude Station

The paper presents a very simple method allowing to detect automatically medium-scale travelling ionospheric disturbances and scintillation effects by observing the high frequency changes in the so-called geometry-free combination of GPS phase measurements. The method has been applied to the GPS measurements gathered at Brussels (mid-latitude European station) during more than 8 years: only a few scintillations were detected during this period but TIDs were very regularly observed. For this reason, we have computed statistics concerning the occurrence of TIDs. The results obtained are in good agreement with the conclusions of other independent studies.

The plasma environment of Mars : from the shocked solar wind down to the ionosphere

Abstract Despite the great number of missions devoted to the exploration of the planet Mars (including: Mariner 4, in 1965; Mars 2, 3, & 5, from 1971 to 1974; Viking landers, in 1976; and Phobos 2, in 1989), actually little is known about the near-Mars plasma environment and its interaction with the interplanetary medium. Mars Global Surveyor has recently confirmed that no intrinsic magnetic field could be invoked to stand off significantly the solar wind. Nevertheless, complex localized magnetic anomalies have been reported. They could be the signature of a past intrinsic field fossilized in rocks. It has long been recognized that the Mars–solar wind interaction strongly depends on solar activity, solar zenith angle, and altitude, as well. Unfortunately, the limited space and time coverages of the observations do not allow us to characterize the full nature of interaction. The solar wind pressure is thought to be balanced by the ionosphere and/or upper atmosphere, in analogy with Venus and comets. While the atmosphere of Venus is 10 000 times denser than that of Mars near the surface, the densities of the neutral atmospheres of the two planets are indeed of the same order of magnitude at altitudes of 250– 450 km . In addition, mass loading of the solar wind by the ionization of suprathermal atmospheric neutrals is likely to play a role comparable to a comet one. Similar to Venus there is evidence that there are important inhomogeneities inside and outside the Martian ionosphere (plasma clouds and holes) and it is likely that there is also a strong axial asymmetry in the plasma distribution around the wake. Erosion processes produced by the solar wind are claimed to be important in the quantification of the Mars atmospheric mass budget. Frictions exerted on the wake planetosphere plasma have been observed. They should not be restricted to the outermost region and might also occur deep inside so that the amount of volatiles lost in the past through this phenomenon could be important.