Variability of foF2 in the African equatorial ionosphere

This paper presents the impact of diurnal, seasonal and solar activity effects on the variability of ionospheric foF2 in the African equatorial latitude. Three African ionospheric stations; Dakar (14.8°N, 17.4°W, dip: 11.4°N), Ouagadougou (12.4°N, 1.5°W, dip: 2.8°N) and Djibouti (11.5°N, 42.8°E, dip: 7.2°N) were considered for the investigation. The overall aim is to provide African inputs that will be of assistance at improving existing forecasting models. The diurnal analysis revealed that the ionospheric critical frequency (foF2) is more susceptible to variability during the night-time than the day-time, with two peaks in the range; 18–38% during post-sunset hours and 35–55% during post-midnight hours. The seasonal and solar activity analyses showed a post-sunset September Equinox maximum and June Solstice maximum of foF2 variability in all the stations for all seasons. At all the stations, foF2 variability was high for low solar activity year. Overall, we concluded that equatorial foF2 variability increases with decreasing solar activity during night-time.     

Effects of physical correlations on long-distance GPS positioning and zenith tropospheric delay estimates

The global positioning system (GPS) has become an essential tool for the high precision navigation and positioning. The quality of GPS positioning results mainly depends on the model’s formulations regarding GPS observations, including both a functional model, which describes the mathematical relationships between the GPS measurements and unknown parameters, and a stochastic model, which reflects the physical properties of the measurements. Over the past two decades, the functional models for GPS measurements have been investigated in considerable detail. However, the stochastic models of GPS observation data are simplified, assuming that all the GPS measurements have the same variance and are statistically independent. Such assumptions are unrealistic. Although a few studies of GPS stochastic models were performed, they are restricted to short baselines and short time session lengths. In this paper, the stochastic modeling for GPS long-baseline and zenith tropospheric delay (ZTD) estimates with a 24-h session is investigated using the residual-based and standard stochastic models. Results show that using the different stochastic modelling methods, the total differences can reach as much as 3–6 mm in the baseline component, especially in the height component, and 10 mm in the ZTD estimation. Any misspecification in the stochastic models will result in unreliable GPS baseline and ZTD estimations. Using the residual-based stochastic model, not only the precision of GPS baseline and ZTD estimation is obviously improved, but also the baseline and ZTD estimations are closer to the reference value.     

Using GPS-SCINDA observations to study the correlation between scintillation,total electron content enhancement and depletions over the Kenyan region

This paper presents the first results of total electron content (TEC) depletions and enhancement associated with ionospheric irregularities in the low latitude region over Kenya. At the low latitude ionosphere the diurnal behavior of scintillation is driven by the formation of large scale equatorial depletions which are formed by post-sunset plasma instabilities via the Rayleigh–Taylor instability near the magnetic equator. Data from the GPS scintillation receiver (GPS-SCINDA) located at the University of Nairobi (36.8°E, 1.27°S) for March 2011 was used in this study. The TEC depletions have been detected from satellite passes along the line of sight of the signal and the detected depletions have good correspondence with the occurrence of scintillation patches. TEC enhancement has been observed and is not correlated with increases in S₄ index and consecutive enhancements and depletions in TEC have also been observed which results into scintillation patches related to TEC depletions. The TEC depletions have been interpreted as plasma irregularities and inhomogeneities in the F region caused by plasma instabilities, while TEC enhancement have been interpreted as the manifestation of plasma density enhancements mainly associated with the equatorial ionization anomaly crest over this region. Occurrence of scintillation does happen at and around the ionization anomaly crest over Kenyan region. The presence of high ambient electron densities and large electron density gradients associated with small scale irregularities in the ionization anomaly regions have been linked to the occurrence of scintillation.     

Determination of the threshold value of F10.7 in the dependence of foF2 on solar activity

By introducing the two-segmented linear regression model instead of the well known quadratic fit, we were able to describe the solar activity dependence of the F2 critical frequency. Saturation features were observed and the corresponding F10.7 values at which this phenomenon occurs were obtained for different hours. The seasonal average values were found to be around 154 sfu, 138 sfu, 177 sfu and 150 sfu for March equinox, June solstice, September equinox and December solstice respectively. These affirmed that saturation phenomenon is more pronounced at the equinoxes than solstices. On the average, the threshold value of F10.7 was obtained to be 154.5 sfu for this station in the African sector of the equatorial region.     

TEC derived from some GPS stations in Nigeria and comparison with the IRI and NeQuick models

Total electron content (TEC) measured simultaneously using Global Positioning System (GPS) ionospheric monitors installed at some locations in Nigeria during the year 2011 (Rz = 55.7) was used to study the diurnal, seasonal, and annual TEC variations. The TEC exhibits daytime maximum, seasonal variation and semiannual variations. Measured TEC were compared with those predicted by the improved versions of the International Reference Ionosphere (IRI) and NeQuick models. The models followed the diurnal and seasonal variation patterns of the observed values of TEC. However, IRI model produced better estimates of TEC than NeQuick at all locations.     

High-Precision Space Astrometry of Gamma-Ray Bursts: II. Relativistic Corrections in Converting the Time Scales

A technique for the determination of cosmic gamma-ray burst coordinates by measuring the time lag between gamma-ray burst signals on different spacecraft in the near-Sun orbits is developed. On the basis of the well-known relativistic time transformation between barycentric, geocentric, and instrumental coordinate frames of reference, the time corrections for specific spacecraft systems are calculated. Among various relativistic corrections, those are determined that are significant for the method in question.     

Method and Results of Direct Measurement of the Interstellar Neutral Helium Abundance and Isotopic Composition

One of the important astrophysical problems is the determination of the abundance of helium isotopes ³He and ⁴He in different regions of the Universe, because this abundance can reflect its history by pointing to the intensity of various possible processes of the creation and decay of light elements. This paper describes the method and results of the determination (for the first time performed by a direct method) of the helium isotopic abundance in the local interstellar medium surrounding the Solar system. The experiment was carried out on the manned Mir space station by long-term space exposure of samples of metal foil with their subsequent recovery to the Earth and detailed laboratory mass-spectrometric analysis. As a result, we succeeded in obtaining an estimation of the ⁴He concentration (about 7.5 × 10^–3 cm^–3) and the isotopic ratio ³He/⁴He (about1.7 × 10^–4) for the local interstellar medium.     

Quiet,Discrete Auroral Arcs—Observations

Space Science Reviews - The icy moons of the outer Solar System harbor potentially habitable environments for life, however, compared to the terrestrial biosphere, these environments are...     

Microwave Diagnostics of Dynamic Processes and Oscillations in Groups of Solar Coronal Magnetic Loops

Low-frequency (LF) modulations of the solar microwave radiation (37 GHz) recorded at the Metsähovi Radio Observatory, are analyzed. Since the intensity of solar microwave radiation, produced by the electron gyrosynchrotron mechanism, is dependent on a value of the background magnetic field [Dulk, G. A.: 1985, Ann. Rev. Astron. Astrophys. 23, 169–224], slow variations of the magnetic field associated with disturbances of the electric current in a radiating source, can modulate the intensity of the microwave radiation. The observed multi-track features of the LF spectra are interpreted as a signature of a complex multi-loop structure of the radiating source. Application of the equivalent electric circuit models of interacting loops allows to explain and reproduce the main dynamical features of the observed LF modulation dynamic spectra.     

Properties of Sharp and Large Changes in the Solar Wind Ion Flux (Density)

The results of a detailed study of large (by 20% and more) and sharp (faster than ten minutes) changes of the ion flux in the solar wind are presented. The data are provided by regular measurements onboard the INTERBALL-1 satellite in the period 1996–1999. Using statistical analysis, we obtained the distribution of these changes in their absolute and relative strength. It is shown that, for a considerable proportion of the events, such sharp and large changes of the ion flux (density) take place under conditions of fairly constant values of the solar wind velocity and of both the magnitude and components of the interplanetary magnetic field.