A time domain analysis, performed upon electron density data obtained on the bottomside of the F peak during equatorial spread F conditions, indicates that the observed k -2 wave number spectrum is due to sharp gradients in the medium and not to plasma turbulence. The gradients are then shown to be unstable to drift waves with a peak growth rate of about 1 s -• which occurs at perpendicular wavelengths near the ion gyroradius. These waves may be responsible for some of the intense backscatter detected by VHF radar during equatorial spread F conditions. The linear theory for collisionless drift waves with wavelengths near the ion gyroradius is discussed in a companion paper. INTRODUCTION Progress toward a better understanding of equatorial spread F has increased greatly in the past several years as a result of both experimental and theoretical research. The purpose of this paper is to discuss a unified model of equatorial spread F which organizes much of the observational data and theoretical results in a self-consistent way. Some of the ideas in this model have evolved from work by other investigators which is not readily available in the literature. G. Haerendel (unpublished manuscript, 1974), for example, in a famous preprint, discussed equatorial spread F in terms of a 'hierarchy of instabilities' in which the Rayleigh-Taylor mode was primary and resulted in the growth of shorter-wavelength modes. Hudson et al. [1973] also discussed a 'two-step' theory for equatorial spread F in which drift waves accounted for the full spectrum of irregularities. In concept, both of these developments are similar to the ideas presented here. Recent work by Woodman and Basu [1978], as well as numerous discussions with R. Woodman, has influenced this investigation. In brief, we suggest that an initial bottomside instability nonlinearly evolves to the point that very steep gradients develop and that these steepened structures are responsible for the k -• power spectra observed by probe experiments. This primary process is very likely the Rayleigh-Taylor instability •'Dungey, 1956; G. Haerendel, unpublished manuscript, 1974; Hudson and Kennel, 1975a, b], perhaps modified or enhanced by the E x B instability [Martyn, 1959; Kelley et al., 1978]. Rocket [Kelley et al., 1976], radar [Woodman and La Hoz, 1976], and satellite [McClure et al., 1977] observations indicate that some of these bottomside structures rise into the topside.We show that the observed gradients on the edges of these rising bubbles should be unstable to drift waves with a linear growth rate of about 1 s -• and a perpendicular wavelength, for maximum growth, near the ion gyroradius. We suggest that these waves are responsible for some of the intense backscatter which occurs at VHF radar frequencies.These ideas are developed in the next section and are followed with a summary and an expanded comparison of the model with theoretical and experimental work. EVIDENCE FOR AND CONSEQUENCES OF STEEPENED STRUCTURES IN EQUATORIAL SPREAD FObserved density and electric field sp...
Ionospheric scintillation is a manifestation of space weather effects that seriously affect the performance and availability of space‐based navigation and communication systems. This paper presents results from an investigation on the characteristics of the phase and amplitude scintillation of Global Positioning System signals at the L1, L2C, and L5 frequencies. Field data obtained by a scintillation monitor installed in São José dos Campos (23.1°S, 45.8°W; dip latitude 17.3°S, declination 21.4°W), Brazil, a station located near the southern crest of the equatorial ionization anomaly, were used for this purpose. The analyzed data were collected during 150 nights from November 2014 to March 2015, an epoch of moderate solar activity close to the recent solar maximum. Only measurements corresponding to an elevation mask of 30° and values above standard threshold levels were used in the analysis. Outstanding characteristics of amplitude and phase scintillation are analyzed and compared in this study. The different characteristics of the scintillation focused in this study include (1) the statistics of their occurrences at the three frequencies; (2) the local time distributions of the amplitude and phase scintillation at different intensity levels; (3) azimuth‐elevation (spatial) distributions at different levels of the standard deviation of phase fluctuations; (4) scintillation enhancement and loss of phase lock conditions due to field‐aligned (longitudinal) propagation; (5) the relationship between amplitude and phase scintillation parameters for the L1, L2C, and L5 frequencies; and (6) the frequency dependence of the amplitude and phase scintillations. Important results on these different characteristics are presented and discussed, and some outstanding problems for future investigations are suggested.
Ionospheric scintillation is a phenomenon that occurs after sunset, especially in the low-latitude region, affecting radio signals that propagate through the ionosphere. Depending on geophysical conditions, ionospheric scintillation may cause availability and precision problems to Global Navigation Satellite System users. The present work is concerned with the development of an extended model for describing the effects of the amplitude ionospheric scintillation on GPS receivers. Using the α-μ probabilistic model, introduced by previous authors in different contexts, the variance of GPS receiver tracking loop error may be estimated more realistically. The proposed model is developed with basis on the α-μ parameters and also considering correlation between amplitude and phase scintillation. Its results are interpreted to explain how a receiver may experience different error values under the influence of ionospheric conditions leading to a fixed scintillation level S 4 . The model is applied to a large experimental data set obtained at São José dos Campos, Brazil, near the peak of the equatorial anomaly during high solar flux conditions, between December 2001 and January 2002. The results from the proposed model show that depending on the α-μ pair, moderate scintillation (0.5 ≤ S 4 ≤ 0.7) may be an issue for the receiver performance. When S 4 > 0.7, the results indicate that the effects of scintillation are serious, leading to a reduction in the receiver availability for providing positioning solutions in approximately 50% of the cases.
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