Strength Of Scintillations Research Articles

Challenges of predicting low-latitude ionospheric scintillations

Two regions of the globe that are most affected by ionospheric scintillations are the auroral region and the low-latitude region encompassing the dip equator. Our present-day dependence on satellite-based communication and navigation has led to a resurgence of interest in the prediction of ionospheric scintillations, which degrade the performance of such systems. In the low-latitude ionosphere, the genesis of intermediate-scale (∼100 m to a few km) irregularities, which cause the most severe scintillations on VHF to L-band trans-ionospheric radio signals, is in the growth of the Rayleigh-Taylor (R-T) plasma instability on the bottom-side of the post-sunset equatorial F layer. Theoretical developments and observations by various ground-based instruments, as well as in-situ measurements by instruments onboard rockets and satellites, have identified several parameters of the ambient ionosphere that play key roles in the occurrence of these irregularities over the dip equator. However, the presence of basic conditions for the linear growth of the Rayleigh-Taylor instability fails to explain the large day-to-day variability in the characteristics of these irregularities, which determines the latitudinal distribution and strength of scintillations, and is a result of the nonlinear evolution of the Rayleigh-Taylor instability. In this paper, the present status of the efforts at prediction of low-latitude scintillations on the basis of a multitude of observations and numerical simulation of the development of the Rayleigh-Taylor instability in the equatorial ionosphere are reviewed. In particular, this review describes how observations of ionospheric scintillations may be used to obtain information about the nonlinear evolution of the Rayleigh-Taylor instability under different ambient conditions, which is yet to be explored through numerical simulation of the phenomenon. Such studies have pointed out important gaps in our knowledge about the evolution of equatorial ionospheric irregularities that are responsible for producing scintillations on L-band signals recorded in the equatorial and low-latitude regions around the globe.

On the occurrence and strength of multi-frequency multi-GNSS Ionospheric Scintillations in Indian sector during declining phase of solar cycle 24

This study presents unique perspectives of occurrence and strength of low latitude ionospheric scintillations on multiple signals of Global Navigation Satellite System (GNSS) and its frequency dependence using continuous observation records of 780 nights. A robust comparative analysis is performed using scintillation index, S4 and its variation during pre-midnight and post-midnight duration from a GNSS receiver located at Waltair (17.7°N, 83.3°E), India, covering period from July 2014 to August 2016. The results, generally exhibit the impact of declining phase of solar cycle 24 on occurrence and strength of scintillations, which, however, is evidently different over different frequencies transmitted from different GNSS systems. A deeper quantitative analysis uniquely reveals that apart from the solar cycle and seasonal effects, the number of visible satellites of a selected GNSS markedly affect the occurrence and also the strength. Processing scheme of adopting 6 hourly time windows of pre-midnight and post-midnight brought a novel result that the strength and occurrence of strong scintillations decrease with declining solar activity during pre-midnight hours but remarkably increase for moderate and weak scintillations during post-midnight. The physical processes that dominate the post-midnight equatorial ionosphere are invoked to explain such variations that are special during declining solar activity. Finally, inter-GNSS signal analysis in terms of the effect of strong, moderate and weak scintillations is presented with due consideration of number of satellite passes affected and frequency dependence of mean S4. The quantitative results of this study emphasize for the first time effect of low latitude scintillation on GNSS signals in Indian zone under changing background solar and seasonal conditions.

L‐band scintillation activity and space‐time structure of low‐latitude UHF scintillations

Spatial correlation function of intensity scintillation patterns produced by the propagation of a UHF signal through irregularities in the nighttime low‐latitude ionosphere is deduced from an analysis of spaced receiver records of such scintillations. This analysis requires that random temporal variations of the irregularity drift speed be taken into account. It is seen from the results that the occurrence of strong scintillations on an L‐band signal requires the presence of short (∼20 m) coherence scale lengths in the UHF scintillation pattern obtained in the plane of the receiver. This condition is satisfied near the crest of the equatorial ionization anomaly (EIA) region, but not near the dip equator. In the decay phase of L‐band scintillations recorded near the crest of the EIA region, the maximum strength of these scintillations at any point in time is found to be correlated with the magnetic eastward drift speed of the pattern of intensity scintillations on an UHF signal recorded in this region, which is determined mainly by the magnetic eastward drift velocity of the ionospheric irregularites. Dependence of the corresponding strength of UHF scintillations on the drift speed indicates that toward the end of the decay phase of L‐band scintillations, the irregularity power spectrum steepens, and the large scale irregularities that remain can cause the UHF signal to be focused in the plane of the receiver, yielding UHF S4‐indices greater than one, while focusing of the UHF signal is less evident at earlier times when there is focusing of the L‐band signal.

Signal frequency dependence of ionospheric amplitude scintillations

The S4 index, which is a measure of the strength of amplitude scintillations, is assumed to vary with signal frequency f according to S4 ∝ f−n, and the frequency exponent n is derived from observed equatorial amplitude scintillations on 40‐ and 140‐MHz signals. A large data base has been used to study the variation of this exponent n(40/140) with the S4 index on the 140‐MHz signal for both daytime and nighttime scintillation events. Although the equatorial E and F region irregularities, which cause the observed scintillation events, have different characteristics, the variation of n(40/140) with S4(140) is found to be almost universal. Theoretical computation of n(40/140) for different values of the irregularity parameters yields results which are in good agreement with these observations.