Satellite Scintillations Research Articles

The observations of localize ionospheric scintillation structure by FORMOSAT-7/COSMIC-2 Tri-band Beacon network

The preliminary results of FORMOSAT-7/COSMIC-2 beacon data are presented in this paper, and the localized scintillation structure where the radio signals are affected by irregularities is recognized. Irregularities in the ionosphere will affect a radio transmission traveling through it. This study looked at a case of beacon satellite scintillation measurement from three stations in Chung-li (25.136N, 121.539E), Tsao-tun (23.981N, 120.697E), and Che-cheng (25.136N, 121.539E) (22.725N, 120.544E). The open-source coherent beacon receiver may receive beacon signals from beacon satellite systems such as the FORMOSAT-3/COSMIC Tri-band Beacon Transmitter (150, 400, 1066.7 MHz) or the FORMOSAT-7/COSMIC-2 Tri-band Beacon Transmitter (150, 400, 1066.7 MHz) (400, 965, and 2200 MHz). In Taiwan, the three stations were located around 360 km apart in a north–south direction. We can recognize the localize scintillation position where the radio signals are affected by abnormalities by projecting the course of the satellite’s orbit. Because the maximum potential height in the F2 layer is 350 km, the height was previously believed to be 350 km. We apply a project method to approximate the realistic height and length from the 3 stations by a scintillation case near midnight in order to improve on this assumption. As a result, the height of scintillation would be confirmed using the project approach, whether in the F layer or the E layer.

GPS phase scintillation associated with optical auroral emissions: First statistical results from the geographic South Pole

Ionospheric irregularities affect the propagation of Global Navigation Satellite System (GNSS) signals, causing radio scintillation. Particle precipitation from the magnetosphere into the ionosphere, following solar activity, is an important production mechanism for ionospheric irregularities. Particle precipitation also causes the aurorae. However, the correlation of aurorae and GNSS scintillation events is not well established in literature. This study examines optical auroral events during 2010–2011 and reports spatial and temporal correlations with Global Positioning System (GPS) L1 phase fluctuations using instrumentation located at South Pole Station. An all‐sky imager provides a measure of optical emission intensities ([OI] 557.7 nm and 630.0 nm) at auroral latitudes during the winter months. A collocated GPS antenna and scintillation receiver facilitates superimposition of auroral images and GPS signal measurements. Correlation statistics are produced by tracking emission intensities and GPS L1 σφ indices at E and F‐region heights. This is the first time that multi‐wavelength auroral images have been compared with scintillation measurements in this way. Correlation levels of up to 74% are observed during 2–3 hour periods of discrete arc structuring. Analysis revealed that higher values of emission intensity corresponded with elevated levels of σφ. The study has yielded the first statistical evidence supporting the previously assumed relationship between the aurorae and GPS signal propagation. The probability of scintillation‐induced GPS outages is of interest for commercial and safety‐critical operations at high latitudes. Results in this paper indicate that image databases of optical auroral emissions could be used to assess the likelihood of multiple satellite scintillation activity.

The application of numerical simulations in Beacon scintillation analysis and modeling

Modeling Beacon satellite scintillation data presents a number of challenges. The dominant ionospheric structure is anisotropic, and it evolves nonuniformly. Moreover, the length and the orientation of the propagation path that intercepts the structure vary continuously. Thus, even under ideal observing conditions, it is difficult to extract unambiguous driving‐point conditions from single‐receiver or multireceiver observations. Statistical models are invariably used to interpret scintillation measurements, but the statistical models themselves require a high degree of statistical uniformity that applies only to segments of the data. These challenges are well known, but evolving computer capabilities have provided new opportunities. Modern computer resources support high‐fidelity simulations that capture the three‐dimensional propagation phenomena in representative propagation environments. Because all aspects of such simulations are known or measurable, one can validate theoretical assumptions and the effectiveness of various analysis procedures. This paper reviews the theory and illustrates the numerical simulation it supports.

Spread F – an old equatorial aeronomy problem finally resolved?

Abstract. One of the oldest scientific topics in Equatorial Aeronomy is related to Spread-F. It includes all our efforts to understand the physical mechanisms responsible for the existence of ionospheric F-region irregularities, the spread of the traces in a night-time equatorial ionogram – hence its name – and all other manifestations of the same. It was observed for the first time as an abnormal ionogram in Huancayo, about 70 years ago. But only recently are we coming to understand the physical mechanisms responsible for its occurrence and its capricious day to day variability. Several additional techniques have been used to reveal the spatial and temporal characteristics of the F-region irregularities responsible for the phenomenon. Among them we have, in chronological order, radio star scintillations, trans-equatorial radio propagation, satellite scintillations, radar backscatter, satellite and rocket in situ measurements, airglow, total electron content techniques using the propagation of satellite radio signals and, recently, radar imaging techniques. Theoretical efforts are as old as the observations. Nevertheless, 32 years after their discovery, Jicamarca radar observations showed that none of the theories that had been put forward could explain them completely. The observations showed that irregularities were detected at altitudes that were stable according to the mechanisms proposed. A breakthrough came a few years later, again from Jicamarca, by showing that some of the "stable" regions had become unstable by the non-linear propagation of the irregularities from the unstable to the stable region of the ionosphere in the form of bubbles of low density plasma. A problem remained, however; the primary instability mechanism proposed, an extended (generalized) Rayleigh-Taylor instability, was too slow to explain the rapid development seen by the observations. Gravity waves in the neutral background have been proposed as a seeding mechanism to form irregularities from which the instability would grow, but the former are difficult to observe as a controlling parameter. Their actual role still needs to be determined. More recently, radar observations again have shown the existence of horizontal plasma drift velocities counter streaming the neutral wind at the steep bottom of the F-region which produces a fast growing instability from which a generalized Rayleigh-Taylor instability can grow. The mechanisms proposed would explain the rapid development of the large and medium scale irregularities that have been observed, including some seen only by radars. Nevertheless, a proper quantitative theoretical mechanism that would explain how these irregularities break into the very important meter scale ones, responsible for the radar echoes, needs to be developed. This paper makes a selective historical review of the observations and proposed theories since the phenomenon was discovered to our current understanding.

A comparison of satellite scintillation measurements with HF radar backscatter characteristics

Abstract. We examine the correspondence between high latitude ionospheric scintillation measurements made at 250MHz with the occurrence of 10MHz HF coherent radar backscatter, on 13 and 14 December 2002. We demonstrate that when the ionospheric intersection point of the scintillation measurements is co-located with significant HF radar backscatter, the observed scintillation, quantified by the S4 index, is elevated. Conversely, when the radar indicates that backscatter is observed away from the intersection point due to movements of the auroral zone, the observed scintillation is low. This suggests that scintillation is highly location-dependent, being enhanced in the auroral zone and being lower at sub-auroral latitudes. The coexistence of scintillation and HF radar backscatter, produced by ionospheric density perturbations with scale sizes of 100s of metres and ~15 m, respectively, suggests that a broad spectrum of density fluctuations is found in the auroral zone.

Field‐aligned 777.4‐nm composite airglow images of equatorial plasma depletions

We present a powerful tool to analyze two‐dimensional field‐aligned images of equatorial plasma depletions taken from mid‐latitudes. By shifting each individual image by the sum of the longitudinal offsets obtained by performing a cross‐correlation between successive images, a single composite image can be formed for the entire night. It is shown that these field‐aligned composite images give information on the observation times of the depletions associated with spread‐F as they pass by Hawaii. In addition, they can be used to estimate the formation time and location of the depletions to the west of the observing site. This technique also yields estimates of the eastward velocity and velocity shear of the depletions. We suggest that the information gained by using these images can be used to predict satellite scintillations at stations to the east of the observing station.

Comparison of the orientation of small-scale electron density irregularities and F region plasma flow direction

Abstract. Results are shown from an experimental campaign where satellite scintillation was observed at three sites at high latitudes and, simultaneously, the F region plasma flow was measured by the nearby EISCAT incoherent scatter radar. The anisotropy parameters of field-aligned irregularities are determined from amplitude scintillation using a method based on the variance of the relative logarithmic amplitude. The orientation of the anisotropy in a plane perpendicular to the geomagnetic field is compared with the direction of F region plasma flow. The results indicate that in most cases a good agreement between the two directions is obtained.Key words: Ionosphere (auroral ionosphere; ionospheric irregularities)

Characteristics of plasma structuring in the cusp/cleft region at Svalbard

Satellite scintillation, all‐sky optical imager, and digisonde observations were coordinated during a cusp campaign conducted at Ny Alesund, Svalbard (78.9°N, 11.8°E 75.7°N corrected geomagnetic latitude, over the period January 4–15, 1997. This paper is focused on a study of the distribution and dynamics of mesoscale (tens of kilometers to tens of meters) electron density irregularities in the dayside auroral region. This study has been performed at Ny Alesund, Svalbard, by measuring the effects of these irregularities on the amplitude scintillation of 250‐MHz transmissions from a quasi‐stationary polar satellite as well as the amplitude and phase scintillation of 1.6‐GHz signals from Global Positioning System (GPS) satellites. These GPS scintillation measurements were augmented by the use of dual‐frequency (1.2 and 1.6 GHz) GPS phase data acquired at the same station by the Jet Propulsion Laboratory for the International GPS Geodynamic Service (IGS). The continuous 250‐MHz scintillation observations explored the daytime auroral ionosphere 2° poleward of Ny Alesund and showed that the scintillation spectra are often broad, as may be expected for irregularities in a turbulent flow region. Such irregularity dynamics were detected poleward of the nominal cusp region over the interval of 0600–1500 magnetic local time. The period of observations included the magnetic storm of January 10–11, 1997, when GPS observations of the IGS detected polar cap patches with total electron contents of 3×1016 m−2 and large‐scale (tens of kilometers) phase variations at the GPS frequency of 1.6 GHz that corresponded to temporal gradients of 2×1016 m−2 min−1. However, amplitude scintillations at the GPS frequency of 1.6 GHz could not be detected in association with these large‐scale phase variations, indicating that the irregularities with wavelengths less than the Fresnel dimension of 400 m were below the detectable limit. This is shown to be consistent in the context of enhanced ionospheric convection determined by digisonde and scintillation spectra.

Equatorial scintillation and systems support

The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day‐to‐day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day‐to‐day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short‐term forecast system based on real‐time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250‐MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on‐line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three‐dimensional maps of irregularity structures and two‐dimensional outage maps for the region.

Parametrization of spectra of plasma bubble induced VHF satellite scintillations and its geophysical significance

Abstract. An important component of ionospheric plasma irregularity studies in the Indian low latitudes involves the study of the plasma bubbles which produce intense scintillations of the transionospheric satellite signals. Many such plasma bubble induced (PBI) scintillation events were identified while recording 244 MHz signal from the geostationary satellite Fleetsat (73°E) at Delhi (28.6°N, 77.2°E) during March-April 1991. This type of scintillations represents changes in plasma processes. These scintillations are spectrally analyzed using an autoregressive (AR) scheme, which is equivalent to maximum entropy method of spectrum analysis, amenable to extracting optimum spectral content from short data lengths (20 – 40 s). Each spectrum is assigned a level of detectability using the final prediction error (FPE) derived from the optimum filter order required to resolve the spectrum. Lower detectability together with a higher order filter indicate a higher level of coherence for the plasma irregularities (discrete structures). Consistent patterns for these scintillations emerge from the present analysis as follows: (1) the initial and final phases of a scintillation patch display quasiperiodic oscillations. Their corresponding spectra show dominant (Gaussian shaped) spectral features with detectability levels of –6 dB to –12 dB and requiring a higher order (>6) AR filter for their spectral resolution. These are most likely associated with discrete "filament-like" or "sheet-like" plasma structures that exist near the bubble walls. (2) Two main features of the scintillation spectra could be positively associated with the well-developed plasma bubble stage: (a) spectra displaying a power-law process with a single component spectral slope between 1.6 to 3.0. Generally such spectra are resolved with a 2nd order filter and have a 1 dB to 6 dB of detectability. (b) Spectra displaying a double slope, indicating an inner and an outer scale regime for the power-law irregularities. These spectra are resolved with higher order filters (>3 but <7) and possess detectability levels of –1 dB to 3 dB. These spectra display finer spectral changes, perhaps indicative of the nature of continuously evolving plasma irregularities. As an example, an analysis of a single scintillation patch is presented to highlight the geophysical significance of the present approach. Some important parameters used in the AR scheme of spectral analysis are given in the Appendix.

Parametrization of spectra of plasma bubble induced VHF satellite scintillations and its geophysical significance

Parametrization of spectra of plasma bubble induced VHF satellite scintillations and its geophysical significance

Scintillations, plasma drifts, and neutral winds in the equatorial ionosphere after sunset

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day‐to‐day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi‐Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced‐antenna satellite scintillation, digisonde, all‐sky imager, and Fabry‐Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all‐sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.

Experimental results on satellite scintillations due to field-aligned irregularities at mid-latitudes

Experiments using multi-station networks receiving signals from the VHF beacon of a geostationary satellite have been carried out in order to clarify the geometrical factor involved in ionospheric intensity scintillations due to field-aligned irregularities. The characteristics of scintillation observed in the daytime agree with the theoretical value expected for weak diffractive scattering by ionospheric irregularities with an elongation of 10 along the geomagnetic field. However, those in the night-time show much marked enhancement along the field-line due to strong refractive scattering by irregularities having the same elongation. Finally, it is shown that the geometrical factor in scintillation at mid-latitudes can be expressed as a function of the propagation angle between the radio path and the geomagnetic field in the ionosphere. The maximum values of the geometrical factor are respectively about 5 in the daytime and 14 at night.

Measurements of ionospheric electric field convection by the long‐line technique

Ionospheric E × B convections are measured by a new technique that uses satellite scintillations. The measurements are for an ionospheric region centred on 39°N, 82°W geographic or 53° invariant latitude. Results are presented for spring equinox 1980. The quiet condition eastward convection drift is approximately ViE=−12 + 43 cos (2π/24)(t ‐ 13.4). During disturbed conditions at nighttime the westward convection becomes large and highly variable. The northward perpendicular E × B convection is approximately ViN=−5 + 13 cos (2π/12)(t ‐ 10.5) + 6 cos (2π/24)(t ‐ 11.5) m/s during quiet conditions. During disturbed conditions the semidiurnal component of the northward convection increases by about a factor of 3 and becomes larger than the diurnal.

Distributions of the irregularities which produce ionospheric scintillations

The distribution of nighttime irregularities which produce satellite scintillation has been examined for a midlatitude location using a large array of receivers. The irregularities are aligned along the earth's magnetic field and appear to extend from top to bottom of the F-region, being preferentially observed near the F-region ionization peak where they produce the strongest scintillations. A new method of mapping the horizontal distribution shows patches of various shapes and sizes but with no systematic structure.

Simultaneous rocket probe, scintillation, and incoherent scatter radar observations of irregularities in the auroral zone ionosphere

Two rocket payloads carrying plasma density probes with high spatial resolution have been flown in the auroral zone during active conditions. Simultaneous Wideband satellite scintillation and Chatanika incoherent scatter radar observations were made in order to study the properties of high‐latitude irregularities and their effects on radio wave transmission. Unlike barium cloud striations and bottomside equatorial spread F, the observed power law dependence of the irregularities does not seem to be due to steepening of kilometer‐scale structures, rather, a turbulent process seems to occur. In addition the power law indexes determined both from the probe and from the scintillation measurements indicates an in situ one‐dimensional spectrum less steep than the k−2 value often reported. Both the probe and the scintillation data indicate absolute electron density fluctuations (Δn2e)½ of several times 109 m−3 during the expansion phase of an auroral substorm, with a layer thickness of several hundred kilometers. The observed S4 levels at VHF were in the range of 0.1–0.4. This level of scintillation, as well as the absolute density fluctuation levels and the power spectral density at the kilometer scale, are shown to be comparable with bottomside equatorial spread F. It is suggested that differences between the power spectral index in the present data set and the other ionospheric experiments mentioned above may be due to a highly conductive E layer and its effects upon the nonlinear evolution of irregularities. During another flight with lower magnetic activity but several bright auroral areas in the trajectory, much lower levels of absolute and relative density fluctuations were observed with a corresponding lower value for S4. Two very sharp changes in electron density were observed (e‐folding scales of 1.45 and 0.7 km) near the field line projected position of the auroral arcs. The associated density spectra were peaked at short wavelengths. The detection of very structured plasma within minutes of the poleward expansion phase of a substorm suggests that the F layer irregularities were formed in the precipitation event. On the other hand, evidence is also presented for production or enhancement of irregularities in the presence of horizontal density gradients which suggests that plasma instabilities also play a role in the production of auroral zone irregularities.

Morphological and ionospheric aspects of quasiperiodic scintillations

Mult́i-satellite scintillations, at a transmission frequency of 150 MHz, and associated vertical-incidence ionosonde data were recorded at Brisbane from November 1973–January 1976. A distinct type of regular amplitude fadings, so-called quasiperiodic (QP) scintillations, was most frequently recorded in a time interval: 2200 to 0200 LT in the southern summer, and to a lesser extent in autumn. The maximum in the occurrence number of QP scintillations coincided with the largest simultaneous increases in the intensities of nocturnal sporadic- E ( Es) and spread- F. QP scintillations were rarely recorded in winter at the time of pronounced Es activity. The majority of QP scintillations occurred at zenith angles in excess of 70°, and at north-west azimuth angles centered on 330°. The azimuth preference in the occurrence of QP scintillations is similar to the prevailing north-west direction of arrival of ionospheric reflections at Brisbane. It appears that the generation of these scintillations is associated with the frontal structure of irregularities responsible for spread- F and Es in southern midlatitudes.

The localization of ionospheric irregularities by the holographic method

The holographic method has been recently introduced to study satellite scintillations. Only one ground station is needed to receive the radio hologram. The object wave scattered or diffracted by irregularities is measured in relation to a harmonic reference wave. The resulting amplitude and phase information can be interpreted, by using the reciprocity theorem, as the one-dimensional radio hologram at the height of the satellite orbit. In this paper the mathematical theory is developed to reconstruct the image. The theory is analogous to that used in acoustical holography to form the image digitally. Since the hologram is only one dimensional, the image can be reconstructed two-dimensionally in the plane which is defined by the vector from the ground station to the satellite and the satellite velocity vector. The Fresnel-Fourier transform is used to form the image in the reconstruction plane at various distances from satellite to ground. The calculations with the computer are carried out by using the fast Fourier transform. The coefficients of the Fresnel-Fourier transform represent the spatial spectrum of the object wave at the desired distance giving the reconstructed image of the irregularities from the radio hologram. The amplitude of the spectrum corresponds to the intensity of the scattered object wave. The coordinates of the irregularities are those given at that distance where the intensity reaches its maximum value, i.e., when the image is ‘focused.’ The irregularities analyzed so far have been localized at heights between 100 and 300 km. The observed irregularity region consists of several small-scale irregularities with cross sections from 50 to 300 m. The strength of these irregularities has not been discussed in the present theory.

Geophysical investigations by scattered radio waves

The application of scattered radio waves to geophysical investigations is reviewed. The occurrence of ground scatter depends on the smoothness of the reflecting layer and limits for the inhomogeneity may be given. Direct backscatter occurs from field‐aligned irregularities in the auroral belt and movements may be derived from Doppler shift. The location and temporal movement of the two predominant precipitation belts are observed. Meteor scatter is strongly aspect sensitive and is used for deriving physical properties of the meteorites. The analysis of scattering in transequatorial short wave propagation points to a gravity wave induced system of eastward travelling field‐aligned inhomogeneities. Incoherent scatter has proved a successful tool for investigating many important parameters of the ionosphere. Satellite scintillations are helpful in mapping the irregularities of the electron content. They may be used for determining the height of the irregularities by radio wave holography. Scattering of microwaves on the ground is becoming a useful tool for geoexploration. Scattering processes in the troposphere are dealt with in a paper by C. G. Little in this issue.

Observations of intense irregularities in the polar F-region by HF backscatter and satellite scintillation measurements

Regions of intense irregularities at the northern cliff of the midlatitude electron density trough have been simultaneously observed by HF backscatter from Lindau, W. Germany and satellite scintillation measurements at Oulu, Finland. There seems to exist a close relation between these regions and the plasmasphere, as both are observed at nearly the same latitude and show the same annual and daily variation of latitude. As the two different observations were made over a longitudinal distance of about 1000 km, a comparison of the two measurements indicates that in 50 per cent of the cases the curtains of strong irregularities deviate from lines of constant invariant latitude by at most ±1.5°.