Quasi-periodic Scintillations Research Articles

Observation and modeling of quasi-periodic scintillations observed at low latitude

Quasi-periodic scintillations are characterized as primary deep fade-out infield strength, associated with regular ringing patterns before and after it. In this paper, observations of quasi-periodic scintillation using geostationary satellite (FLEETSAT) transmissions operating at frequency 250 MHz at low latitude ground station, Varanasi (geomag. lat 14° 55' N, long. 154°E) are reported. The results indicate that the quasi-periodic scintillations are most likely produced by plasma blobs/bubbles present in the E and F-region of the ionosphere. The various characteristics features of the quasi periodic scintillations are discussed after the autocorrelation, power spectrum and scintillation index analysis. The computed horizontal scale size of the quasi periodic scintillation producing irregularity varies from 100 m to 1300 m which shows that the irregularities are of intermediate-scale sizes. The spectral index obtained from the slopes of power spectrum varies from -2 to -8. The observed fading patterns, especially the modulation of the diffraction pattern (fading envelope) can be explained by considering an obstacle called radio lens in the ionosphere elongated in one direction. We have simulated successfully the amplitude versus time plot of quasi periodic scintillation patches and found that our theoretical and experimental results of quasi periodic scintillation patches compares well with each other and also with the earlier published works.

Quasiperiodic scintillation and data interpretation: Nongeophysical GPS amplitude fluctuations due to intersatellite interference

Spectrograms of high‐rate Global Positioning System (GPS) power data at the L1 frequency (1.57542 GHz) commonly show patterns of fluctuation frequency with respect to time that resemble quasiperiodic scintillation observations. These fluctuation patterns have probably not been noted in the past because of the low sampling frequencies and coarse quantization of power measurements in most GPS receivers. We demonstrate that the source of these fluctuations is usually not edge‐type diffraction from steep ionospheric gradients, as is often hypothesized for quasiperiodic scintillation, despite some features consistent with ionospheric generation. Instead, in most cases these spectrogram patterns derive from intersatellite signal interference between pairs of coarse/acquisition (C/A) codes broadcast on L1. As evidence of this mechanism, we show that most spectrogram features appear on pairs of satellite tracking channels and that the patterns can be reproduced using a multisatellite GPS simulator. We also model the spectrogram patterns, both with regard to the basic GPS C/A code correlation process and the differential Doppler shift conditions that cause the fluctuations. In this latter respect, the GPS quasiperiodic scintillation observations bear resemblance to a mechanism discussed by L. A. Hajkowicz for traditional satellite beacons. Despite the complex nature of the intersatellite interference phenomenon, we can model when the spectrogram patterns are expected to appear. Thus one can screen out these time‐varying interference patterns to find potential occurrences of actual ionospheric edge diffraction in spectrograms. The intersatellite interference phenomenon is also of general interest to GPS radio engineering.

Simultaneous observations of ionospheric quasiperiodic scintillations from short and long meridional baselines using VHF transmissions from Transit satellites

For the first time it was possible to observe regular quasiperiodic scintillations (QPS) in VHF radio-satellite transmissions from orbiting satellites simultaneously at short (2.1 km) and long (121 km) meridional baselines in the vicinity of a typical mid-latitude station (Brisbane; 27.5°S and 152.9°E geog. and 35.6° invar.lat.), using three sites (St. Lucia-S, Taringa-T in Brisbane and Boreen Pt.-B, north of Brisbane). A few pronounced quasiperiodic (QP) events were recorded showing unambiguous regular structures at the sites which made it possible to deduce a time displacement of the regular fading minimum at S, T and B. The QP structure is highly dependent on the geometry of the ray-path from a satellite to the observer which is manifested as a change of a QP event from symmetrical to non-symmetrical for stations separated by 2.1 km , and to a radical change in the structure of the event over a distance of 121 km . It is suggested the short-duration intense QP events are due to a Fresnel diffraction (or a reflection mechanism) of radio-satellite signals by a single ionospheric irregularity in a form of an ellipsoid with a large ionization gradient along the major axis. The structure of a QP event depends on the angle of viewing of the irregular blob from a radio-satellite. In view of this it is suggested that the reported variety of the ionization formation, responsible for different types of QPS, is only apparent but not real.

Radio Signatures of November 1998 Leonid Meteor on Transionospheric VHF Satellite Signal

This letter presents, to our knowledge for the first time, two cases of scintillations of the 244 MHz satellite beacon signal recorded from the geostationary satellite FLEETSATCOM (FSC) (Long: 73°E) at the Haringhata Field Station (HFS) (Lat: 22.97°N Long: 88.50°E Dip: 32°N) of the University of Calcutta on the night of November 16/17, 1998, which corresponds to the peak period of the Leonid meteor shower in terms of its rate and visual magnitude. Scintillations produced by the sporadic-E layer (E s) generated by meteoric ionization are transient and quasi-periodic in nature. This type of scintillations have much shorter durations (∼30–100 s) than those normally observed at night in the equatorial latitudes (∼5 min to couple of hours). It is characterized by a primary deep fade out in field strength, associated with regular ringing patterns before and after it. Generation of quasi-periodic scintillations is believed to be due to a small-scale (scale size ∼100–1000 m) density enhancement or blob which acts as a radio lens in the path of the transionospheric signal. Assuming one-dimensional density irregularities, the peak phase deviation introduced in the radio wave passing through is estimated. The observed scintillation patterns have been simulated considering a series of isolated ionospheric irregularities of different strengths and scale sizes. Critical frequency of the E s layer (f 0E s) measured by the Ahmedabad ionosonde (Lat: 23.03 0N Long: 72.40 0E Dip: 34°N) for the same night shows two isolated spikes which may be due to a sudden increase in plasma density caused by meteoric ionization. In addition, the meteor visual magnitude and cumulative mean flux have been calculated using the meteor intensity data available on the Internet.

Possible ambiguity in defining the ionospheric origin of quasiperiodic scintillations

Abstract It is found that quasiperiodic scintillation events (QPS) originating in the ionosphere have a close counterpart in non-ionospheric QP scintillations (QPR and QPA). The presence of QPA, resulting from an interference of radio signals from two satellites, has been known for some time but rarely referred to in publications. The addition of a new type of regular scintillations (QPR) which have an extended ringing structure, modulation envelope and central maximum, makes it necessary to evaluate the quasiperiodic scintillation data carefully. Unlike QPA, the new type of quasiperiodic scintillations can be generated by an interference effect between a ground transmitter and the Doppler-shifted radio transmission from a satellite. A close resemblance between quasiperiodic events recorded from VHF television transmission, reported to be caused by a Fresnel diffraction effect, and QPR events may indicate their common origin. A careful analysis of the different types of the regular fading, presented here, may help workers in this field to distinguish between the trivial and real types of the specific regular scintillation activity.

Characteristics of small‐scale ionospheric irregularities as deduced from scintillation observations of radio signals from satellites ETS‐2 and Polar Bear 4 at Irkutsk

This paper presents some new results on the small‐scale inhomogeneous ionospheric structure obtained at a facility for spaced‐antenna reception of transionospheric signals from ETS‐2 and Polar Bear 4 near Irkutsk (Eastern Siberia, 52°N, 104°E). A technique based on transferring time spectra of scintillations to spatial spectra using measured horizontal irregularity drift velocities is used to obtain an estimate of the mean spatial spectrum of midlatitude scintillations. Two different methods were used to determine the inclination index of the scintillation spectrum, which was found to be equal to −2, in agreement with the value recently predicted for small‐scale F region irregularities generated through mapping of small‐scale, turbulent electric fields from the E region to the F region. Drift velocities of the diffraction pattern, and also the altitudes at which ionospheric irregularities are located, agree well with results obtained by other authors for midlatitudes. Using simultaneous measurements for a geostationary satellite and an orbiting satellite, the supposition about the existence of the southern boundary of the scintillation region has been confirmed. Finally, analysis of quasi‐periodic (QP) scintillations and simultaneously determined diffraction pattern velocities is used to show that the height of isolated irregularities giving rise to QP scintillations corresponds to the maximum of the ionospheric F2 region.

Observations of random and quasi-periodic scintillations at southern mid-latitudes over a solar cycle

An extended period (1973–1985) of recording of random and Fresnel type quasi-periodic (QP) scintillations in southern mid-latitudes, using satellite beacon transmissions at a frequency of 150 MHz, has provided some new information on the morphology of scintillation-producing irregularities. It has become evident that a pronounced daytime increase of the random type of scintillations in the southern winter (at 1200–1600 LT) occurs throughout the solar cycle and becomes a distinct daytime maximum during the years of sunspot minimum. Scintillations are most intense in the pre-midnight period in the southern summer (2000–2400 LT). There is a gradual decline in scintillation activity by about 40% from the period of sunspot maximum to the period of sunspot minimum. It appears that a specific type of sporadic- E, so-called constant height Es ( Esc), is responsible for daytime scintillation activity in winter. Night-time scintillations are strongly correlated with the presence of the range-spread type of spread- F, but not so with the frequency-spread type. There are two peaks in the occurrence of QP scintillations, predominantly in the southern summer: in the late morning (0800–1000 LT) and in the pre-midnight period (2000–2200 LT). The daytime QP scintillations occur mainly polewards of the station, whereas the night-time scintillations are recorded predominantly equatorwards. There is a distinct increase in the occurrence number of QP scintillations with a decrease in the sunspot number.

Drift analysis of random and quasiperiodic scintillations in the ionosphere

Radio signals in the VHF/UHF range from the geostationary satellite ATS-6 were recorded using a system of three spaced antennas at Slough. Simultaneously, the integrated electron content (TEC) was monitored between the satellite and ground. Full correlation analysis and similar fade techniques were used to deduce the drift velocities of irregularities responsible for random and quasiperiodic (QP) ‘ringing’ scintillations. Similar drift velocities were found for the disturbances responsible for both types of scintillations at the times when QP and random scintillations occurred in a sequential pattern. A southward-drifting disturbance was responsible for rare, multiple QP scintillations which were followed by large scale fluctuations in electron density. In general, QP-scintillation-producing irregularities drifted southward, with velocities whose median magnitude and azimuth were 64 m s −1 and 178°E of N respectively. The sequential occurrence pattern of QP-random scintillations as well as the time delay between occurrences of large fluctuations in TEC and QP scintillations, appear to be consistent with a reflection model of generation of the ringing fading of the signal.

A horizontal structure of random and quasiperiodic scintillations over a wide range of latitudes

VHF amplitude transmissions, from a large number of non-synchronous satellite passes, were recorded to deduce ionospheric scintillation occurrences from low to auroral latitudes during a geomagnetically quiet period. An intense region of random scintillationproducing irregularities was centred near a mid-latitude station. Simultaneously, the records obtained from satellites moving in meridional planes indicated the presence of pronounced, regular fading, or quasiperiodic (QP) scintillations, at large zenith angles and equatorwards of the station. Other stations, positioned away from scintillationproducing irregularities, recorded signals which were free of QP scintillations. It is suggested that reflection and interference of the transmitted radio waves, due to the presence of tilted ionization surfaces embedded in the random scintillation-producing structure, are responsible for the preferential occurrence pattern of the regular fading.

Mid-latitude quasi-periodic scintillations of satellite beacon signals

Simultaneous observations of amplitude fluctuations at two separated stations and of the relative phase of the arriving signals are used to study quasi-periodic scintillations of the kind that are sometimes called ‘ringing irregularities’. Since one set of measurements is sufficient to locate the irregularity it is possible to assess the utility of the simple model proposed by Elkins and Slack by using it in a computer simulation to predict the temporal behaviour of the other observed quantities. The resulting agreement leaves no doubt that scintillations arise in the ionosphere by a mechanism having the gross features of the one suggested, although the additional redundant observations permits the derivation of information about the ionospheric irregularities not encompassed by the Elkins and Slack model. This simple geometric model which assumes specular reflection is extremely useful and the more complete scattering analysis is needed only where detailed information about the irregularity is required. Evidence is also presented for the occasional occurrence of large angle scattering from ionospheric 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.

Reply to the paper on quasi-periodic scintillations of radio satellite signals

Reply to the paper on quasi-periodic scintillations of radio satellite signals

Quasi-periodic scintillation of radio-satellite signals

Quasi-periodic scintillation of radio-satellite signals has been attributed to a variety of possible causes; these include site effects, diffraction by ionospheric irregularities, and interference between transmissions from two satellites or between a single satellite and ground transmissions. Evidence is presented showing that fading produced by interference effects can readily be distinguished from scintillation of ionospheric origin, one distinct class of which is confirmed by experiment to originate in the ionospheric E-region. The suggested grouping of all quasi-periodic amplitude fluctuations under the broad classification ‘QP Scintillations’ is not satisfactory, and this term should be restricted to fluctuations originating in the ionosphere.

Radio transmissions from two satellites as a possible cause of quasiperiodic scintillations in amplitude recordings

A number of reports have been written on the occurrence and causes of regular fadings, or quasiperiodic (QP) scintillations, in amplitude and phase of satellite radio-transmissions in the VHF range. QP scintillations have been associated with the E- or F-region ionospheric phenomena. Further analysis of the amplitude data suggests that QP scintillations may be closely linked with an interference effect of electromagnetic waves simultaneously transmitted by two satellites moving within the beamwidth of the receiving antenna. A certain type of QP scintillations could be due to an interference effect between the satellite and ground transmissions.