Radio Wave Scintillation Research Articles

Novel Measurement Architectures Addressing Unresolved Issues Concerning Radio Wave Scintillation and Ionospheric Irregularities

Novel Measurement Architectures Addressing Unresolved Issues Concerning Radio Wave Scintillation and Ionospheric Irregularities

Interplanetary scintillation observation and space weather modelling

Interplanetary scintillation (IPS) refers to random fluctuations in radio intensity of distant small-diameter celestial object, over time periods of the order of 1 s. The scattering and scintillation of emergent radio waves are ascribed to turbulent density irregularities transported by the ubiquitous solar wind streams. The spatial correlation length of density irregularities and the Fresnel radius of radio diffraction are two key parameters in determining the scintillation pattern. Such a scintillation pattern can be measured and correlated between multi-station radio telescopes on the Earth. Using the “phase-changing screen” scenario based on the Born approximation, the bulk-flow speed and turbulent spectrum of the solar wind streams can be extracted from the single-station power spectra fitting and the multi-station cross-correlation analysis. Moreover, a numerical computer-assisted tomography (CAT) model, iteratively fit to a large number of IPS measurements over one Carrington rotation, can be used to reconstruct the global velocity and density structures in the inner heliosphere for the purpose of space weather modelling and prediction. In this review, we interpret the underlying physics governing the IPS phenomenon caused by the solar wind turbulence, describe the power spectrum and cross correlation of IPS signals, highlight the space weather application of IPS-CAT models, and emphasize the significant benefits from international cooperation within the Worldwide IPS Stations (WIPSS) network.

Space Diversity Mitigation Effects on Ionospheric Amplitude Scintillation With Basis on the Analysis of GNSS Experimental Data

Ionospheric density irregularities embedded in Equatorial Plasma Bubbles, with scale sizes varying from several hundred kilometers to several tens of meters, may cause amplitude and phase scintillation of transionospheric radio waves, degrading the performance and availability of space-based communication and navigation systems. A recent computer simulation study, based on ionospheric irregularities detected by the Planar Langmuir Probe onboard the Communication/ Navigation Outage Forecasting System satellite, analyzed the mitigation effects from space diversity on amplitude scintillation of transionospheric signals received on the ground. The present work, based on experimental data, will confirm and extend the previous results, indicating, in statistically quantitative terms, how space diversity, effective on uplink and downlink ground-satellite paths, particularly in the strong and saturated scintillation regimes, depends on Ionospheric Pierce Point dip-latitude and distance intervals, as well as on a well-known amplitude scintillation index.

Towards the possibility to combine LOFAR and GNSS measurements to sense ionospheric irregularities

Inhomogeneities within the ionospheric plasma density affect trans-ionospheric radio signals, causing radio wave scintillation in the amplitude and phase of the signals. The amount of scintillation induced by ionospheric irregularities typically decreases with the radio wave frequency. As the ionosphere affects a variety of technological systems (e.g., civil aviation, financial operations) as well as low-frequency radio astronomy observations, it is important to detect and monitor ionospheric effects with higher accuracy than currently available. Here, a novel methodology for the detection and characterization of ionospheric irregularities is established on the basis of LOFAR (Low-Frequency Array) scintillation measurements at Very High Frequency (VHF) that take into account the lack of ergodicity in the intensity fluctuations induced by scintillation. The methodology estimates the S4 scintillation index originating from irregularities with spatial scales in the inertial sub-range of electron density fluctuations in the ionosphere. The methodology is illustrated by means of observations that were collected through the Polish LOFAR stations located in Bałdy, Borowiec and Łazy: its validation was carried out by comparing LOFAR VHF scintillation observations with independent Global Navigation Satellite Systems (GNSS) observations that were collected through a high-rate receiver located near the LOFAR station in Bałdy as well as through geodetic receivers from the Polish ASG-EUPOS network. Two case studies are presented: 31 March 2017 and 28 September 2017. The comparison between LOFAR S4 observations and independent ionospheric measurements of both scintillation and rate of change of Total Electron Content (TEC) from GNSS reveals that the sensitivity of LOFAR and GNSS to ionospheric structures is different as a consequence of the frequency dependency of radio wave scintillation. Furthermore, it can be noticed that observations of LOFAR VHF scintillation can be utilised to detect plasma structures forming in the mid-latitude ionosphere, including electron density gradients occurring over spatial scales that are not necessarily detected through traditional GNSS measurements: the detection of all spatial scales is important for correct monitoring and modelling of ionospheric processes. Hence, the different sensitivity of LOFAR to ionospheric structures, in addition to traditional GNSS ionospheric measurements, allows us to expand the knowledge of ionospheric processes.

Interpretation of Radio Wave Scintillation Observed through LOFAR Radio Telescopes

Abstract Radio waves propagating through a medium containing irregularities in the spatial distribution of the electron density develop fluctuations in their intensities and phases. In the case of radio waves emitted from astronomical objects, they propagate through electron density irregularities in the interstellar medium, the interplanetary medium, and Earth’s ionosphere. The LOFAR radio telescope, with stations across Europe, can measure intensity across the VHF radio band and thus intensity scintillation on the signals received from compact astronomical objects. Modeling intensity scintillation allows the estimate of various parameters of the propagation medium, for example, its drift velocity and its turbulent power spectrum. However, these estimates are based on the assumptions of ergodicity of the observed intensity fluctuations and, typically, of weak scattering. A case study of single-station LOFAR observations of the strong astronomical source Cassiopeia A in the VHF range is utilized to illustrate deviations from ergodicity, as well as the presence of both weak and strong scattering. Here it is demonstrated how these aspects can lead to misleading estimates of the propagation medium properties, for example, in the solar wind. This analysis provides a method to model errors in these estimates, which can be used in the characterization of both the interplanetary medium and Earth’s ionosphere. Although the discussion is limited to the case of the interplanetary medium and Earth’s ionosphere, its ideas are also applicable to the case of the interstellar medium.

The dependence of the Irregularity Parameter Characterizing the Equatorial Ionospheric Irregularities on the background Ionospheric Density

The low-latitude ionosphere is characterized by large-scale instabilities in the post-sunset hours due to the distinct geometry of the earth’s magnetic field lines at the equator. The magnetic field lines are horizontal at the equator contributing to the high vertical drift velocity of the plasma bubbles growing from the bottomside of the ionospheric F-region. The phenomenon, commonly known as equatorial spread F, is an important problem in aeronomy as it can cause radio wave scintillation effects representing the most critical impacts of space weather on man-made technologies, such as satellite communications and global navigation satellite systems (GNSS). Here, we report results on the dependence of the peak heights of the irregularities at the magnetic equator, also called as apex-altitude, on solar flux by analyzing in-situ observations made on-board the Communications/Navigations Outage Forecasting System (C/NOFS) satellite mission. Our analysis indicates the median of the peak-height distributions of the irregularities increases linearly from about 491 km at solar minimum to 737 km during solar maximum. The Physics-Based Model (PBMOD) has been used to confirm the space-based observational results and we find the field-line integrated conductivity is the key parameter which controls the peak-heights of the irregularities. In this investigation, we also seek to understand the possible dependence of the irregularity parameter characterizing the equatorial ionospheric irregularities on the background ionospheric density.

Ionospheric irregularities and scintillations: a direct comparison of in situ density observations with ground-based L-band receivers

Ionospheric irregularities can affect satellite communication and navigation by causing scintillations of radio signals. The scintillations are routinely measured using ground-based networks of receivers. This study presents observations of ionospheric irregularities by Langmuir probes on the Swarm satellites. They are compared with amplitude scintillation events recorded by the Global Positioning System-Scintillation Network and Decision Aid (GPS-SCINDA) receiver installed in Mbarara (Lat: 0.6^{circ }hbox {S}, Lon: 30.8^{circ }hbox {E} , Mag. lat: 10.2^{circ }hbox {S}). The study covers the years from 2014 to 2018 when both data sets were available. It was found that the ground-based amplitude scintillations were enhanced when Swarm registered ionospheric irregularities for a large number of passes. The number of matching observations was greater for Swarm A and C which orbited at lower altitudes compared to Swarm B. However, some counterexamples, i.e., cases when in situ electron density fluctuations were not associated with any observed L-band amplitude scintillation and vice versa, were also found. Therefore, mismatches between observed irregularity structures and scintillations can occur just over a few minutes and within distances of a few tens of kilometers. The amplitude scintillation strength, characterized by the S4 index was estimated from the electron density data using the well-known phase screen model for weak scattering. The derived amplitude scintillation was on average lower for Swarm B than for A and C and less in accordance with the observed range. Irregularities at an altitude of about 450 km contribute strongly to scintillations in the L-band, while irregularities at about 510-km altitude contribute significantly less. We infer that in situ density fluctuations observed on passes over or near Mbarara may be used to indicate the risk that ionospheric radio wave scintillations occur at that site.

Simultaneous Observations of SAR Arc and Its Ionospheric Response at Subauroral Conjugate Points (L ≃ 2.5) During the St. Patrick's Day Storm in 2015

AbstractDuring the St. Patrick's Day storm on 17 March 2015, an all‐sky imager at King Sejong Station (KSS; geo: 62.2°S, 58.8°W; mag: 50.2°S) captured diffuse aurora and a stable auroral red (SAR) arc. Ground‐based Global Positioning System observations were also simultaneously conducted at KSS, but there was no significant increase in the total electron content's rate‐of‐change index or the ionospheric scintillation indices around the SAR arc region. Auroral activities including a SAR arc were also detected by the all‐sky imager at Millstone Hill Station (geo: 42.6°N, 71.4°W; mag: 52.5°N), which is magnetically conjugate to KSS. The total electron content's rate‐of‐change index map around Millstone Hill Station, too, indicates that the ionospheric irregularities occurred only near the diffuse aurora and not in the SAR arc. The northern SAR arc is broader than the southern one and also shows a multiplicity pattern, which may be due to latitude structure within the seasonally dependent midlatitude ionospheric trough. These conjugate observations, despite their hemispheric differences, validate the classical SAR arc mechanism of emission driven by heat conduction from the inner magnetosphere that does not generate small‐scale ionospheric irregularities that can affect Global Positioning System radio signals. Previous studies of SAR arcs and very high frequency radiowave scintillations did show a positive correlation.

Effect of moderate geomagnetic storms on equatorial plasma bubbles over eastern Africa in the year 2012: Evolution and electrodynamics

Equatorial plasma bubbles (EPBs) are common features of the equatorial and low-latitude ionosphere and are known to cause radio wave scintillation which leads to the degradation of communication and navigation systems. Although these structures have been studied for decades, a full understanding of their evolution and dynamics remains important for space weather mitigation purposes. In this study, we present cases of EPBs occurrences around April and July 2012 geomagnetic storm periods over the African equatorial sector. The EPBs were observed from the Communications/Navigation Outage Forecasting System (C/NOFS) and generally correlated well to the ionospheric irregularities observed from the Global Positioning System total electron content (GPS-TEC) measurements (rate of TEC change, ROT). This study revealed that the evolution of the EPBs during moderate storms is controlled by the strength of the daytime equatorial electrojet (EEJ) currents regardless of the strength of the equatorial ionization anomaly (EIA), the latter is observed during the July storm case in particular. These effects were more evident during the main and part of the early recovery phases of the geomagnetic storm days considered. However, the evening hours TEC gradients between regions of the magnetic equator and ionization crests also played roles in the existence of ionospheric irregularities.

ОСОБЕННОСТИ ПОВЕДЕНИЯ СПЕКТРАЛЬНОГО ИНДЕКСА ИОНОСФЕРНЫХ МЕРЦАНИЙ ПО НАБЛЮДЕНИЯМ КОСМИЧЕСКИХ РАДИОИСТОЧНИКОВ В ДЕКАМЕТРОВОМ ДИАПАЗОНЕ ВОЛН

PACS numbers: 94.20.dm, 94.20.Vv, 94.20 ws Purpose: With relation to radio astronomy, ionospheric scintillation of discrete cosmic radio sources is an undesirable effect. At the same time, ionospheric scintillations carry information on the ionospheric turbulence and are a separate object of study. From observations at the URAN-4 radio telescope of the power cosmic radio sources (3C144, 3C274, 3C405, and 3C461) at 20 and 25 MHz, the ionospheric scintillation parameters: index, characteristic period and spectral index were estimated. Correlation between spectral index measured from scintillation time spectrum and spectral index of space spectrum of irregularities have allowed considering the first of them as an index of ionospheric irregularity structure changing. Peculiarities of the ionospheric scintillation spectral index behavior compared with ionospheric scintillation index are analyzed for several time intervals. Design/methodology/approach: For analyzing the ionospheric scintillation data, the statistical methods were used. Findings: Estimations of ionospheric scintillation parameters were obtained during 1998–2007. It has been shown that spectral index mean values for each radio source depend on height at which it is observed that was not found for scintillation index. Comparison of yearly mean trends does not show the correlation of spectral index with the solar and geomagnetic activity indices (i.e. with the F10.7–flow of solar radiation at the wave length of 10.7 cm and Ap -index). The daily-seasonal dependence is clearly seen for the monthly mean values. In general, the maximum spectral index values were revealed at night, but small increase was also observed in day-time during 9–11 hours. For different time intervals, the behavior of ionospheric scintillation parameters obtained for the radio source been observed at greater angles of elevation (3C405 and 3C461) remarkably differ from that for the radio sources observed at lower angles of elevation. Conclusions: Using the data obtained as a result of long-term monitoring of the group of discrete cosmic radio sources at the URAN-4 radio telescope made it possible to investigate the behavior of spectral indices of ionospheric scintillations over a large time interval. The peculiarities found in the ionospheric scintillation spectrum index behavior require further analysis for explaining within the limits of present conceptions of radio wave propagation in such an irregularity medium as ionosphere. Key words: discrete radio sources, ionospheric scintillations, scintillation index, spectral index, daily-seasonal dependence Manuscript submitted 22.10.2018 Radio phys. radio astron. 2019, 24(1): 44-54 REFERENCES 1. SOLODOVNIKOV, G. K., NOVOZHYLOV, V. I. and FATKULLIN, M. N., 1990. Radio wave propagation in multiscale irregularity ionosphere . Moskow, Russia: Nauka Publ. (in Russian). 2. YEH, K. C. and LIU, C.-H., 1982. Radio Wave Scintillations in the Ionosphere. Proc. IEEE . vol. 70, no. 4, pp. 324–360. DOI: https://doi.org/10.1109/PROC.1982.12313 3. SOLODOVNIKOV, G. K., SINELNIKOV, V. M. and KROKHMALNIKOV, YE. B., 1988. Remote Sensing of the Earth’s Ionosphere Using a Beacon Satellite . Moskow, Russia: Nauka Publ. (in Russian). 4. YAKOVLEV, O. I., 1985. Radio wave propagation in space. Moskow, Russia: Nauka Publ. (in Russian). 5. DEREVYAGIN, V. G., ISAEVA, E. A., KRAVETZ, R. O., LITVINENKO, O. A. and PANISHKO, S. K., 2005. Observations of power cosmic radio sources on the radio telescope URAN-4 during 1998-2004. Astron. Astrophys. Trans . vol. 24, no. 5, pp. 421–424. DOI: 10.1080/ 10556790600631652 6. LYTVYNENKO, O. A. and PANISHKO, S. K., 2015. Seasonal variations of the ionosphere scintillation parameters obtained from the long observations of the power cosmic radio sources at decameter wave range. Odessa Astronomical Publications . vol. 28, no. 2, pp. 235–237. DOI: https://doi.org/10.18524/1810-4215.2015.28.71031 7. KARETNIKOV, V. G., ed 2008. Odessa Astronomical Calendar: 2009 . Odesa, Ukraine: Astroprint Publ, 2008. vol. 10, pp. 158–163. (in Russian). 8. SPATZ, D. E., FRANKE, S. J. and YEH, K. C., 1988. Analysis and interpretation of spaced receiver scintillation data recorded at an equatorial station. Radio Sci . vol. 23, no. 3, pp. 347–361. DOI: https://doi.org/10.1029/RS023i003p00347 9. AARONS, J., 1982. Global morphology of ionospheric scintillations. Proc. IEEE . vol. 70, no. 4, pp. 360–378. DOI: https://doi.org/10.1109/PROC.1982.12314

Ionospheric Scintillation Activity Over Ilorin, Nigeria

AbstractScintillation of radio waves in the L‐band frequency is a regular occurrence at the equatorial and auroral regions at night most especially during high solar activity periods. Scintillation is caused by plasma density irregularities, and this could cause loss of lock of Global Navigation Satellite System (GNSS) signals leading to impairment of the applications that rely on this system. A study on the occurrence of scintillation activity over Ilorin (latitude = 8.48°N, longitude = 4.67°W, and geomagnetic latitude = 1.89°S), Nigeria was done using S4 index data from NovAtel GPStation‐2 receiver (2009–2012) and NovAtel GPStation‐6 receiver (August 2013 to December 2016) which are both located at this station. The solar maximum period of the solar cycle 24 is located well within the period of this investigation; hence, this study provides opportunity to see the occurrence pattern of scintillation during different seasons as well as the pattern from low solar activity to solar maximum. The results obtained showed that scintillation occurs between 21:00 LT and 04:00 LT at the peak of the occurrence in 2014. The time window of occurrence decreases with decrease in solar activity. Similarly, scintillation activity was observed to be more regular during high solar activity and it has two peaks of occurrence in March and October. A solar activity trend was observed in scintillation occurrence; scintillation activity increases with increase in the level of solar activity.

Ionospheric TEC Weather Map Over South America

Ionospheric weather maps using the total electron content (TEC) monitored by ground based GNSS receivers over South American continent, TECMAP, have been operationally produced by INPE´s Space Weather Study and Monitoring Program (Estudo e Monitoramento Brasileiro de Clima Especial, EMBRACE) since 2013. In order to cover the whole continent, 4 GNSS receiver net-works, RBMC/IBGE, LISN, IGS and RAMSAC, in total ~140 sites, have been used. TECMAPs with a time resolution of 10 minutes are produced in 12 hour time delay. Spatial resolution of the map is rather low, varying between 50 and 500 km depending on the density of the observation points. Large day to day variabilities of the equatorial ionization anomaly (EIA) have been observed. Spatial gradient of TEC from the anomaly trough (TECu 80) causes a large ionospheric range delay in the GNSS positioning system. Ionospheric plasma bubbles (IPB), their seeding and development, could be monitored.These plasma density (spatial and temporal) variability causes not only the GNSS based positioning error but also radio wave scintillations. Monitoring of these phenomena by TEC Mapping becomes an important issue for Space Weather concern for high technology positioning system and telecommunication.

Off‐great‐circle paths in transequatorial propagation: 1. Discrete and diffuse types

AbstractThere is mounting evidence that plasma structure in nighttime equatorial F layer evolves from large‐scale wave structure (LSWS) in the bottomside F layer. This process cannot be ignored because equatorial plasma bubbles (EPBs) arise from large‐amplitude LSWS; and, because intense radiowave scintillations are associated with EPBs, understanding the LSWS‐to‐EPB process is a crucial step toward reliable Space Weather Forecasting. In this regard, the transequatorial propagation (TEP) experiment appears to be the most useful among available research instruments. After a lapse of 30 years, the TEP experiment has been resurrected; a goal of this research is to understand TEP measurements well enough so that they can be used to diagnose the LSWS‐to‐EPB process. Toward this end, new results are presented in two companion papers. Herein (P1), off‐great‐circle (OGC) propagation paths are shown to consist of two types, discrete and diffuse. The new findings include the following: (1) a generalized multireflection model that can explain most of the observed properties; (2) the discrete type is supported by multireflections from an unstructured upwelling, (3) the diffuse type is supported by reflections from plasma structure in EPBs; and (4) the observed east‐west (EW) asymmetry can be explained in terms of a distorted upwelling or plasma structure along the west wall of an upwelling. In Paper 2 (P2), a second form of observed EW asymmetry is explained in terms of plasma structure, which is not aligned with the geomagnetic field. The findings strongly confirm a close relationship between upwellings, ESF patches, and OGC paths.

''DOUBLE-HUMPED EFFECT'' IN THE TURBULENT COLLISION MAGNETIZED PLASMA

Statistical moments of the spatial power spectrum of multiple scattered ordinary and extraordinary waves in the turbulent collision magnetized plasma with aligned anisotropic electron density irregularities are investigated using modify smooth perturbation method taking into account diffraction effects. Correlation function and variances of the phase fluctuations are obtained for arbitrary correlation function of the electron density fluctuations. Double-humped Effect is investigated analytically and numerically using the anisotropic Gaussian spectral function of electron density irregularities for the polar ionospheric F-region applying the experimental data. Investigation of the anisotropy of the F layer irregularities is of great interest in ionospheric physics. It is well known that the irregularities are elongated in the direction of the geomagnetic field of lines, but especially at high latitudes they are also found to be anisotropic in the plane perpendicular to the external magnetic field vector. The anisotropy is usually investigated by correlation analysis of radio signals from satellites, observed by space receiver on the ground. The fluctuations in amplitude and phase (scintillations) of radio waves passing through the ionosphere are caused by spatial irregularities in the electron density. The spatial fluctuations in phase cause fluctuations of the angle of arrival at the observation points. Broadening of the spatial power spectrum (SPS) of scattered electromagnetic (EM) waves in the turbulent collision magnetized plasma for both power-law and anisotropic Gaussian correlation functions of electron density fluctuations was analyzed in (1) using the complex ray (-optics) approximation. Double-humped Effect, the features of the SPS, spectral width and shift of its maximum of multiple scattered EM waves in the turbulent collisionless magnetized plasma were considered in (2, 3). The influence of the collision frequency between plasma particles on the statistical characteristics of scattered EM waves in the turbulent magnetized plasma with both electron density and external magnetic field fluctuations was investigated in (4). The present paper reports the results of an analysis of the SPS, which is related to fluctuations of the radio refractive index in the F region. Analytical expressions of the correlation function of phase fluctuations of scattered both ordinary and extraordinary waves are obtained for the arbitrary correlation functions of electron density fluctuation using modify smooth perturbation method taking into account the diffraction effects. The Double-humped Effect in the collision magnetized plasma with elongated electron density irregularities is considered for the first time. Formation of a gap in the SPS of scattered radiation is analyzed numerically for the polar ionospheric F -region using the anisotropic Gaussian spectral function of electron density fluctuations. This spectrum contains angle of aligned plasma irregularities to the direction of an external magnetic field and anisotropy factor. The numerical calculation is computed based on experimental data. Finally, paper concludes the obtained results and their significance.

Observation of polar cap patches and calculation of gradient drift instability growth times: A Swarm case study

AbstractThe Swarm mission represents a strong new tool to survey polar cap patches and plasma structuring inside the polar cap. In the early commissioning phase, the three Swarm satellites were operated in a pearls‐on‐a‐string configuration making noon‐midnight transpolar passes. This provides an unparalleled opportunity to examine the potential role of the gradient drift instability (GDI) process on polar cap patches by systematically calculating GDI growth times during their transit across the pole from day to night. Steep kilometer‐scale gradients appeared in this study as initial structures that persisted during the approximate 90 min it took a patch to cross the polar cap. The GDI growth times were calculated for a selection of the steep density gradients on both the dayside and the nightside. The values ranged from 23 s to 147 s, which is consistent with recent rocket measurements in the cusp auroral region and provides a template for future studies. Growth times of the order of 1 min found both on the dayside and on the nightside support the existing view that the GDI may play a dominant role in the generation of radio wave scintillation irregularities as the patches transit the polar cap from day to night.

CIGALA: A FP7 INNOVATIVE ACTIVITY TO TACKLE THE THREAT OF IONOSPHERIC SCINTILLATION TO GNSS OPERATIONS IN LATIN AMERICA

Drifting ionospheric electron density irregularities may lead to the scintillation of transionospheric radio waves, as in the case of signals broadcast from artificial satellites. Scintillations can not only degrade signal quality but also cause receiver loss of lock on GNSS satellites, therefore posing a major threat to GNSS based applications demanding high levels of accuracy, availability and integrity. The problem is particularly acute in Latin America and will be further amplified with the next solar maximum, expected to occur in the 2012-2014 time frame. The CIGALA (Concept for Ionospheric Scintillation Mitigation for Professional GNSS in Latin America) project, led by Septentrio N.V. and cofunded by the European GNSS Supervisory Authority (GSA) through the European 7th Framework Program, will tackle this problem. The aim of the CIGALA project is to develop ionospheric scintillation mitigation countermeasures to be implemented in Septentrio's professional multi frequency multi-constellation GNSS receivers, to be tested in Latin America. The project will leverage research and development activities coordinated between European and Brazilian experts and will involve a wide scale ionospheric measurement and test campaigns that will be conducted in Brazil with the support of several local academic and industrial partners. The overall strategy adopted and the current status of the project is presented and preliminary results discussed.

Analysis of strong ionospheric scintillation events measured by means of GPS signals at low latitudes during disturbed conditions

Drifting structures characterized by inhomogeneities in the spatial electron density distribution at ionospheric heights cause the scintillation of radio waves propagating through. The fractional electron density fluctuations and the corresponding scintillation levels may reach extreme values at low latitudes during high solar activity. Different levels of scintillation were observed on experimental data collected in the Asian sector at low latitudes by means of a GPS dual frequency receiver under moderate solar activity (2005). The GPS receiver used in these campaigns was particularly modified in firmware in order to record power estimates on the C/A code as well as on the carriers L1 and L2. Strong scintillation activity was recorded in the post‐sunset period (saturatingS4 and SI as high as 20 dB). Spectral modifications and broadening was observed during high levels of scintillation possibly indicating refractive scattering taking place instead of diffractive scattering. A possible interpretation of those events was attempted on the basis of the refractive scattering theory developed by Uscinski (1968) and Booker and MajidiAhi (1981).

Study of night time VHF ionospheric scintillations near the crest of equatorial Appleton anomaly Indian Station, Udaipur (24.6°N, 73.7°E)

This paper presents the results of percentage occurrence of night time VHF ionospheric scintillations characteristics of radio beacon signals of 244/250 MHz transmitted from Fleet Satellite (positioned at 73°E Longitude) received over Udaipur (24.6°N, 73.7°E, Dip angle 35°) during different epochs of solar activity periods from March 1986 to April 2000. The long term observations of VHF scintillation phenomenon, spanning a solar cycle (1986 to 2000) which covers low, mid as well as high solar activity period, has shown the consistent nocturnal temporal hourly, seasonal and solar cycle variations in scintillation occurrences. The night time VHF radio wave scintillations has been found to be at maximum percentage occurrence during the equinoxes months and less during winter and the least during summer months in all the epochs of solar activity period. The monthly variations of percentage occurrences of VHF ionospheric scintillation activities are seen to be increased during high solar activity years and also showed that highest values of percentage of occurrences in equinoctials months during high solar activity years. The prominent feature of discrete patchy nature of night time VHF amplitude scintillations specifically, over Equatorial Appleton anomaly region, is also presented in the light of seasonal and solar activity dependence of scintillation occurrences and patches duration. The results of present observations are also compared with the earlier works of similar stations.

Modeling of ionospheric scintillation at low-latitude

The presence and movement of plasma density fluctuations in the F-region of the ionosphere are studied by monitoring phase and amplitude of radio waves propagating through the region. In this paper, we have used weak scattering theory and assumed the plasma density fluctuations to behave like phase changing diffraction screen. Appropriate relations for scintillation index S 4, and phase variance δϕ are derived and computed for different parameters of the plasma density irregularities of the ionosphere. SROSS-C2 satellite in situ measurements of plasma density fluctuations, which provide direct information about the structure and morphology of irregularities that are responsible for scintillation of radio waves, were used first time to develop a scintillation model for low latitude. It is observed that the scintillation index S 4 and phase variance δϕ depends on the strength of the plasma turbulence. Finally, the results obtained from modeling are compared and discussed with the available recent results.

Correlation of in situ measurements of plasma irregularities with ground‐based scintillation observations

We present here the first comparison of in situ ion density fluctuations measured by the Planar Langmuir Probe on Communication\Navigation Outage Forecasting System (C/NOFS) with coincident ground‐based measurements of scintillation from the Scintillation Decision Aid (SCINDA) network and coherent scatter radar located on Christmas Island (3°N, 157°W) over a 15 month period from May 2008 through June 2009. The Planar Langmuir Probe on C/NOFS measures absolute ion densities from 10 to 108 cm−3 at a rate up to 1024 Hz. The instrument is conceptually similar to ion traps and retarding potential analyzers that have flown on many past satellite and rocket missions. However, the present design includes advances in electronic capability compared with past designs. Initial results from this study show that in situ density fluctuations observed on magnetic flux tubes that pass over Christmas Island can be used as an indicator of ionospheric radio wave scintillation at that site. This is true even when the measurements are made at horizontal ranges of over 1000 km away from the ground site as long as the field line apex altitudes are less than ∼600 km.