GHz Scintillations Research Articles

Relationships between ionospheric parameters derived from ionosonde observations and characteristics of post-sunset GHz scintillation during high solar activities (2012−2013) at Sanya (18.3°N, 109.6°E), China

In this study, we presented the characteristics of post-sunset GHz scintillation occurrence and ionospheric parameters derived from ionosonde observations, including the critical frequency of F2-layer (foF2), peak height of F2-layer (hmF2), minimum virtual height of F-layer (h’F), scale height around the F2-layer peak (Hm), virtual height (h’F5) and true height (hF5) measured at 5 MHz, in high solar activity years (2012-2013) of solar cycle 24 at Sanya (18.3°N, 109.6°E; dip lat.: 12.8°N), China. Furthermore, we investigated the relationships between the equinoctial asymmetry of scintillation and these ionospheric parameters. In addition, the growth rates of Rayleigh-Taylor instability were calculated on the basis of the ionosonde measurements and models, respectively. The results showed that the equinoctial asymmetry of scintillation onset time would be associated with the scale length of vertical electron density gradient (L), which affects the growth of Rayleigh-Taylor instability at the bottom of F-layer. The seasonal variations of foF2, Hm and scale length of vertical electron density gradient would be the reason to cause the seasonal variations of scintillation occurrence, and the equinoctial asymmetry of scintillation occurrence rate over low latitude can be related to the background electron density and the vertical drifts around sunset in F-layer. The correlations between the scintillation strength represented by the daily maximum S4 and the daily maximum values of foF2, hmF2, h’F, Hm, and also the drifts over low latitudes were weak, which need to be further studied.

Signatures of equatorial plasma bubbles in VHF satellite scintillations and equatorial ionograms

Since their discovery in the 1970s, equatorial plasma bubbles (EPBs) have been invoked to explain the propagation of VHF signals on trans‐equatorial circuits at night, and blamed for highly detrimental scintillation of VHF and GHz trans‐ionospheric communications signals in equatorial regions. Over the last four decades, the properties of EPBs have been deduced by multiple techniques such as incoherent scatter radar, 630 nm airglow, depletions in GPS total electron content observations, VHF and GHz scintillations, and HF observations by ionosondes. The initiation and evolution of EPBs have by now been successfully modeled and a good understanding developed of the underlying physics. However, different communities tend to concentrate on a single observing technique, without regard to whether the different techniques provide a consistent physical picture. In contrast, this paper discusses two very different types of observations made on a night‐by‐night basis during the COPEX campaign of late 2002 in Brazil, namely, VHF scintillations and ionograms, and shows that the two methods of observation can provide a consistent interpretation of the properties of EPBs. For example, an EPB seen as an eastward drifting scintillation event can also be seen as an extra ionogram reflection trace that moves closer to and then away from the ionosonde site. The scintillations are attributed to strong gradients across the walls of an EPB, whereas the extra ionogram traces are attributed to oblique reflection of the ionosonde signals from the walls of the EPB.

The linear dependence of GHz scintillation on electron density observed in the equatorial anomaly

Abstract. The dependence of scintillation caused by equatorial bubbles on the electron density in the post sunset anomaly cannot generally be determined because the irregularities that produce the scintillation obscure the measurement of the electron density. This work addresses this problem with the introduction of a new technique based on the finding that anomaly electron density is so regular that its LT distribution can be fit by a polynomial over its post-sunset extent. By means of this polynomial, gaps in the data caused by the irregularities can be inferred where otherwise not measurable. Observations are of amplitude scintillation at 1.5 GHz from the Marisat satellite and of NmF2 measured by ionospheric soundings at Ascension I. in Mar 2001 during solar maximum. The result is that the well defined maximum of the S4 scintillation index is a linear function of coinciding NmF2. Evidence that supports this result has been found in 3 differing cases: The first finds that both S4 and NmF2 decrease linearly with LT; the second, that both occurrence of scintillation and NmF2 increase linearly with F10.7 cm solar flux; the third finds a linear relation between occurrence of scintillation at 137 MHz and NmF2. The importance of these results is that scintillation is caused by the bubble, but its magnitude is determined by the magnitude of the electron density that the bubble intersects. A further implication is that, given the unpredictability of bubbles, if NmF2 could be predicted, the maximum level of S4 possible could be predicted if a bubble were to occur.

Equatorial and low latitude spread-F irregularity characteristics over the Indian region and their prediction possibilities

To study the occurrence characteristics of equatorial spread-F irregularities and their latitudinal extent, simultaneous digital ionosonde data (January–December 2001) from Trivandrum (8.2°N), Waltair (17.7°N) and Delhi (28.6°N) and 4 GHz scintillation data from Sikandarabad (26.8°N) and Chenglepet (10.4°N), and 250 MHz scintillation data from Bhopal (23.2°N) for equinoxes period are analysed. It is noted that except summer months, occurrence of spread F is always maximum at Trivandrum, minimum at Delhi and moderate at Waltair. During equinoxes and winter months. Their occurrences at higher latitude station are always conditional to their prior occurrences at lower latitudes indicating their association with the generation of equatorial plasma bubble and associated irregularities. Scintillation occurrences also follow the similar pattern. During the summer months, the spread-F occurrences are highest at equatorial location Trivandrum, moderate at Delhi and minimum at Waltair and seem to be caused by irregularities generated locally especially over Delhi. To gain forecasting capability, night-to-night occurrences of spread-F/scintillation at these locations are examined in relation to post sunset rise of h’F and upward ExB drift velocity over the magnetic equator using Trivandrum ionosonde data. It is noted that except the summer months, the spread-F at Trivandrum, Waltair and Delhi are observed only when equatorial ExB (h’F) is more than about 15 m/s (325 km), 20 m/s (350 km) and 25 m/s (375 km), respectively. With these threshold values their corresponding success rate of predictions are more than 90%, 50% and 15% at the respective locations. Whereas in the case of GHz scintillations near equator are observed only when ExB (h’F) is more than 15 m/s (325 km), whereas for low latitude, the same should be 30 m/s (400 km) and their success rate of prediction is about 90% and 30%, respectively. The intensity of 4 GHz scintillation at low latitude is also found to be positively correlated with equatorial upward ExB drift velocity values, whereas correlation is poor with that of equatorial scintillations. In conclusions, near magnetic equator threshold values of ExB or h’F can be successfully used for the night-to-night prediction of spread-F/scintillations occurrences, whereas these are necessary but not sufficient for their prediction at higher latitudes. For that some other controlling parameters like background electron density, neutral winds, gravity waves, etc. should also be examined.

GPS TEC and scintillation measurements from the polar ionosphere during the October 2003 storm

Severe ionospheric storms occurred at the end of October 2003. During the evening of 30 October a narrow stream of high electron concentration plasma crossed the polar cap in the antisunward ionospheric convection. A GPS scintillation receiver in the European high arctic, operating at 1.575 GHz, experienced both phase and amplitude scintillation on several satellite‐to‐ground links during this period. Close examination of the GPS signals revealed the scintillation to be co‐located with strong gradients in Total Electron Content (TEC) at the edge of the plasma stream. The gradient‐drift instability is a likely mechanism for the generation of the irregularities causing some of the scintillation at L band frequencies during this storm. The origin of the high TEC is explored and the possible implications of the work for scintillation forecasting are noted. The results indicate that the GPS scintillation over Svalbard can originate from traceable ionospheric plasma structures convecting from the American sector.

Evolution and dynamics of equatorial plasma bubbles: Relationships to ExB drift, postsunset total electron content enhancements, and equatorial electrojet strength

The growth in altitude/latitude of equatorial plasma bubbles was monitored, using simultaneous recordings of VHF scintillations at five locations situated between 3° and 23°N magnetic latitudes along a common meridian (84°E) during February 1980. The onsets of postsunset scintillation were mostly abrupt in character, and their occurrence at higher latitudes was conditional on their prior appearance at lower latitudes, indicating a causal link to irregularities associated with rising equatorial plasma bubbles. The day‐to‐day occurrence and the latitudinal, and effectively altitudinal, growths are examined in relation to the prereversal enhancement in h′F during sunset hours and its rate of rise, the onset of a postsunset secondary maximum (PSSM) in ionospheric electron content (IEC), and equatorial electrojet strength (EEJ) variations. It is observed that the bubble and associated irregularities, after its onset over the magnetic equator, reached the highest altitudes/latitudes only on those days when a prior PSSM in IEC is observed there in addition to high values of h′F, dh′F/dt and bubble rise velocity; otherwise it will be confined to near equatorial latitudes only. Also, the equatorial h′F, dh′F/dt, magnitude of PSSM and intensity of 4 GHz scintillations at low latitude are all showing positive correlation with daytime EEJ strength variations. It is concluded that, after the initial development of a bubble, the ExB drift and the PSSM play an important role in the subsequent growth and evolution, and EEJ is a useful parameter for the prediction of the development.

Rain fade dependence model of long-termscintillation amplitude distribution at 12.5 GHz

An extension of current scintillation stochastic models is described to examine the alteration of the scintillation amplitude distribution with the concurrent rain fade depth on earth-space paths. Descriptive statistical results of 12.5GHz scintillation at two sites show that the long-term magnitude of tropospheric scintillation steadily increases with worsening rain attenuation.

Ku-band wave propagation and communication experiments using SIRIO-1 satellite in Beijing Earth station

The wave propagation and communication experiments using SIRIO-1 satellite were performed during the Sino-Italian cooperative experimental campaign in 1983–1984 in an Earth station in Beijing. The main research topics included down-link rain attenuation at 11.5 GHz, correlation between attenuation and rainfall rate, attenuation rate, frequence scaling between 11.5 and 17.1 GHz, microwave radiometer and amplitude scintillation phenomenon. The results are outlined and some conclusions are drawn.

Observations and modeling of multi‐frequency VHF and GHz scintillations in the equatorial region

Amplitude scintillation spectra and statistics obtained simultaneously at VHF (257.55 MHz), L band (1541.5 MHz), and C band (3945.5 MHz) from the MARISAT communications satellite (15°W) are presented. The measurements were made at Ascension Island (7°58′S, 14°25′W, 30°S DIP) over a high elevation propagation path within a few degrees of the magnetic meridian plane. The multi‐frequency data and the unique propagation geometry enable us to study the structures of the equatorial ionospheric irregularities closely. Numerical simulation is used to model a specific event. Comparisons between the modeling results and the observations indicate that the data are consistent with the recently measured (in situ) two‐component power law irregularity spectrum with a shallow large‐scale regime and a steeper small‐scale regime, and a spectral break at about 1‐km scale size.

Nighttime VHF and GHz scintillations in the East‐Asian sector of the equatorial anomaly

Measurements made during solar maximum years in the East Asian sector of the equatorial anomaly show different seasonal patterns of night‐time scintillation occurrence at 137 MHz at Lunping and 4 GHz at Hong Kong. These seasonal variations are very similar to that observed at the equatorial station in Guam, indicating strong coupling between equatorial and anomaly irregularities. Model computations indicate that the GHz scintillation observed at Hong Kong is much stronger than one would expect based on VHF scintillation measured at Lunping. It is suggested that this might be an indication of strong latitudinal dependence of scintillation in the anomaly region. We also discuss the possible difference in local ionospheric conditions that were responsible for generating GHz scintillation causing irregularities in the anomaly region and at the equator.

High resolution topside in situ data of electron densities and VHF/GHz scintillations in the equatorial region

Coordinated in situ measurements of high‐resolution (spatial resolution 35 m) irregularity structures in the topside ionosphere using the retarding potential analyzer (RPA) on the AE‐E satellite and scintillation measurements using the geostationary satellite Marisat transmissions at 257 MHz and 1.54 GHz from Ascension Island were made in December, 1979 during the recent solar maximum period. The in situ irregularity spectra could be classified into two categories: those having or not having spectral breaks. The first category of spectra is characterized by two spectral indices: one above and the other below a scale‐length that lies typically between 500 m and 1 km. The spectral index in the long scale‐length end (10 km to ∼1 km) varies between −1 to −1.5, whereas the index in the short scale‐length end (∼1 km to 70 m) is between −3 to −3.5. In the second category, the power law spectral index of the irregularities ranges between −2 to −2.5 over the entire scale‐length range of 70 m to 10 km. The in situ measurements reveal that in the early evening hours, the E‐W gradient scale‐lengths can be as small as 35 m, the sampling interval of the RPA instrument. In the pre‐midnight and post‐midnight hours, the gradient scale length becomes larger, but scale lengths of several hundred meters are frequently encountered. The temporal variation of 1.54‐GHz scintillation magnitudes are found to track the E‐W bubble structure, scintillations being minimum when the ray traverses the center and large when it crosses the eastern and western walls of the bubble. On the other hand, scintillations at 257 MHz remain saturated during an encounter with a bubble and focussing is often observed, but the autocorrelation interval tracks the bubble structure, being large when the ray path is deep within the bubble where the perturbation is weaker. The power spectra of weak 1.54‐GHz scintillations are found to be consistent with the predictions of weak scatter theory based on irregularities having the observed in situ spectra. Significant differences are noted in the spectra of strongly scattered 257‐MHz scintillations. In this case the decorrelation bandwidth extends beyond 1 Hz, and the spectral slope of the transitional portion of the spectrum becomes as steep as −6. The moderate to strong GHz scintillations sometimes show a dual slope power spectrum with the lower frequencies exhibiting a slope of −1.5 and the higher frequencies showing a slope of −5.5. The observations of steep structures in irregularities with scale‐length of a few hundred meters near midnight, implying relatively slow erosion of sharp gradients are discussed in terms of plasma processes in the topside ionosphere.

Equatorial scintillations—a review

The scintillation technique, as is well known, provides an integrated measure of phase and amplitude fluctuations imposed on radio signals over a wide range of frequencies during their propagation through the ionosphere. The large amplitude of equatorial irregularities necessitates the use of frequencies in the GHz band to obtain unambiguously the temporal variation of irregularity intensity and the effect of irregularity anisotropy. Recent observations of equatorial scintillations will be reviewed with an emphasis on GHz measurements. The steep spatial gradients observed in in-situ data and their relationship to intense GHz scintillations will be explored. Co-ordinated measurements of equatorial irregularities by such techniques as radar backscatter, in-situ rocket and satellite, total electron content and 6300 Å airglow will be discussed, insofar as they provide a better understanding of the scintillation phenomena. While it is difficult to critically assess results that are so recent and constantly evolving, we have attempted to focus attention on the outstanding problems that still remain in the field.

Coordinated study of equatorial scintillation and in situ and radar observations of nighttime F region irregularities

A coordinated set of Atmospheric Explorer E (AE‐E) satellite in situ, VHF radar backscatter, and scintillation measurements performed during 1977 over a common ionospheric volume is used to study the relationship between the plasma depletions or bubbles, the extended 3‐m irregularity structures known as plumes and bursts of scintillation activity or patches in the nighttime equatorial F region. It is shown that in the early evening hours saturated VHF and UHF scintillation and 3‐dB fluctuations (propagation path in the east‐west plane where the effect of anisotropy is least) at 1.54 GHz are obtained in association with strong 50‐MHz radar backscatter, implying near simultaneous (∼min) excitation of large scale (∼km to 100 m) and 3‐m irregularities. In this early developed phase of irregularities, the AE‐E in situ measurements detect strong fluctuations of ion concentration with very sharp spatial gradients. The power spectra of these fluctuations in the scale length range of 10 km‐100 m conform to a power law variation with one‐dimensional spectral index ∼1.5. The postmidnight period, on the other hand, is found to be characterized by an absence of radar backscatter and presence of only marginal GHz scintillation activity. However, strong VHF scintillations and somewhat weaker UHF activity is found to persist. The AE‐E measurements indicate that during the late phase, the spatial gradients of ion concentration become shallower and the spectral intensity of the irregularities <500 m decreases by about two orders of magnitude as compared with that recorded during the onset phase. The irregularity spectrum conforms to a quasi‐gaussian description with correlation lengths of the order of a kilometer, which corresponds to the Fresnel dimensions of VHF scintillation observations. The implications of the observed spatial structures and the level of ambient concentration on the generation of 3‐m irregularities and scintillation modeling are discussed.

Severe disturbances of VHF and GHz waves from geostationary satellites during a magnetic storm

Severe night-time scintillations of VHF and GHz waves (0.136, 1.7, 4 and 11.5 GHz) emitted from the three Japanese geostationary satellites were observed during a geomagnetic storm on February 15, 1978. The GHz scintillations were strongly enhanced when irregular fluctuation of total electron content (TEC) was superposed on the background TEC which was varying sharply with time. This indicates a close relationship between the scintillations and irregularities. Detailed analysis shows that the one-dimensional wavenumber power spectrum of irregularities obeys a form of k −2 and that the scintillation spectra vary as ƒ s −3 . The scintillation index, S 4, seems to have a frequency dependence of ƒ −0.5 for 0.136 ⪅ ƒ ⪅ 1.7 GHz, f −1 for 1.7 ⪅ ƒ ⪅ 4 GHz and ƒ −2 for 4 ⪅ ƒ ⪅ 11.5 GHz. It is found that this scintillation event was accompanied with large decrease in the plasmaspheric electron content, which continued for one and a half days after the storm onset, and with appearance of localized electron density trough in the F-region ionosphere.

1.7 GHz scintillation measurements at midlatitude using a geostationary satellite beacon

Preliminary results of 1.7 GHz scintillation measurements made in Japan using a geostationary satellite during May–August in 1977 are presented. Since the propagation path below about 100km altitude is nearly parallel to the geomagnetic field, it was possible to observe irregularities along the field direction. The scintillation activity was enhanced at night in June and the maximum peak-to-peak variation of scintillations observed was about 2.3 dB. Simultaneous measurements of the total electron conterrt show that irregular electron density structures play an important role in 1.7 GHz scintillations.

Early results from the DNA Wideband satellite experiment—Complex‐signal scintillation

A multifrequency (ten spectral lines between VHF and S band) coherent radio beacon is presently transmitting continuously from a 1000‐km, high‐inclination orbit for the purpose of characterizing the transionospheric communication channel. Its high phase‐reference frequency (2891 MHz) permits direct observation of complex‐signal scintillation, and its very stable, sun‐synchronous orbit allows repeated pre‐midnight observations at low latitudes and near‐midnight observations at auroral latitudes. We present here early results of the observations; salient points include the following. First, most of the data are consistent with phase‐screen modeling of the production of ionospheric scintillation, including an ƒ−2 frequency dependence for phase variance. Second, propagation theories invoking weak, single scatter seldom are adequate, because even moderate intensity scintillation usually is accompanied by phase fluctuations comparable to or greater than a radian. Third, under conditions producing GHz scintillation (near the geomagnetic equator), lower frequencies show marked diffraction effects, including breakdown of the simple ƒ−2 behavior of phase variance and loss of signal coherenceacross a band as narrow as 11.5 MHz at UHF.

Statistics of VHF and L-band scintillation at Huancayo, Peru

A preliminary study of simultaneous scintillation measurements at VHF and GHz recently made at Huancayo, Peru, using transmissions from the MARISAT satellite is reported. It is shown that GHz scintillations occur when VHF scintillation undergoes an explosive growth within a time scale of the order of a few minutes. In the fully developed condition, VHF scintillation is located in the multiple scatter domain whereas at GHz weak scatter condition prevails. A study of the amplitude distribution, power spectrum and autocorrelation function of scintillations at VHF and GHz is carried out to explore the type of signal statistics in the weak and multiple scatter regimes.

Model computations of power spectra for ionospheric scintillations at GHz frequencies

Recently, two models of ionospheric electron density irregularities have been proposed to interpret the GHz scintillation phenomenon. One is an irregularity slab of thickness about 200 km or so around the F-region peak with electron density fluctuations of the order of 20% about its background value. The other assumes an ensemble of field-aligned irregularities throughout the large part of the magnetosphere as the cause for signal fading. Power spectra of GHz scintillation signals for these two models are investigated in this paper from the viewpoint that comparisons between experimental results and model computations may yield important information about the irregularities.

Correlated measurements of scintillations and in‐situ F‐region irregularities from OGO‐6

Scintillation estimates obtained from in‐situ irregularity measurements by OGO‐6 are compared with simultaneous 137 MHz and 6 GHz scintillations recorded at equatorial stations in the American and African sectors. In this first such correlated study, it is found that the equatorial irregularity amplitudes in these sectors are so large that for a moderately thick irregularity layer the commonly observed monotonic power law spectrum with outer scale dimensions much larger than the Fresnel dimension at VHF can explain the observed GHz scintillations and can locate the VHF band in the saturated scintillation regime. This finding is in contrast to other numerical modelling attempts which usually call for artificial tailoring of irregularity spectra to account for the observed GHz scintillations.