Scintillation Level Research Articles

Solar activity dependence of interplanetary disturbances

Interplanetary scintillation measurements obtained inside 200 R⊙ using the Ooty Radio Telescope during August 1986–April 1991 have been analysed to study the interplanetary disturbances (or events) and their occurrence rates at various phases of the solar cycle. The disturbances are identified by the increase in the level of scintillation compared with the expected value. In total, 735 events have been identified. The results show a rate of 0.49 events per day near solar maximum and a low rate of 0.16 events per day during minimum of activity. The results are compared with coronal mass ejection (CME) rates and transients rates obtained from the Doppler scintillation measurements.

An improved model of equatorial scintillation

One of the main limitations of the modeling work that went into the equatorial section of the Wideband ionospheric scintillation model (WBMOD) was that the data set used in the modeling was limited to two stations near the dip equator (Ancon, Peru, and Kwajalein Island, in the North Pacific Ocean) at two fixed local times (nominally 1000 and 2200). Over the past year this section of the WBMOD model has been replaced by a model developed using data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data collected under the auspices of the USAF Phillips Laboratory Geophysics Directorate) which provide a greater diversity in both latitude and longitude, as well as cover the entire day. The new model includes variations with latitude, local time, longitude, season, solar epoch, and geomagnetic activity levels. The way in which the irregularity strength parameter CkL is modeled has also been changed. The new model provides the variation of the full probability distribution function (PDF) of log (CkL) rather than simply the average of log (CkL). This permits the user to specify a threshold on scintillation level, and the model will calculate the percent of the time that scintillation will exceed that level in the user‐specified scenario. It will also permit calculation of scintillation levels at a user‐specified percentile. A final improvement to the WBMOD model is the implementation of a new theory for calculating S4 on a two‐way channel.

Effects of the refractive index spectral model on the irradiance variance of a Gaussian beam

The normalized irradiance variance of a Gaussian beam in the weak-fluctuation regime is examined numerically with the use of various spectral models for refractive index fluctuations. The Kolmogorov, Tatarskii, and modified von Kármán spectral models are chosen as conventional models, while the Hill numerical spectral model and the Andrews analytic approximation to the Hill model are selected to feature the characteristic bump at high wave numbers. The latter two models are known to predict higher scintillation levels than conventional spectral models when the Fresnel zone and the inner scale are of comparable size. Outer scale effects appear minimal near the centerline of the beam but can reduce off-axis scintillation significantly. Inner scale effects are prominent on axis as well as off axis, although they sometimes tend to diminish near the diffractive beam edge. Analytic approximations are developed for the irradiance variance based on the Kolmogorov, modified von Kármán, and Andrews spectral models. These analytic expressions are generally in excellent agreement with numerical results.

Irregularity structures in the cusp/cleft and polar cap regions

A case study of the temporal behavior of ionospheric scintillations and their frequency spectra in the cusp/cleft and polar cap regions is presented. These measurements were made at Søndrestrom and Thule, Greenland, using the 243‐MHz transmissions from quasi‐stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDAR) high‐latitude plasma structure (HLPS)/solar terrestrial energy program (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were also performed at Søndrestrom, which defined the dynamic ionospheric environment in the cusp/cleft region. The availability of the radar results has enhanced this case study. It is found that scintillations at Søndrestrom are abruptly enhanced about an hour before magnetic noon when the propagation path to the satellite entered the cusp/cleft region. Subsequently, a series of enhanced and reduced scintillation activity was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spaced‐antenna scintillation measurements at Søndrestrom detected considerable velocity shear, and the frequency spectra showed flat low‐frequency portions, implying the presence of turbulent plasma flows. A comparison with the ISR observations indicates that the temporal variation of scintillation was caused by the poleward convection of alternate regions with high‐ and low‐ionization density, the density depletions being caused by channels of high zonal flows associated with velocity shear. The level of scintillation observed in the low‐density regions imply the presence of small‐scale irregularities with considerable irregularity amplitude. In contrast to the above behavior, the polar cap scintillations exhibit deep minima between the transit of successive “patches” of ionization, and their frequency spectra imply the absence of turbulent plasma flows. It is postulated that in the cusp/cleft and polar cap regions, the gradient‐drift instability mechanism generates the observed small‐scale irregularities associated with discrete density enhancements, whereas a shear‐driven instability, such as the nonlinear collisional Kelvin‐Helmholtz (K‐H) instability mechanism, may generate the irregularities in the intervening low‐density regions.

Solar cycle variation of interplanetary disturbances observed as Doppler scintillation transients

Interplanetary disturbances characterized by plasma that is more turbulent and/or moves faster than the background solar wind are readily detected as transients in Doppler scintillation measurements of the near‐Sun solar wind. Systematic analysis of over 23,000 hours of Pioneer Venus Orbiter Doppler measurements obtained inside 0.5 AU during 1979‐1987 have made it possible for the first time to investigate the frequency of occurrence of Doppler scintillation transients under solar minimum conditions and to determine its dependence on solar cycle. On the basis of a total of 142 transients, Doppler scintillation transient rates vary from a high of 0.22 in 1979 (one every 4.6 days) to a low of 0.077 transients/d in 1986 (one every 13 days), a decrease by almost a factor of 3 from solar maximum to solar minimum. This solar cycle variation, the strongest yet of any solar wind Doppler scintillation property, is highly correlated with both solar activity characterized by sunspot number and the coronal mass ejection rates deduced from Solwind and Solar Maximum Mission (SMM) coronagraph observations. These results indicate that coronal mass ejections and Doppler scintillation transients are closely related not just during solar maximum, as occasional individual comparisons have shown in the past, but throughout the entire solar cycle, and strengthen the notion that the Doppler scintillation and optical transients are different manifestations of the same physical phenomenon. The magnitudes of the transients, as described by the ratio of peak to pretransient scintillation levels (EF for enhancement factor), and their distribution with heliocentric distance also vary with solar cycle. While EF tends to diminish with increasing heliocentric distance during high solar activity, it is more evenly distributed during low solar activity. EF is also lower during solar minimum, as 13% of the transients during solar maximum have values exceeding 23, the highest EF observed during solar minimum. These results are consistent with the fact that occasional major fast‐moving interplanetary shocks that are observed during solar maximum are very rare during solar minimum.

Possible contribution of a solar transient to enhanced scintillation of a quasar

Observations of the quasar 2314 + 038 were carried out during 16–21 December, 1985 at a solar elongation (e) around 85 ‡, when the plasma tail of comet Halley swept in front of it. These observations have shown a two-fold increase in scintillation index (m) as compared to the expected levels of scintillation for the source, computed using the well-known RKH model. Spacecraft data and the geomagnetic indices available during the period show that a shock-front had reached the earth on the 18 December, the day when maximum increase in scintillation was recorded. The possible contribution of such a shock-front to the enhancement has been shown to be not greater than 15 percent. Hence, the major contribution to the enhancement came from the plasma tail of a comet.

Interplanetary magnetic field control of drifts and anisotropy of high‐latitude irregularities

Recently, much attention has been focused on the control exerted by the north‐south component of the interplanetary magnetic field (IMF) on the nature of large‐scale plasma structures in the polar cap ionosphere. In this paper we investigate whether the above IMF control also extends to the small‐scale irregularities of plasma density associated with the large‐scale structures. For this purpose, we have performed spaced‐receiver scintillation measurements at Thule and Sondrestrom, Greenland, using the 250‐MHz transmissions from quasi‐geostationary polar beacon satellites. Under IMF Bz northward conditions, moderate levels of amplitude (S4 < 0.6) and phase scintillations are observed with highly variable decorrelation times. Spaced‐receiver drifts under this situation show dramatic reversals of the true drift of the diffraction pattern from antisunward to sunward with moderate values of the axial ratio ranging between 4 and 12. For southward Bz we detect, in the central polar cap, a series of large magnitude scintillation (S4 ∼ 1) structures drifting at speeds of the order of 500 m s−1 in the antisunward direction indicating the passage of large‐scale ionization structures in the F region. In these cases the apparent drift speed of the diffraction pattern can only be determined as the pattern on ground is highly anisotropic (axial ratios 15 to 40) which makes it difficult to determine the true drift velocity. However, with suitable orientation of our antenna baseline we find that the apparent drift gives a fair estimate of the actual velocity. We also demonstrate that when these irregularities associated with the ionization patches in the F region transit across the polar cap and are observed at Sondrestrom, in conjunction with underlying E region ionization caused by auroral particle precipitation (as sensed by simultaneous incoherent scatter radar measurements of densities and temperatures), the irregularity anisotropy is much reduced. This reduction of anisotropy is possibly a result of increased cross‐field diffusion due to coupling with the E region having enhanced density. The true drift of the diffraction pattern measured at Sondrestrom on one evening agrees remarkably well with the simultaneous incoherent scatter radar measurements of F region plasma drifts, both sets of drifts showing equatorward and eastward motion varying between 0.5 and 1 km s−1.

Observations of weak scattering during AATE

Under ice measurements of acoustic phase and amplitude were made in the Beaufort Sea during April 1985 in the AIWEX acoustic transmission experiment (AATE). Pulses of 2, 4, 8, and 16 kHz were sent every 0.49152 s over a 6.4-km path between a moored transmitter and a depth cycling receiver array. The receiving aperture covered 153 m during down cycles that were repeated every 4.8 min for 12 days. The observations exhibit levels of intensity scintillation much weaker than those observed in the open ocean. At the lower frequencies, the scintillation levels are representative of statistics predicted by Born or Rytov theory. Measurements made of the internal wave field by AIWEX investigators exhibit fluctuations a factor of 10 to 20 below those observed in the open ocean. The observed travel time fluctuations and second and fourth moments of the field are consistent with predictions made using the observed environmental statistics. Tests comparing variations in time with the vertical variability have been made.

A synoptic study of Doppler scintillation transients in the solar wind

This paper presents the first synoptic study of Doppler scintillation transients caused by the passage of interplanetary disturbances. It is based on 40,000 hours of 2.3‐GHz Doppler scintillation measurements surrounding 15 superior conjunctions of planetary spacecraft during 1979–1983, a period that includes solar maximum. A total of 148 separate transients covering a heliocentric distance range of 5–179 RS were detected: 122 by one spacecraft and 26 by more than one. The enchancement in scintillation level (termed EF for enhancement factor) of the transients varied from 2 to 100. The multispacecraft measurements of 22 disturbances showed that they attenuated and expanded as they propagated outward from the Sun with propagation speeds varying from 200 to 2000 km/s. The propagation speed was highly correlated with EF (correlation coefficient of 0.91), showing that EF serves as a good indicator of propagation speed, at least for large EF. The frequency of occurrence was highest near the Sun (within 40 RS), where it was 0.29 transients/d, and decreased with radial distance, a reflection of the radial evolution of the transients and the sensitivity of the Doppler scintillation measurements to transients. Since the Doppler scintillation measurements are most effective near the Sun, they ideally complement in situ spacecraft observations farther away. An earlier study of nine interplanetary shock waves based on scintillation, Solwind coronagraph, and Helios plasma measurements showed a significantly wider range of shock velocities close to the Sun than near 1 AU. The results of this synoptic study indicate that this is apparently a long‐term feature of the solar wind around solar maximum.

Low‐frequency modulation of plasmas and soft electron precipitation near the dayside cusp

On December 2, 1983, the HILAT satellite trajectory was approximately aligned with the noon‐midnight meridian while crossing the northern polar cusp/cleft region. During that time we observed (1) an increase in scintillations of radio signals, (2) strongly variable fluxes of precipitating low‐energy electrons, (3) gradients in the thermal ion density, (4) fluctuations in the east‐west component of the ion drift velocities superimposed on the noon sector convection pattern, and (5) field‐aligned currents. In the region corresponding to the highest level of radio scintillations we also detected fluctuations in electron fluxes on a time scale of half a second superimposed on a low‐energy inverted‐V event. The analysis of these measurements, which provide information on both large‐ and small‐scale dynamics of the dayside cusp, leads to the following conclusions: (1) the large‐scale convection pattern is characteristic for the northern hemisphere high‐latitude ionosphere when the interplanetary magnetic field has negative Y and Z components; (2) the most intense scintillations and the cusp field‐aligned currents occur on recently opened magnetic field lines; (3) plasma created by increased fluxes of precipitating low‐energy electrons convects antisunward with velocity of ∼0.5 km/s; (4) enhanced chemical reactions due to the Joule heating associated with large electric fields maximize at the equatorward edge of the cusp, leading to the net depletion in total electron content and plasma density measurements; (5) plasma irregularities throughout the F region are formed due to a combined effect of structured soft electron precipitation and current‐convective instabilities; and (6) if the subsecond fluctuations of the ion drift measurements seen during the inverted‐V event are due to static variations in the ambient electric field, there are potential drops of a few tens of volts between the altitude of the satellite (∼830 km) and the lower ionosphere.

F layer irregularity observations of the SAR arc event of March 5–6, 1981

Over several years, various observers have recorded irregularities in the F layer of the ionosphere at latitudes which were identified as the plasmapause region. During the stable auroral red arc of March 5–6, 1981, intense irregularities were recorded using amplitude fluctuations of satellite beacon signals. In the case of this SAR arc optical intensity was well correlated with irregularity intensity. The location of the small scale (∼1 km) irregularities was centered at plasmapause latitudes; scintillation levels tapered off in the same longitude sector at trough latitudes and were of lower than normal intensity and of different nature at auroral latitudes. Reported optical measurements indicated no visible aurora until areas four L shells poleward. Low energy (<12 eV) electron precipitation accompanied by elevated electron temperatures have been reported within the region covered by SAR arcs. High neutral gas temperatures have also been reported. Scintillation of moderate level is not uncommon for the plasmapause region. In March 1981, approximately 60% of the nights had scintillation levels > 6 dB at 137 MHz lasting over one hour. These irregularities, frequently of lower intensity than those accompanying the March 5–6 SAR arc, are probably initiated by low energy precipitation at these latitudes. However the role of convection is not as yet clear in this latitude region.

Equatorial F-layer irregularity patches at anomaly latitudes

Recent studies of the physics of F-layer irregularities in the equatorial ionosphere have been concerned with the development of plumes or patches. A series of observations in the equatorial anomaly region in a year of high solar flux has been analyzed for the radio propagation effect of scintillations. The observations were made on patches in the developing, mature and decay phases. Although irregularities develop on the west wall of the patches, the intensity of scintillation does not appear to diminish within the patch; the patches contain bursts of high level activity. Patch characteristics at microwave wavelengths match airglow depletion images when two considerations are introduced, i.e. the westward tilt of the patch as shown by optical and radar observations and the effective path length of the irregularities affecting the radio propagation path. Using optical images of depletions the effective thickness of the layer of irregularities above the peak of the F2-layer can be estimated; it is relatively short, i.e. of the order of 70 km for the gigaHertz frequencies and 150 km for the 257 MHz transmissions. The total path length is 110 km for the microwave frequencies and 220 km for the lower levels of scintillation at 257 MHz. The decrease in microwave scintillations compared to meter wavelength observations in the midnight and post-midnight time period in these anomaly observations is due to the combination of decay of electron density as well as the relatively rapid decay of smaller scale irregularities, as has previously been noted in observations at the magnetic equator.

Radio-scintillation observations of interplanetary disturbances

Recent developments in the studies of interplanetary disturbances by scintillation (IPS) techniques are briefly reviewed. The turbulent post-shock region of an interplanetary disturbance produces transient enhancements in the scintillation level and the flow speed in many cases. An empirical method to determine three-dimensional angular distribution of propagation speed of the disturbance on the basis of IPS measurements of post-shock flow speeds is applied to 17 events which took place in 1978–1981. Among them, four representative examples including two events which were associated with disappearing solar filaments are described in detail. Several disturbances had oblate configurations; the latitudinal extent is smaller than the longitudinal extent. On an average, the angular distribution of propagation speed at 1 AU heliocentric distance is quasi-isotropic over a longitudinal range of 100° centered at the normal of relevant solar phenomenon. The net excess mass and energy in an interplanetary disturbance associated with a disappearing solar filament can be comparable to those of an interplanetary disturbance associated with a large solar flare.

Propagation model of laser beams in turbulence

A closed set of three simultaneous partial differential equations is derived for the solution of the average irradiance and the irradiance variance of focused laser beams in turbulence. The equations are uniformly valid for arbitrary scintillation levels. Examples of solutions calculated by finite difference techniques are shown to fit typical atmospheric and laboratory data well. Analytic solutions derived for asymptotic conditions are in good agreement with the related classic perturbation results. The model also provides interesting physical insight into the phenomena of saturation and supersaturation.

Observations of tropospheric phase scintillations at 5 GHz on vertical paths

We report here observations of tropospheric phase fluctuations at 5 GHz on the near‐vertical paths relevant to a number of astronomical and geophysical measurements. Using the Very Large Array near Socorro, New Mexico, the spatial structure function of phase was measured in the scale size range 1–35 km. The 23 data sets reported here were taken over a 2.5‐year interval. During 15 observing sessions there is a clear detection of tropospheric phase fluctuation, while only upper limits can be constructed on the remaining 8 days. The measured structure functions vary as (baseline)x, with the average value of x being 1.4. Where comparisons with other microwave observations (e.g., water vapor radiometry) are possible, there is reasonably good agreement in both the level and shape of the power spectrum of these tropospheric phase fluctuations. Some implications of these observations for precision Doppler tracking of spacecraft and geodesy/radio interferometry observations are discussed. Although the level of these tropospheric scintillations is highly time variable, very low frequency gavitational radiation searches using Doppler tracking of distant spacecraft will have, if uncalibrated, tropospheric propagation noise at a typical level Δƒ/ƒ∼ 5 × 10−15 for 1000‐s integration times.

On the application of phase screen models to the interpretation of ionospheric scintillation data

Phase screen models have been used for nearly three decades to facilitate the application of the scintillation theory to data interpretation. Only recently, however, as large amounts of multifrequency phase‐coherent beacon data have been analyzed, has the full potential of the model been realized. This paper presents a review of the application of the phase screen model to the interpretation of data from the Defense Nuclear Agency's Wideband satellite (international designation P76‐5), which was operational from May 1976 until August 1979. The signal structure and experiment configuration allowed measurements of temporal, spatial, and frequency coherence for a wide variety of scintillation conditions. The derived scintillation parameters are in excellent agreement with the phase screen theory. Moreover, the inferred irregularity structure has been confirmed by recent in situ measurements. The principal results can be summarized with simple algebraic formulas for the scintillation level, spatial/temporal coherence, and frequency coherence or coherence bandwidth. With appropriate manipulations, the formulas fully accommodate the propagation angle dependence in highly anisotropic media.

Global morphology of ionospheric scintillations

Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

Microwave equatorial scintillation intensity during solar maximum

A comparison of scintillation levels at 1.5 GHz made from the Appleton anomaly region of the magnetic equator and from the region close to the magnetic equator (termed the electrojet latitudes) showed increased F region irregularity intensity over the anomaly region during years of high sunspot number. Peak to peak fading greater than 27 dB was noted from Ascension Island (through a dip latitude of 17°) in the anomaly region while only 7–9 dB from Natal, Brazil, and Huancayo, Peru, were noted, the last two paths being close to the magnetic equator. The hypothesis advanced is that the dominant factor responsible for the intense gigahertz scintillation is the traversal of the propagation path through the anomaly region. During years of high sunspot numbers the high levels of ΔN constituting the F region irregularity structure are due to (1) very high electron density in the anomaly region (compared to the electrojet region) and (2) the late appearance of these high electron densities (to 2200 local time) in the anomaly region. The patches or plumes of irregularities seen in the postsunset time period then produce high ΔN; scintillation excursions are proportional to this parameter. The postulation of vertical irregularity sheets in the patches was examined to determine the possibility of this being an important factor in the difference between electrojet and anomaly scintillation levels. Older gigahertz data from the sunspot maximum years 1969–1970 were reanalyzed, and more recent observations from other studies were also reviewed. It was found that through the anomaly region, high scintillation indices were noted at a variety of azimuths of the propagation path rather than just along a path closely aligned with the magnetic meridian. A more complete evaluation of the geometrical factor, which must be of considerable importance in determining the absolute value of the scintillation intensity, awaits further observations.

UHF scintillation activity over polar latitudes

Polar cap ionospheric irregularities have been monitored for several years from Thule Air Base, Greenland using 250 MHz satellite beacon scintillation measurements. The seasonal pattern of the polar cap irregularities shows very high intensity levels during the winter and lower levels during the summer (sunlit) months. This behavior is similar to in‐situ polar cap electric field measurements which show larger fluctuations in the winter than in the summer, an effect which may be related to E layer conductivity changes. A striking contrast was noted between high scintillation levels observed during 1979/80, a year of high solar flux, and much lower levels observed during 1975, a year of low solar flux. This variation may be related to a corresponding solar cycle variation in polar cap F layer electron density. The data reveal little difference between periods of high and low Kp, and only a weak diurnal variation in any season. Direct optical and ionosonde measurements indicate that these scintillations are produced by ionospheric irregularities in the polar cap F‐region. Results of spaced receiver drift measurements indicate that the small scale of irregularity drift was antisunward. Intense irregularities are associated with discrete sun aligned F layer auroras. A weak background level of scintillation persisted in the high solar flux years.

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.