Phase Scintillation Data Research Articles

Ionospheric Scintillation Data Inversion to Characterize the Structures Associated With a Series of Polar Cap Patches

AbstractIn this study, we present the results of an inversion of ionospheric phase scintillation data to characterize the plasma density irregularity parameters for the structures associated with a series of Polar Cap Patches. The parameter estimates obtained during the inversion suggests that the irregularities associated with Polar Cap Patches are predominantly composed of moderately elongated electron density rods aligned with the earth's magnetic field which in some instances are interbedded within sheet and wing like density structures. Analysis of the spatial and temporal distribution of the axial ratio (AXRs), which are the ratios of irregularity elongation parallel and perpendicular to the field, indicates that the measured phase scintillation indices increase roughly proportionally with AXR values for the rods but remain roughly constant for wings and sheets. These findings indicate that while wings and sheets can produce phase fluctuations, it is the apparent existence of rods that mark the occurrence of plasma processes that lead to the formation of field‐aligned irregularities that produce phase scintillations which are most significant.

An empirical L-band scintillation model for a mid-latitude station, Weihai, China during the low solar activity period

Mid-latitude ionospheric scintillation has been studied in very poor proportion as compared to the equatorial and high latitude ionospheric scintillation. Mid-latitude ionospheric scintillations are often associated with either day time photo-ionization or due to the storm enhanced density. Using the phase screen model and the wave propagation theory in random media, we have identified the orientation of the ionospheric irregularities over Weihai with the local geomagnetic field. Amplitude and phase scintillation data observed using global positioning system (GPS) scintillation receiver deployed at the mid-latitude observation station Weihai, have been used along with K-index derived from the horizontal magnetic field component of the local magnetometer. The proposed model uses the scintillation indices relationship with the local K-index. We identified the scintillation dependence over local K-index during geomagnetic quiet and disturbed condition. This dependence coefficient is used on the real scintillation data for modeling. The presented scintillation model has been validated by comparing it to the real observations. The co-relation coefficient is more than 90% during the disturbed as well as quiet geomagnetic conditions.

A characterization of intermediate‐scale spread F structure from four years of high‐resolution C/NOFS satellite data

AbstractPower law spectra have been invoked to interpret equatorial scintillation data for decades. Published analyses of intensity and phase scintillation data typically report power law spectra of the form q−p with 2.4 < p < 2.6. However, in situ rocket and satellite measurements of equatorial spread F have shown evidence of spectra with two power law components. Strong scatter simulations and recent theoretical results have shown that two‐component power law spectra can reconcile simultaneous equatorial scintillation observations from VHF to S‐Band. The Communication/Navigation Outage Forecasting System (C/NOFS) satellite Planar Langmuir Probe generated a multiyear high‐resolution sampling of equatorial spread F, but published analyses to date have reported only single‐component power laws over scales from tens of kilometers to 70 m. This paper summarizes the analysis of high‐resolution C/NOFS data collected over the four year period 2011 to 2014. Following an earlier investigation of several months of C/NOFS data by the authors of this paper, the extended data set revealed a pattern of occurrence of two‐component spectra in the most highly disturbed data sets. The results confirm a known inverse correlation between turbulent strength and spectral index. The new results are interpreted as an equatorial spread F life cycle pattern with two‐component spectra in the early development phase giving way to single‐component spectra in the decay phase.

Development of high latitude phase fluctuationsduring the January 10, April 10–11, and May 15, 1997magnetic storms

GPS satellite transmissions have been used to study the development of moderateionospheric phase irregularities. The use of the multi-station, multi-path observations of the GPSbeacons has allowed the study of the time development of irregularities as a function of latitudeand longitude of individual storms. The basic storms studied were those of January 10, April10–11, and May 15, 1997. The results from studying these storms showed the unique nature ofeach storm. For the three storms, data were available from four stations near 65° CorrectedGeomagnetic Latitude (CGL); the stations ranged from Fairbanks to Tromso. In addition, datafrom higher latitude stations are analysed. For the January storm, irregularity development startedat Fairbanks. Then as magnetic midnight approached longitudes to the west, the storm effectsreached the Tromso–Kiruna longitudes. For the April magnetic storm, at 65° CGL, irregularitydevelopment maximized at approximately the same UT at stations ranging in longitude fromFairbanks to Kiruna. For this storm, the development of irregularities was dominated by stormtime. The May storm irregularities were dominated by magnetic local time once the stormcommenced. With both total electron content and rate of change of total electron content (phasefluctuations) available, it was noted that over periods of minutes, clumps of irregularities wereaccompanied by increases in TEC. In addition total electron content increased over large areasduring maximum magnetic activity in the auroral oval. During the storms, ionograms showed thatthe altitude of maximum electron density fluctuated; at times the dominant maximum frequencywas noted in the E layer and at other times in the F layer. This fluctuation of electron density ineach layer during storms led to the conclusion that the turbulent activity within the auroral regiondominated the development of irregularities. The irregularities that are noted on trans-ionosphericpaths are therefore thought to be in both the E and the F layers with a combination of structuredhard and soft electron precipitation and coupling initiating the turbulent activity. The generalpositive correlation of one periods phase scintillation data with the Ultra Violet Imagerobservations as shown on POLAR indicates the importance of 100–200 km precipitation.However the very high occurrence of spread F at high latitudes as shown by both ground andsatellite ionosondes indicates the considerable contributions of F layer irregularities.

Coronal radio sounding: Non‐Gaussian turbulence in the source regions of slow and fast solar wind

Structure functions of arbitrary order are analyzed using differential phase scintillation data obtained during the fourth solar conjunction of the Ulysses spacecraft in February–March 1995. The scaling features of these structure functions clearly discriminate between electron density fluctuations observed in coronal holes versus coronal streamers. The second order structure function, which is related to the power spectrum, shows essentially a scaling exponent ζ(p) = α −2, where p=2 and α is the spectral index of the three‐dimensional wavenumber spectrum of electron density fluctuations. Higher‐order structure functions provide evidence that these fluctuations are scale dependent and do not follow Gaussian statistics.

Chaotic behavior of ionospheric scintillation: Modeling and observations

The chaotic, but nondeterministic, structure of the ionosphere and radio wave amplitude and phase scintillation measured on the ground have been simulated using the phase screen model with the power law spectrum of random Gaussian phase fluctuations. Comparison of the information dimension calculated for phase fluctuations on a screen with that for amplitude scintillation at the receiver shows that the original chaotic structure in the ionosphere is completely masked by the propagation effects. Hence the ionospheric turbulence attractor (if it exists) cannot be reconstructed from amplitude scintillation data. On the other hand, measured phase scintillation data adequately reproduce the assumed chaotic structure in the ionosphere. Results of the attractor reconstruction for amplitude scintillation observed at high latitudes show an excellent agreement with our simulation.

Structure of ionospheric irregularities from amplitude and phase scintillation observations

The mutual coherence function Γ2, or the second moment of the complex amplitude of a radio wave which traverses through equatorial F region irregularities, is computed from amplitude and phase scintillation data. Theoretically, the equation satisfied by the coherence function has an analytic solution over the whole range of scintillation strength. This solution is directlly related to the structure function for the phase fluctuations produced by the irregularities. Hence the shape of the correlation function for variations in the total electron content along the signal path can be derived from the computed values of Γ2. With a suitable power‐law model for the irregularities, an “intermediate break scale,” 10, as well as the rms density fluctuation are deduced from a comparison of computed values for short time lags with those expected from theory. During a postsunset scintillation event, 10 is found to increase with local time. In the context of the generalized Rayleigh‐Taylor instability, which is the likely source of the irregularities, this increase may be attributed to a decline in the effective electric field prevailing in the region of the irregularities.

Scintillation measurement of density fluctuations in a tokamak plasma (abstract)

Recent theoretical results for line integrated measurements of phase and amplitude perturbations introduced on a laser beam probing a randomly fluctuating plasma are compared with phase scintillation data obtained on the LT-4 tokamak. It is shown that an estimate of the transverse velocity profile along the line-of-sight can be obtained from single shot measurements of the phase perturbations.

Chaotic behaviour of ionospheric turbulence from scintillation measurements

Ionospheric amplitude and phase scintillation data have been analysed to estimate the ‘information dimension’ associated with the attractor of the system. For weak scintillations, both amplitude and phase data yield identical results which demonstrate that spatial fluctuations of electron density in the ionosphere may be characterized by a few degrees of freedom. Stronger scintillations are attributed to steepened density irregularities which cause ‘focusing’ of the incident radio wave. This results in the amplitude scintillations exhibiting higher dimensional chaos but spatial fluctuations in ionospheric density still involve low dimensional chaos.

Morphology of phase and intensity scintillations in the auroral oval and polar cap

The first long‐term measurements of phase scintillations at high latitudes from a quasi‐stationary Air Force satellite at 250 MHz are reported. The measurements were made from Goose Bay, Labrador covering the corrected geomagnetic latitude range of 64°–72°N and Thule, Greenland covering the corrected geomagnetic latitude range of 85°–89°N. Data from December 1979, March–April 1980 and March–April 1982 are presented from both stations. The sunspot numbers during these specific time periods were approximately the same. The scintillation magnitudes were higher during the vernal equinox at both stations than during the winter solstice. The Goose Bay data showed a well ordered diurnal variation with a nighttime maximum and daytime minimum, whereas the Thule data showed no such ordering. The limited phase scintillation data set from Thule is augmented by intensity scintillation measurements made during 1979–1982 using the same quasi‐stationary satellites. The long‐term Thule data shows that the major variation in scintillation activity is annual with minimum scintillations observed in the local summer, as was determined earlier by Aarons et al. (1981). In contrast to the pronounced annual variation, the dependence of overall scintillation activity on sunspot cycle is not greatly evident during 1979–1982 when the sunspot number varied approximately between 100 and 200. It was found that both Goose Bay and Thule could show phase scintillations of the order of 10 rad (with 82‐s detrend period) even during magnetically quiet times. One such case study, conducted during March 1982, is presented and second‐order parameters such as phase aad intensity spectral strengths and slopes and intensity decorrelation times are discussed for this event at both stations. The use of geostationary satellite data shows the importance of enhanced magnetospheric convection velocities on the observation of large phase scintillation values at both sites. This underscores the need for incorporating irregularity dynamics in the currently developed static models of ionospheric scintillation.

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.

Spectral characteristics of medium‐scale equatorial F region irregularities

The spectral characteristics of equatorial F region irregularities with scale sizes from a few kilometers to several hundred kilometers have been measured using data from the Atmospheric Explorer‐E satellite. The spectra admit the expected power law characterization with a mean spectral index near 2. There is, however, a systematic decrease in the spectral index with increasing perturbation strength which has not previously been reported. Nearly simultaneous phase scintillation data from the Wideband satellite were also spectrally analyzed for comparison with the AE‐E data. The same systematic decrease of the spectral index with increasing perturbation strength was found in the measured phase spectral indices. A completely unambiguous comparison between the in situ and scintillation deduced turbulence levels cannot be made. However, for realistic propagation model parameters, the spectral characterization of turbulence based on the phase scintillation data is generally consistent with that observed in situ.

The time structure of transionospheric radio wave scintillation

The time structure of a representative set of weakly and strongly scintillating transionospheric beacon signals is analyzed. Under conditions of weak scatter, the coherence time of the signal intensity is a monotonic function of the Fresnel radius divided by the effective scan velocity. The shape of the function, however, is controlled by the power law index. Data from a Peruvian station show evidence of a slightly steeper spectral distribution than do data from Kwajalein in the Marshall Islands. Under conditions of strong scattering, the intensity coherence time depends only on the perturbation strength. The strong scatter data show remarkably little dispersion when they are plotted against the perturbation strength. The data are all consistent with a phase spectral slope somewhat less than 3, which is independently verified by using phase scintillation data.

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.