Global Morphology of Scintillation

Region-specific examinations of the scintillation effect have been published for both high-latitude [Basti et al., 1985] [Weber et al., 1985] [Aarons et al., 1988] [Basu et al., 1988b], and equatorial latitude zones [Basu and Basu, 1981] [Mullen et al., 1985]. Figure 4-22 indicates the nature of scintillation at UHF and L-Band for equatorial regions [Basu, 2003].

27 March 2000

UHF Scintillation

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Figure 4-22: Characteristics of scintillation as observed at Ascension Island in March 2000. The scintillation is "saturated" for both bands for the first half of the record. The L-Band scintillation virtually disappears for the last part of the record. We note that the scintillation is patchy in nature. [Sa. Basu, 2003]

Figure 4-22: Characteristics of scintillation as observed at Ascension Island in March 2000. The scintillation is "saturated" for both bands for the first half of the record. The L-Band scintillation virtually disappears for the last part of the record. We note that the scintillation is patchy in nature. [Sa. Basu, 2003]

The nature of the ionospheric structures, which give rise to radiowave scintillation, has been the subject of numerous theoretical studies and experimental campaigns. Other aspects important are dependencies on solar and magnetic activity. The regionally-averaged scintillation activity increases over the equatorial region as the sunspot number increases. This is particularly true for the equatorial anomaly region, that area approximately 20 degrees from the magnetic equator, where the most intense GHz scintillation is noted. For the high-latitude regions, the same is true. Even when the magnetic conditions are held constant, scintillation activity is observed to be higher during years of enhanced solar flux [Aarons et al., 1980]. Although sunspot number and magnetic activity are not perfectly correlated, we still find that the number and intensity of magnetic storms increases with an increase in solar activity. This general observation may be specialized somewhat for specific instances of scintillation that arise within the auroral zone and polar cap regions. We find that scintillation intensity is directly related to magnetic index within the auroral region [Rino and Matthews, 1980] whereas, within the polar cap, irregularity intensity (which is proportional to scintillation) is more closely related to solar flux. It is interesting to note that within the polar cap the irregularity intensity is vanishingly small during years of low solar flux [Buchau et al., 1985].

The most interesting aspects of the current drive to understand the problem have been attacked from the view point of three differing observational effects of irregularities: (i) radar backscatter of small-scale structures (meters), (ii) scintillation caused by ionospheric inhomogeneities of intermediate scale (hundreds of meters), and (iii) detection of large-scale electron content variations (blobs, bubbles, plumes and patches, of the order of many kilometers). Much of the current scientific activity is focused on the scintillation cause-and-eftect relationships, both in the auroral zone and the equatorial region. The auroral physics is more complex but the equatorial region is of continuing interest since the scintillation effects are more intense. Clearly, the instabilities that give rise to plume development are of major concern in understanding the scintillation problem. The scintillation that occurs at both equatorial and high latitudes is thought to arise from one of several related interchange instabilities (i.e., Rayleigh-Taylor, gradient drift, ExB, current convective, and flux-tube interchange) plus structured low energy electron precipitation and shear mechanisms. These are described lucidly by Tsunoda [1988],

Plumes are very large-scale depletions in ionization, frequently of the order of several hundred kilometers and extending from 250 km to 800 km in altitude. These are major sources of scintillation near the equatorial anomaly. Are large scale diminutions responsible for similar scintillation events at high latitudes? Probably not. In fact, large-scale increases in electron density (i.e., blobs and patches) are thought to be the cause of auroral zone scintillation. Also, in this region a high correlation exists between intense irregularity levels and the variations of the earth's magnetic field, as represented by the planetary magnetic activity index Kp. When magnetic storms occur, the irregularities in the auroral region spread in latitude and become more intense [Rino and Matthews, 1980], A considerable advance in the total understanding of ionospheric scintillation phenomenology as well as the underlying physical processes involved has been achieved through utilization of data sets obtained via the WIDEBAND DNA-002 program [Fremouw et al., 1978] [Fremouw et al., 1985a] as well as the HiLat Satellite [Fremouw et al., 1985b] [Fremouw, 1985], Fig. 4-23 is a model of high-latitude scintillation proposed by Aarons [1982].

The modest scintillation observed at midlatitudes is now thought to be a combination of the extension of the auroral effects of magnetic storms and substorms plus the after effect of the decay of ions in the earth's ring current, which produce both Stable Auroral Red Arcs after a magnetic storm and irregularities at sub-auroral latitudes. Nevertheless, it has been shown that sporadic E patches can give rise to some isolated scintillation events, while they are unlikely to be observed at UHF and above.

Latitude

Figure 4-23: Model ofhigh latitude irregularity structures. From Aarons [1982]. 4.4.2.3 Modeling of the Scintillation Channel

Latitude

Figure 4-23: Model ofhigh latitude irregularity structures. From Aarons [1982]. 4.4.2.3 Modeling of the Scintillation Channel

A considerable amount of effort has been directed toward the development of algorithms to describe the effect, with the ultimate objective of communication channel modeling. The approach has been to deduce the morphology from all available scintillation data and to derive the channel properties from the hypothesis of a two-component signal statistical model [Fremouw and Lansinger, 1982], It has been demonstrated that signal amplitude, signal phase, and the angle-of-arrival of the wave fluctuates during scintillation episodes. The generally accepted parameter for amplitude scintillation is the so-called S4 index [Whitney et al., 1969] [Fremouw et al., 1980], which is defined as the square root of the variance of received power normalized by the mean power. The probability density function for signal amplitude is well described by a Nakagami distribution, characterized by a single parameter m, which reduces to a Rayleigh distribution when scintillation is saturated (i.e., S4 ~ 1) [Aarons et al., 1988]. The phase is modeled generally as a Gaussian (normal) distribution [Fremouw et al., 1980].

The frequency dependence of moderate amplitude scintillation is consistently observed to vary as f / J over a range of frequencies between VHF and L band [Fremouw et al., 1985a] and phase scintillation varies as f"J. As the scintillation intensifies, the amplitude scintillation drops off more gradually with frequency, and the parameter S4 tends toward unity (i.e., Rayleigh fading conditions).

The power spectra of scintillation typically reflect the nature of the underlying inhomogeneity wave number spectra. For an inhomogeneity one-dimensional form power-law spectrum of the form k'p, the spectra of the fluctuations (in both amplitude and phase) behaves as F{l~p> at the higher frequency part of the spectrum, where p is of the order of 3.5 and F is the fluctuation frequency. At the lower frequency part of the spectrum, the amplitude scintillation exhibits a peak at the so-called Fresnel frequency and is diminished below this frequency [Fremouw et al., 1980]. On the other hand, phase scintillation suffers no such filtering action and the lowest frequency terms dominate the phase effects. These are associated with the largest scale irregularities in TEC. Figure 4-24 shows typical amplitude and phase spectra at 138 MHz at Poker Flat Alaska. [Secan, 1998].

Models have been developed to describe the global scintillation behavior. The currently available model, WBMOD, provides estimates of signal statistical parameters based upon the efforts of many investigators over the years [Secan et al., 1987], However, as of this writing there are still areas omitted in this model, including the intense polar and equatorial aftomaly scintillations.

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