In order to understand the situation fully, it is first necessary to present a clear definition of the extent of the mid-latitude region. This presents a problem in that the mid-latitude region is defined, by exclusion, to be the region of the Earth's ionosphere between the high and low latitude ranges. The boundaries of the high and low latitude regions of the ionosphere can be uniquely distinguished by examining several indicators but it is important to note that these boundaries are not fixed in time, showing diurnal, seasonal and other periodicites. To step around this problem in the discussions that follow, it is sufficient to outline the geomagnetic latitudinal ranges that shall pertain to the labels of high, low and mid-latitudes.
Hajkowicz (1994), in a review of scintillations over a sunspot maximum, presents a classification system that distinguishes between ionospheric irregularities originating in the equatorial, auroral and mid-latitude regions viewed by observation posts located in the southern mid-latitude range. Adopting this system as a convenient labelling criterion, we are presented with three types of scintillation:
N type scintillations are defined to occur in the vicinity of the polewards boundary of the equatorial scintillation region. (In the case of Hajkowicz's study, this corresponded with the sub-ionospheric points to the north of his stations and hence his use of the designation N). N type scintillations have a maximum in their occurrence during periods of heightened solar activity when the equatorial scintillation belt creeps polewards as the equatorial electrojet intensifies. Heightened activity is found to occur at sunspot maximum and a seasonal periodicity is observed, peaking for the summer-autumn period (in Hajkowicz's study this was for the austral summer-autumn period.) N type scintillations are more frequently observed at night although increased activity is often observed during the summer daytime. This daytime maximum is closely correlated with the well known increase in Es occurring in the summer daytime.
The observation of N type scintillations is strongly affected by the position of the receiver location and the geometry of the antenna. In true mid-latitudes, for observations confined to small zenith angles (less than 45°), N type scintillations are rarely observed. If however one were to consider the case of small elevation angles, N type scintillation is likely to be observed frequently. The observation of geostationary satellites from mid-latitude regions must therefore take into account the position of the equatorial scintillation oval when examining the causitive mechanism behind the scintillation occurrence. For specific geometrical alignments, such studies are susceptible to the enhancement of scintillation phenomena associated with the aspect angle sensitivity discussed by Sinno and Minakoshi (1983).
P type scintillations are so called because the electron density irregularities responsible for the disturbance to the transionospheric signal occur in confined patches within the mid-latitude ionosphere. The patches have been found to consist of rod-like field-aligned irregularities (FAIs) associated with disruption to the F2 layer in the form of range spread-F. As the only true mid-latitude ionospheric scintillations, it is important to note that their specific morphology is quite different to that of the N and S types. While the N and S types occur more frequently in the perturbed ionosphere during sunspot maximum, the P type scintillations are noted to occur within the quiescent night-time ionosphere of geomagnetic mid-latitudes during sunspot minimum. This pattern of behaviour is strongly correlated with the maximum in spread-F occurrence during the same time frame, as commented on by (Bowman and Hajkowicz, 1990) and others. A subsiduary maximum can also be found during sunspot maximum during daylight hours which is thought to be associated with sporadic-E, although this enhancement is relatively minor. A subset type PS, where the patches lie to the south of the station away from magnetic zenith, demonstrates similar occurrence patterns as the main P type.
S type scintillations are found to be associated with the formation of irregularities in the vicinity of the equatorwards edge of the auroral scintillation belt. S type scintillations commonly produce the most intense scintillations recorded at mid-latitude stations. The occurrence maximum for this type of scintillation is found to show a close correlation with the maximum in the 10.7 cm solar radio flux that takes place for solar maximum. This enhancement peaks during the sunspot maximum which reflects the migration of the boundary of the auroral scintillation oval to lower latitudes during heightened solar activity. A seasonal dependence with maximum during the austral summer-autumn period is noted to take place, coupled with a diurnal enhancement in the early evening and mornings when the ionosphere is perturbed by effects associated with the sunrise and sunset terminator. The incidence of S type scintillations during the summer daytime is found to coincide with the well known pattern of occurrence for sporadic-E. The minimum occurrence of this type of scintillation is observed during solar minimum during the winter. Most scintillations of this type occur during the night-time. No significant increase in scintillation activity can be found for periods with larger values of the average planetary magnetic index Ap, although sensitive dependence is observed during the equinoctial months of September and March.
Quasi-periodic (QP) scintillations are so named because the patterns found in records of amplitude scintillations have features with regular structures with a definite period. QP scintillations differ from random scintillations in that random scintillations appear ``noisy'' while QP scintillations closley resemble the sinc waveform that is well known from optical interference studies. The ``ringing'' structure of QP scintillations should possess a deep central minimum for events caused by ionospheric irregularities, while QP events with a central maxima are attributable to spurious radio frequency interference from ground transmitters or other satellites. The most widely accepted view on the production of the QP scintillations is that they are caused by reflection from the frontal structure of the irregularities in a manner that has been described previously in looking at the scattering model. This view is supported by the fact that QP scintillations tend to occur for low elevation angles and where the ray path coincides with the direction of the slope of the frontal irregularities. While it is possible that QP scintillations may be caused by irregularities in the E or the F region, the prevalent thinking on the matter suggests that irregularities associated with sporadic-E disturbances are the most likely culprit. The relatively high number of QP scintillations recorded during the day-time, when compared to P type random scintillations, further supports this notion. A final possibility is that QP scintillations may take place following the passage of a TID, either from the frontal structure of the TID or from irregularities produced in its wake.
Investigations of the small-scale structure of the ionosphere conducted by scintillation studies frequently draw upon information gleaned from sources other than the record of the received amplitude of the transionospheric signal. More traditional ionospheric investigative procedures such as the use of ionosonde data and TEC records are particularly useful in this context in that the known morphology of gross features in the mid-latitude ionosphere can be incorporated into the total body of knowledge available to researchers. This type of information plays a vital role in the formulation and evaluation of theories that describe the manner in which ionospheric disruptions over a range of scale sizes develop. The role played by field-aligned irregularities and their occurrence in conjuction with large scale disturbances is central to the process of describing scintillation phenomena.
The occurrence of field-aligned irregularities in the F2 region of the mid-latitude ionosphere is well known. These scintillation producing irregularities have especially been found to develop at the same time as range spread-F has been detected by topside and bottomside ionosonde measurements. Early results of comparitive analysis of spread-F studies and scintillation occurrence suggested that the scintillation irregularities were associated with frequency spread-F but this has since been ruled out as a selective bias by the ionosonde technique in finding frequency rather than the range spread-F.
The simultaneous occurrence of scintillations and range spread-F during the night-time ionosphere of the geomagnetic mid-latitude region during solar minimum is thought to explain the relatively high number of scintillations recorded during this time. Measurements in the high and low latitude regions at similar times do not display a concurrent increase in the number of scintillations detected. As was pointed out in the preceding section, the P type scintillations which are the only true mid-latitude scintillations display a similar occurrence enhancement while the S and N type scintillations, belonging to the high and low latitude regions do not. The vertical extent of spread-F in mid-latiude regions, in the order of 60 km, is consistent with the axial ratios determined for the irregularities from scintillations studies of P type scintillations.
An examination of field-aligned irregularities on the cusp of the auroral scintillation oval by Wernik et. al found that the S type scintillations were produced by rod-like inhomogeneities with axial ratios between 5:1:1 and 10:1:1. By contrast, a study by MacDougall (1990) of P type scintillations within the mid-latitude ionosphere established that these ratios may be significantly understated. Theoretical considerations indicate that the weak irregularities thought to be responsible for the scintillation activity may have axial ratios of up to 63:1:1 with the cross-sectional width in the order of 0.8 km. During observations at mid-latitude stations in the United States, results were obtained that indicated preferential elongation of the irregularities in the direction of the magnetic field with mean axial ratios of 44.5:1:1 and minor half-axis widths of 0.3 ± 0.13 km. Speculation that weak, extended, ever-present irregularities aligned with the direction of the magnetic field was raised in the 1960's by such workers as Jones (1969), Parkin (1968) and Singleton and Lynch (1962) during observational campaigns conducted at sites operated by the University of Queensland in the vicinity of Brisbane. These three studies all found that the irregularities responsible for the scintillation activity were due to the presence of field-aligned irregularities in the kilometric range at F region heights, with a marked enhancement of scintillation activity for small aspect angles. Although the studies concluded that the irregularities could be rod-like or sheet-like, this result is likely to be due to the increased sensitivity of the relatively low frequency signals to the presence of sheet-like irregularities. Modern studies operating using higher frequency signals in excess of 100 MHz conclude that the irregularities are rod-like and not sheet-like, as found by topside ionosonde studies that obtain ducted echoes (Dyson 1967).
Similarly, modern studies also differ in their interpretation of the specific ray propagation interactions that produce the scintillations. Earlier studies tended to favour the notion that the interference arose when signals encountered the irregularities at grazing incidence angles and reflection from the surface of the excess electron density distribution with the undeviated portion of the ray were responsible for producing the amplitude scintillation patterns produced. While this theory has not been discounted and is still widely used in qualitative studies, contemporary thinking leans toward the use of diffracting phase screens, be they of the Briggs and Parkin type or power law form. All studies demonstrate a pronounced enhancement in scintillation activity where the ray path coincides with the field point. This aspect angle sensitivity has been noted by many observers, such as Sinno and Minakoshi, although the techniques used to interpret data vary widely. Older studies concentrated on confining the data sets used to variations in the zenith (Briggs and Parkin 1963) or azimuth (Jones 1969) angle while modern studies, using computer calculations to find the aspect angle as a function of zenith and azimuth angle, are not restricted in the same manner.
The manner in which the irregularities form is still a point of contention. The regions of the ionosphere where they form is well described (mainly the F region although some form in the E region) but the specific processes that drive their production are undecided at this time. A satisfactory explanation for the spatial and temporal integrity of the electron density enhancements is not yet fully formed. The most likely candidate at this time would appear to be the generation of differentiated turbulence by the passage of TIDs, probably AGWs moving up from the neutral atmosphere, which halt the diffusion process that is expected to nullify the formation of irregularities beyond 3-4 seconds. Some indicators exist to suggest that this process is aided by electric fields that act in the direction opposite to the gravity forces at work. A magnetic bottling effect that is well known from the theory of plasmas then enhances the concentration of the irregularities in the direction of the Earth's magnetic field. Tomographic analysis of this situation is expected to reveal more information on this topic in the future.
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Last updated 28/12/1997 by Mark Keir