Scintillation Boundary Research Articles

Comparison of boundaries for HF auroral backscatter and VHF scintillation

A study is described of the equatorwards boundary of high-latitude ionospheric irregularities using measurements obtained by two different experimental techniques. Auroral traces observed on backscatter ionograms from an HF radar have been used to identify the boundary of the decametre scale-size irregularities responsible for the coherent backscatter. Simultaneous observations of scintillation on the 150 MHz signals from NNSS satellites have enabled the boundary for irregularities in the sub-kilometre scale regime to be located. Comparisons of the results from the two techniques indicate that reconciliation between boundary positions can be made in only about 50% of the cases considered. In general the scintillation boundary lies on average well polewards of that identified by the HF radar, suggesting that caution should be exercised in the mapping of irregularity boundary positions.

Scintillation near the F layer trough over northern Europe

Results are presented of amplitude and phase scintillation observations made during a two and a half year period at Lerwick in the Shetland Islands, a station in the subauroral zone in the vicinity of the F layer trough and the scintillation boundary. Transmissions from the multisatellite Navy Navigation Satellite System have been used to obtain both latitudinal and temporal coverage for the observations. More than 19,000 satellite passes have been monitored yielding in excess of 750,000 values of S4 and σф for amplitude and phase scintillation, respectively. The paper presents examples of scintillation occurrence as a function of latitude for both long term and diurnal time scales. A dominant feature is the enhanced occurrence of scintillation in response to increased solar activity. This is accompanied by an advance in the scintillation boundary to lower latitudes on a scale comparable to that associated with geomagnetic activity. The position of the premidnight minimum in boundary latitude has moved by some 10 deg in response to the increased solar activity between autumn 1987 and autumn 1989. The paper also discusses the importance of propagation and irregularity geometry to scintillation measurements and by comparison with model studies concludes that the observations are consistent with scintillations arising from irregularities of a field‐aligned rodlike nature with low aspect ratio rather than sheets extended along L shells.

Broadband lane structures in the decametric spectra of Jupiter and the scintillation boundary

Dynamic spectra of Jovian noise storms at 20–30 MHz are compared with the scintillation indices of the overhead radio source Cas A at 45 MHz. Of six intense storms observed, four that displayed broadband lane patterns occurred on nights when the scintillation index of Cas A was unusually low, while two that did not display broadband lane structures occurred on nights when the scintillation index was high. It is evident that the scintillation boundary was located north of Oulu on the nights when the broadband lane patterns occurred and was therefore not involved in the lane-producing mechanism.

Dominant configurations of scintillation‐producing irregularities in the auroral zone

Recent scintillation observations have disclosed the existence of sheetlike electron density irregularities aligned along L shells in the auroral zone ionosphere. In this paper we exploit the aspect sensitivity of phase scintillation to identify the dominant three‐dimensional configuration of irregularities in four latitude/time zones: (1) equatorward of the high‐latitude scintillation boundary on the nightside of the earth, (2) poleward of the nightside boundary, (3) equatorward of the boundary on the day side, and (4) poleward of the dayside boundary. We find the sheetlike irregularities to be confined to region 2, with few exceptions. The dominant configuration in the other zones, including zone 4, is that of axially symmetric rodlike irregularities aligned along the magnetic field.

A comparison of the relative locations of the mid-latitude electron density trough and the scintillation boundary

The mid-latitude electron density trough position and the scintillation boundary have been compared for magnetically quiet periods by using the data returned by Ariel 3 and Explorer 22 satellites. The scintillation boundary is found southward of the trough during daytime, but at night the positions are reversed.

Ionospheric mid-latitude trough and the abrupt scintillation boundary

THE mid-latitude trough1,2 in electron density and the equatorward limit of small-scale F-region irregularity structure apparent in the scintillation of satellite and radio star signals, the scintillation boundary3,4, are both features of the transition region between the high and mid-latitude ionospheres. There are broad similarities between the morphology and statistical behaviour of the trough and the irregularity boundary5–7 and, although the individual behaviour of the scintillation boundary and the mid-latitude trough may be related to particle precipitation patterns or plasmapause movements, there is a definite local time variation in their mean relative latitudinal positions when simultaneous observations of the two phenomena are analysed8. We have examined this variation in more detail, using data from simultaneous observations of trough and scintillation boundary positions.

High latitude ionospheric irregularity model

Modifications were made to the global scintillation model that was developed by Fremouw (Stanford Research Institute), to better represent recent high latitude scintillation data. These modifications were made primarily in the high latitude term of the model and include a provision for variation in the position of the scintillation boundary with respect to magnetic activity. The resulting model is compared with published curves of scintillation activity with respect to latitude and is found to produce a considerably better agreement with those curves than the original model. A modification of the model is made to represent the finding that the northern and southern polar regions of the scintillation activity are not symmetrical with respect to each other.

The dependence of high-latitude ionospheric scintillations on zenith angle and azimuth

Scintillation observations of Transit 4A and Explorer 22 made at Hamilton, Massachusetts, and Thule and Narssarssuaq, Greenland, are employed to determine, for high latitudes, the nature of the variation of scintillation index with the zenith angle and azimuth of the direction of propagation at the Earth's surface. The variations found are shown to be consistent with irregularities which, on the average, are field aligned but not axially symmetric. The average elongation along the field is found to decrease from a factor of 7 at Hamilton to 2·5 at the highest latitude station, Thule. Slight broadening of the irregularities in a direction normal to the magnetic meridian plane is found which maximizes at twice the dimension in this plane at Narssarssuaq. The smallest dimension of the irregularities (i.e. in the magnetic meridian plane) decreases slightly from 1 km at Hamilton to 0.7 and 0.8 km at Narssarssuaq and Thule, respectively. The strength of the irregularities, defined in terms of the irregularity r.m.s. incremental electron density and the effective thickness of the irregular region, is shown to increase by a factor of at least 3 at the edge of the scintillation boundary in the vicinity of 60° invariant latitude.

A descriptive model ofFlayer high-latitude irregularities as shown by scintillation observations

Observations in the northern hemisphere of the amplitude fluctuations of radio signals from satellites and radio stars being used as a data base, a model is sketched of the high-latitude F layer irregularity region. On the nightside, irregularity intensity increases from a low level at its equatorward boundary position of ∼57° at 2200 hours LT to a higher level within the auroral oval. A high level of irregularity intensity exists over the corrected geomagnetic pole. However, the data base for quantitative measurements at latitudes poleward of the auroral oval is small. On the dayside the scintillation boundary is in the region of 70° for quiet to moderate magnetic conditions. During all hours an increase in magnetic activity brings on deeper fading across the entire high-latitude region, from the equatorward scintillation boundary to the geomagnetic pole. Precipitation of electrons with energies of the order of a few hundred electron volts is not thought to be the mechanism for the production of small-scale irregularities. In this energy range, electron precipitation shows a low occurrence in the polar cavity, whereas scintillations remain high in this region. Measurements of suprathermal electrons show polar patterns closer to the scintillation observations than measurements of high-energy electrons. Irregularities observed in satellite observations of the thermal electrons (R. Sagalyn, private communication, 1973) at high latitudes including polar areas show geomagnetic occurrence patterns similar to those shown in this paper. In their measurements of turbulent isotropic electric fields at 400 km Kelley and Mozer (1972) have found variations with latitude in the boundary region, the auroral oval, and over the pole that resemble the scintillation variations of this model.

A catalog of ionosphericFregion irregularity behavior based on Ogo 6 retarding potential analyzer data

Review of in situ data obtained with the aid of the retarding potential analyzer on board Ogo 6 which reveal the nature of ionospheric irregularities in the total ion concentration above 400 km. Except for the high-latitude regions, the ionosphere is usually observed to be very smooth in the daytime, but considerable structure is observed at night, particularly near the equator and at Atlantic longitudes. Although most of the irregularities observed appear to be stochastic in nature, many nearly monochromatic waveforms are observed near the equator. The topics discussed include the midlatitude scintillation boundary, large-amplitude equatorial irregularities, the fluctuation spectrum of typical F-region irregularities, the lower edge of the equatorial F region, sinusoidal waveforms, ground glass irregularities, breaking wave irregularities, and regions of smooth and irregular ionization inside the polar cap.

Relative Movements of Mid-Latitude Trough and Scintillation Boundary

THE ionospheric mid-latitude trough has been investigated by means of satellite in situ probes1 and topside sounders2,3. Its behaviour has also been studied by measurements of total electron content4 which show it to exhibit a diurnal variation reaching its lowest latitude shortly after local midnight. Rycroft and Thomas3, however, after removing variations due to changes in magnetic activity, have shown a movement towards lower latitudes as dawn progresses. Studies of the motion towards the equator of the trough with increasing magnetic activity have led Rycroft and Thomas to infer that the ionospheric trough and the magnetospheric plasmapause are basically related phenomena. Liszka and Turunen5 have shown that the polewards boundary of the trough follows the movement of the equatorwards boundary of the precipitation of 1 KeV auroral electrons and they propose a mechanism for the production of the trough in terms of the increase in temperature resulting from the influx of precipitating particles.

Characteristics of the abrupt scintillation boundary

An abrupt onset of scintillation on 40-MHz beacon satellite records occurs within the invariant latitude range 36–63° along the New Zealand meridian. Equatorward boundaries of the irregular regions giving rise to the scintillation occur most often near 56° invariant. The boundaries at the lower latitudes occur mainly at night. The latitude of the boundaries is greater during the day than at night and most of the irregular regions extend deep into the polar cap. The edges of the regions tend to be aligned along the magnetic field and more diffuse during the day than at night. There is also a tendency for the edges to be more diffuse at the higher latitudes. A preliminary analysis suggests that the scintillation boundary may be associated with gradients in the mid-latitude ionisation trough. The two phenomena show differing behaviour with magnetic activity however, and this could possibly be explained if the trough gradients change with magnetic activity. The poleward boundaries of other irregular regions show different characteristics, being confined to 40–45° invariant and more or less uniformly distributed throughout the day. These are far less common than observations of the equatorward boundary of the scintillating region that extend to higher latitudes.

On annual movements of the scintillation boundary of satellite signals

It is suggested that the scintillation boundary of satellite signals in the subauroral ionosphere exhibits annual movements. The boundary seems to be located closest to the pole at the December solstice and closest to the Equator at the June solstice. These movements are suggested to be linked with recently proposed annual changes in the size of the plasmasphere. The scintillation boundary seems also to be associated with the mid-latitude trough in the electron density of the F-layer, being possibly located near the equatorward wall of the trough.

Scintillation boundary during quiet and disturbed magnetic conditions

Earlier studies of ionospheric scintillations outlined the lower boundary of the high-latitude region where intense scintillations at 40 MHz were observed. The quiet-day scintillation boundary reached a lower position of 57° invariant latitude at 2200 LT. Dyson's (1969) recent observations with a Langmuir probe have verified the existence of a lower latitude boundary of small-scale irregularities. The boundary concept has been extended to include the effects of magnetic storms. Observations of satellite beacon signals at 40 and 54 MHz during 1961 to 1966 indicate that the mean change in the lower boundary latitude of the irregularity region is a decrease of approximately 1.6° per unit change in local K index. This is quite similar to the change of 1.8° per unit change in Kp noted for the trough position by Rycroft and Thomas (1970). In examining the data available from high-inclination and synchronous satellites, it was noted that the change in latitude with K index is a function of time. The maximum change of latitude as a function of K index, approximately 2° to 3° per unit K, occurred between 0300 and 0600 LT; the minimum change, about 1° per unit K, occurred over a broad interval from 1600 to 0200 LT. If the irregularity structure is produced by an interaction of the plasmapause with the ionosphere, the morphologic behavior of this region of the magnetosphere can be studied by reviewing the large inventory of scintillation and spread F data that has been amassed in the course of ionospheric research.

The High‐Latitude F‐region Irregularity Structure During the October 30–November 4, 1968, Magnetic Storm

A study of multi‐station scintillation observations of the 137‐MHz transmissions from ATS‐3 during the magnetic storm of October 30 to November 4, 1968, was made to separate the diurnal pattern of high‐latitude irregularities from storm‐time effects. The night maximum was maintained during magnetic storms, but the pattern was modulated by storm‐time effects. Daytime scintillation indices were increased at specific times during the magnetic storm; a universal time correlation was observed for stations in Europe and in North America. Peak scintillation activity shifted from before midnight (under quiet magnetic conditions) to the 0130 to 0430 time period. A sharp boundary of less than 2° was noted between scintillating and non‐scintillating regions. 40‐MHz transmissions from Explorer 22 during one period of the storm verified that the irregularity structure extended from 48° to 50° GN to auroral latitudes. Using records of 54‐MHz transmissions from Transit 4A taken between 1962 and 1965, as well as the ATS‐3 data, it was found that during magnetic storms the lower edge of the scintillation boundary, which bounds the irregularity structure, moves below its quiet positions to lower latitudes, particularly in the midnight‐to‐dawn time period. It was found that the observations were more adequately ordered by invariant latitudes than geomagnetic (dipole) latitudes used in scintillation papers. Latitude variations in position of the boundary are compared with the trough. It is found that the irregularity‐boundary changes as a function of K index and as a function of local time. Latitude variations in the position of the boundary are compared with the trough. Akasofu's concept of nearly open field lines in the plasmapause during magnetic storms might account for the increased scintillation indices as well as for the lower position of the scintillation boundary during magnetic storms.