Observations of random and quasi-periodic scintillations at southern mid-latitudes over a solar cycle

An extended period (1973–1985) of recording of random and Fresnel type quasi-periodic (QP) scintillations in southern mid-latitudes, using satellite beacon transmissions at a frequency of 150 MHz, has provided some new information on the morphology of scintillation-producing irregularities.It has become evident that a pronounced daytime increase of the random type of scintillations in the southern winter (at 1200–1600 LT) occurs throughout the solar cycle and becomes a distinct daytime maximum during the years of sunspot minimum. Scintillations are most intense in the pre-midnight period in the southern summer (2000–2400 LT). There is a gradual decline in scintillation activity by about 40% from the period of sunspot maximum to the period of sunspot minimum. It appears that a specific type of sporadic-E, so-called constant height Es (Esc), is responsible for daytime scintillation activity in winter. Night-time scintillations are strongly correlated with the presence of the range-spread type of spread-F, but not so with the frequency-spread type.There are two peaks in the occurrence of QP scintillations, predominantly in the southern summer: in the late morning (0800–1000 LT) and in the pre-midnight period (2000–2200 LT). The daytime QP scintillations occur mainly polewards of the station, whereas the night-time scintillations are recorded predominantly equatorwards. There is a distinct increase in the occurrence number of QP scintillations with a decrease in the sunspot number.     

Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum

A 5-yr study (1987–1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20–55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found: so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorward edge of auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988–1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.     

Post-midnight equatorial scintillation activity in relation to geomagnetic disturbances

VHF amplitude scintillation measurements made during the period April 1978 through December 1982 at Calcutta (23°N, 88.5°E; 32°N dip), situated near the northern crest of the Appleton Anomaly in the Indian sector, have been used to study the association of post-midnight (as well as post-sunrise) scintillations with the occurrences of the maximum negative excursion in the variation of the Earth's horizontal magnetic intensity. The post-midnight scintillation has been found to be related to the maximum negative excursion occurring in the 0000–0600 LT interval. No such relation is observed with the pre-midnight excursions. Scintillation with onset between 0000 and 0300 LT shows remarkable correspondence with the occurrence of negative excursion (18 out of 20 available cases). Magnetic conditions with Dst < −150 nT have been found to be most effective in producing the above scintillation activity. From the present observations, a threshold value of the maximum negative excursion of Dst for producing scintillation may be obtained, Dst < −75 nT being significantly associated with the post-midnight scintillation occurrences. The results are interpreted in terms of the reversal of the equatorial horizontal electric field, under magnetically disturbed conditions, due to a coupling of the high latitude and magnetospheric current systems with the equatorial electric field.     

VHF radio scintillations at Bombay

The simultaneous recordings of the amplitude scintillations of VHF radio signals from nearby geostationary satellites FLEETSAT (at 73°E long.) and SIRIO (at 65°E) received at Bombay (geog. lat. 19°N, geog. long. 73°E, mag. lat. 15°N) have revealed systematic time shifts in the starting and the ending of the individual scintillation events. The ionosphere crossover points of the two transmission paths were separated by only 80 km in the east-west direction, which was smaller than the average size of the irregularity patches. Scintillations normally started after 1930 h, reached a maximum at 2200 h and slowly decreased till 1000 h, after which no scintillations were observed. The speed of the irregularity patches computed from the time shifts of these events was about 150 m s⁻¹ in the early hours of the night, decreasing to about 100 m s⁻¹ by midnight and showing much lower velocities in the post-midnight hours.     

Quasi-periodic scintillation events at southern auroral latitudes

Quasi-periodic (QP) radio scintillations were observed during (1987) on 244 MHz and 1.5 GHz geostationary satellite transmissions in the southern auroral zone from Davis station (68.6°S, 78.0°E geographic, 74.6°S Aλ) in Antarctica. Three distinct types of OP events were identified, with occurrence times mainly restricted to the period 18-00 MLT. The substantial loss of signal associated with these events appears to be an important factor in determining the reliability of satellite links on 1.5 GHz in auroral regions. Previous observations at mid-latitudes of QP scintillations have noted a preference for large zenith angles and equatorward azimuths. It is demonstrated that a height transition in a densely ionized layer can produce QP scintillations in a manner analogous to a dense column of ionization but at lower ionization densities, as well as demonstrating a zenith angle and azimuthal dependence that is more consistent with observations than a column of ionization. At the occurrence times noted, the raypath may be intersecting the poleward edge of the trough where sporadic-E is a regular feature. QP scintillation events may result when the E_s-layer is height modulated by the passage of acoustic-gravity waves originating in the auroral zone.     

Simultaneous observation of the quasi-two-day variations in the lower and upper ionosphere over Europe

The large scale character of the observed quasi-two-day fluctuations in the whole ionosphere (from D- uptoF-region maximum) over Europe is shown. The study is based on the lower and upper ionospheric data obtained in Sofia (42.9°, 23.4°E), Ebre Observatory (40.9°N, 0.5°E) and El Arenosillo (37.1°N, 6.7°W) during two summer intervals: June–August 1980 and 1983. The obtained prevailing periods for the F-region fluctuations are 52–55 h and the mean amplitude is higher than 1 MHz. It was found that the fluctuations propagate westward with a mean phase velocity between 4.6 and 6° /h. The quasi-two-day variations in the F-region maximum are probably generated by flucutations in the mesospheric, neutral wind. During the time when well developed quasi-two-day fluctuations exist in the mesospheric neutral wind, similar variations are observed in the lower ionosphere also. Possible mechanisms for generating the D- andF-region electron density fluctuations from these oscillations in the neutral wind are proposed.     

Drift analysis of random and quasiperiodic scintillations in the ionosphere

Radio signals in the VHF/UHF range from the geostationary satellite ATS-6 were recorded using a system of three spaced antennas at Slough. Simultaneously, the integrated electron content (TEC) was monitored between the satellite and ground. Full correlation analysis and similar fade techniques were used to deduce the drift velocities of irregularities responsible for random and quasiperiodic (QP) ‘ringing’ scintillations. Similar drift velocities were found for the disturbances responsible for both types of scintillations at the times when QP and random scintillations occurred in a sequential pattern. A southward-drifting disturbance was responsible for rare, multiple QP scintillations which were followed by large scale fluctuations in electron density. In general, QP-scintillation-producing irregularities drifted southward, with velocities whose median magnitude and azimuth were 64 m s⁻¹ and 178°E of N respectively.The sequential occurrence pattern of QP-random scintillations as well as the time delay between occurrences of large fluctuations in TEC and QP scintillations, appear to be consistent with a reflection model of generation of the ringing fading of the signal.     

Spatial and temporal distributions of midlatitude ionospheric scintillations observed by low-altitude satellites

Spatial and temporal distributions of ionospheric scintillations have been observed at Kashima (36.0°N, 140.7°E) using VHF and UHF signals from low-altitude satellites. From these observations, three different types of prevailing ionospheric scintillations seen from Japan are identified. Scintillations of type I are rather weak scintillations, occur most frequently during the daytime in summer and are primarily associated with the sporadic E-layer. However, considerable occurrences of type I scintillations are also observed during the night in summer and autumn, not necessarily due to the sporadic E-layer but occasionally due to F-layer irregularities which originate from localized midlatitude processes. Type II scintillations are much stronger than type I and occur near the equatorward horizon during spring, summer and autumn. Their occurrences start after sunset, reach a maximum before midnight and decrease subsequently, with a tendency for negative and positive correlations with the magnetic and solar activities, respectively. It is concluded that type II scintillations are the midlatitude aftermath of equatorial plume-associated irregularities and cause trans-equatorial propagation of VHF waves. From observations of type I and II scintillations, the boundary between midlatitude and equatorial scintillations is clearly identified. Type III scintillations are as strong as type II and appear only during magnetically active periods. They can be regarded as another aspect of the severe scintillation events observed on gigahertz waves from geostationary satellites as reported by Tanaka (1981).     

Spectral characteristics of magnetic storm-induced F-region scintillations extending into daytime

Magnetic storm-induced F-region scintillations extending into daytime were recorded over Bombay, situated near the anomaly crest region in India, on 12 November 1991. The scintillations at 244 MHz using the radio beacon onboard FLEETSAT (73°E), lasted till 1312 h IST (77.5°E). Observations at Trivandrum, situated close to the magnetic equator also show strong daytime scintillations lasting till 1030 h. The scintillation event followed a sudden commencement at 1748 h UT (2318 h IST) on 11 November 1991 and the ionosonde observations, both over Ahmedabad in the anomaly crest region and Kodaikanal near the magnetic equator, show large upward drift of about 50 m s⁻¹ around 0300 h IST. The scintillation index S₄, autocorrelation function and power spectra have been computed from the digital data recorded at Bombay. The time variation of S₄ shows large fluctuations with a periodicity of about half an hour. The 50% decorrelation time of the signal fluctuations is of the order of seconds. The spectral index n, of the temporal power spectra, where P(F)αF⁻ⁿ, varies between 1.5 and 5.0, with a mean value of 3.0, and shows a dependence on the S₄ index. These features are similar to those reported for night time scintillations recorded over Ahmedabad.     

Equatorial scintillations—a review

The scintillation technique, as is well known, provides an integrated measure of phase and amplitude fluctuations imposed on radio signals over a wide range of frequencies during their propagation through the ionosphere. The large amplitude of equatorial irregularities necessitates the use of frequencies in the GHz band to obtain unambiguously the temporal variation of irregularity intensity and the effect of irregularity anisotropy. Recent observations of equatorial scintillations will be reviewed with an emphasis on GHz measurements. The steep spatial gradients observed in in-situ data and their relationship to intense GHz scintillations will be explored. Co-ordinated measurements of equatorial irregularities by such techniques as radar backscatter, in-situ rocket and satellite, total electron content and 6300 Å airglow will be discussed, insofar as they provide a better understanding of the scintillation phenomena. While it is difficult to critically assess results that are so recent and constantly evolving, we have attempted to focus attention on the outstanding problems that still remain in the field.     

Small-scale ionospheric structures associated with mid-latitude spread-F

Slant-F traces on ionograms recorded by a modern ionosonde in a sunspot-minimum period have revealed the existence of field-aligned irregularities at times of spread-F occurrence. This appears to be the first investigation in a mid-latitude region around 36° (geomagnetic) to detect these irregularities at F₂-region heights using an ionosonde. Although such traces were observed frequently near sunspot minimum they were seldom recorded for periods close to sunspot maximum. Also, for a specific spread-F event in August 1989, both the ionograms from the modern ionosonde and scintillations of 150 MHz transmissions from a Transit satellite indicate the existence in the ionosphere of periodic structures (period around 11 min). The scintillation recording also included rapidly fading signals indicative of small-scale structures. The satellite had a path close to the magnetic meridian which passed through the recording station (Brisbane, Australia). Because of the enhanced signal fluctuations in the scintillation recording on this occasion it seems likely (with the support of other evidence on the ionograms) that the small-scale structures present were field-aligned.     

spread-F ionospheric irregularities and their relationship to stable auroral red arcs at magnetic mid-latitudes

The signature of the stable auroral red arc (SAR arc) as it appears on ionograms is described. The key features are a very significant increase in the amount of spread-F and a reduction in the maximum plasma density compared with regions just equatorward and poleward of the SAR arc Identification of the SAR arc signature is made by using complementary data from the global auroral imaging instrument on board the Dynamics Explorer-1 satellite.At sunspot minimum there is a positive correlation between the occurrence of spread-F on ionograms from Argentine Islands, Antarctica (65°S, 64°W; L = 2.3) and magnetic activity. In contrast, at sunspot maximum there is a weak negative correlation when the K magnetic index is less than 6. but a significant increase in spread-F occurrence at K ⩾ 6. Detailed study of ionograms shows that there are two distinct regions where considerable spread-F is observed. These are the region where SAR arcs occur and the poleward edge of the mid-latitude ionospheric trough. They are separated by a region associated with the trough minimum, where comparatively little spread-F is seen. It is suggested that the movement of these features to lower latitudes with increasing magnetic and solar activity can explain the lack of correspondence between variations of spread-F occurrence as a function of magnetic activity at sunspot maximum compared with that at sunspot minimum at Argentine Islands.     

Large simultaneous disturbances (LSDs) in the Antarctic ionosphere

A high frequency radio Doppler experiment was deployed in the Antarctic Peninsula region, centred on Argentine Islands (65°15′S, 64°16′W; L = 2.3), to investigate the morphology and sources of ionospheric disturbances. The experiment consisted of a three-transmitter dual frequency network which permits horizontal and vertical propagation velocities to be estimated over a north-south baseline of 200 km and an east-west baseline of 100 km.A new class of ionospheric disturbance has been observed, in the period range 10 min−1 h. These disturbances are characterised by unusually good correlation between perturbations on all available Doppler signals, but are apparently non—propagating and occur simultaneously at each reflection point. Several of these events display large (2 Hz at about 5 MHz transmitted frequency) Doppler shifts, thus we have labelled them Large Simultaneous Disturbances (LSD_s).Criteria for identification of LSD_s are established and the analysis of one event is described in detail. The occurrence statistics of the LSD_s are presented, including their seasonal and diurnal distributions.There is no clear general relationship between LSD_s and local geomagnetic field perturbations. However, examination of the magnetic indices AE and I_RC indicates that there is a loose association between the occurrence and amplitude of LSD_s and magnetic activity.Several possible mechanisms for the generation of LSD_s at middle latitudes are reviewed. The most likely explanation is that high latitude electric fields penetrate to magnetic middle latitudes and drive the ionospheric plasma via the E × B drift.     

Diurnal tide in the Antarctic and Arctic mesosphere/lower thermosphere regions

The behaviour of the diurnal tide at 95 km over various years between 1965 and 1986 is studied using radar data from Heiss Island (81°N), Mawson (67°S), Molodezhnaya (68°S) and Scott Base (78°S). The observations are also compared with the model results of FORBES and HAGAN [(1988) Planet. Space Sci. 36, 579] for the same latitudes. There are substantial fluctuations in amplitude and phase at all stations, particularly in winter. Phase fluctuations can be as large as a uniform random distribution over the 24-h cycle. In summmer the phases of the meridional components are well defined and suggest the presence of a dominant symmetric mode. The meridional amplitudes are larger in summer whereas the zonal components have a greater variation and show no significant variation with season.     

Ionospheric control of polarization of low-latitude geomagnetic micropulsations at sunrise

Continuous observations of low-latitude Pc3 and Pc4 geomagnetic micropulsations were carried out at ASO (22.0°N, 198.0° geomagnetic coordinates) from November 1979 to July 1980 to confirm the ionospheric control of polarization characteristics of low-latitude pulsations presented by Saka etal. (1980). The present study confirms the previous result that D-component amplitude starts to increase with sunrise. From the present study, the following results are obtained : (1) the D-component amplitude, which is much smaller than the H-component amplitude before sunrise, increases as much as that of the H-component after sunrise, and this brings about the tilting of the major axis of the polarization ellipse from north to northwest; (2) the onset-time of the D-component increment (or tilting of the major axis) coincides with the appearance of the E-layer in the ionosphere within an hour, and the time of the coincidence shifts from season to season, in parallel with the change of sunrise ; and (3) the ellipticity of the polarization in the horizontal plane is not affected appreciably by sunrise.It is suggested that the Hall conductivity increment associated with the E-layer sunrise enhancement affects the characteristics of the D-component on the ground.     

Very-low frequency signals (2–10 kHz) received at Palmer Station,Antarctica, from the 21.4 km dipole antenna at Siple Station, 1400 km distant

Radio signals transmitted from the unique experimental VLF transmitter at Siple Station (76°S, 84°W), Antarctica, as well as VLF signals from communication and navigation systems and waves that propagate in the ionosphere and magnetosphere in the whistler mode, are regularly received and analysed at Palmer Station (65°S, 64°W), Antarctica. The amplitude and polarization properties of the Siple signals are predicted using a ray optics analysis. The amplitude of the signal received from Siple varies with frequency; observed nulls in the signal spectrum, where thesignal amplitude/alls 5–10 dB below what might be expected, are explained by the ray analysis. The amplitude spectrum is observed to be very sensitive to ionospheric conditions. Whereas the arrival bearings of signals from VLF transmitters other than Siple are found to be within 5° of their expected values, which is consistent with their expected vertical polarization and the operation of the DF system, an approximately 90° anomaly in the apparent arrival bearing of the signals from Siple is attributed to the essentially horizontal polarization of the received signal. The anomaly is found to be consistent with the theory of operation of the DF system. Occasional anomalies greater than 90° are explained in terms of a combination of polarization error and a smaller multi-path error. Siple two-hop signals and whistlers propagating on a common magnetospheric path showed arrival bearings and other properties consistent with a path end point within 200km of Siple. This suggests that these signals were received at Palmer with essentially vertical polarization.     

Electrojet irregularity parameters from daytime equatorial scintillations

The second moment of the complex amplitude or the mutual coherence function (MCF) for transionospheric VHF radio waves transmitted from the geostationary satellite ATS-6 is computed from daytime amplitude and phase scintillations recorded at an equatorial station, in order to study the structure of electrojet irregularities. The shape of the correlation function for fluctuations in the integrated electron content along the signal path is deduced by using a theoretical relationship between this correlation function and the MCF which is based on the assumption that the irregularities are “frozen”. Further, using a power-law spectrum to describe the electrojet irregularities, the outerscale l_o associated with the spectrum as well as the r.m.s. density fluctuation are estimated from theoretical fits to the computed values. The irregularity drift speeds V_o transverse to the signal path, for the scintillation events studied here, are derived from power spectra of weak scintillations. On the basis of a relationship between l_o and V_o suggested by a linear theory of the gradient-drift instability, the effective Hall conductivity is estimated to be about five times the effective Pedersen conductivity in the electrojet region.     

Ionospheric sources of Pi(c) pulsations

Data collected at Macquarie Island (invariant latitude 64.5°S) shows average delays of 0.6 s and 0.2 s between D and H component Pi(c) pulsations, respectively, and the N₂⁺1NG (0,1) optical band pulsations. The dominant phase of the cross-correlations between the H component pulsations and the optical pulsations is consistent with the H component fluctuations being generated by increases in a westward electrojet. The D component fluctuations are consistent with either an increase in an equatorward Pedersen current or an increase in a field aligned current. The polarization of the Pi(c) pulsations may be explained in terms of the altitude of the respective currents.     

Annual variations in the electron content and height of the F layer in the northern and southern hemispheres,related to neutral composition

Total electron content (TEC) data is presented for similar sites at ±35° latitude, and conjugate sites at ±20°, for several years near solar maximum. Comparison with the MSIS atmospheric model shows that the large seasonal anomaly at 35°N (an increase of 80% in TEC from October to April) is fully explained by changes in neutral composition. The small seasonal anomaly at 35°S also agrees with the MSIS model. Composition changes fail to account for the generally higher TEC in the northern hemisphere; this suggests the presence of an overall south-to-north atmospheric wind. Eastern declinations also contribute to enhanced TEC in the northern hemisphere, in the Pacific zone. The MSIS model predicts a semiannual variation of about ±25% in TEC at all sites, while observed changes are only about ±8%; thus we require some enhanced loss process near the equinoxes, particularly in September and October.Peak height calculations assuming a constant pressure level give a large semiannual variation in the F2 region: this is replaced by an annual variation when h_m F2 is calculated from diffusion theory. Heights calculated from the MSIS model are similar to observed values at ±35° latitude on summer days. A decrease of about 20km in observed heights on winter days is attributed to a poleward neutral wind; this wind also reduces the observed TEC. At night the height changes correspond to an equatorial wind, which is largest in summer and equinox. Observed day time TEC is greater at 20°N than at 20°S at all times of year, suggesting a northward transequatorial wind which is strongest near January and gives increased TEC and decreased peak height at 20°N.     

Annual and semiannual variations of the geomagnetic field at equatorial locations

For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0° and 30° north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements (H, D and Z) were included in the study.The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in Z having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude.The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The Sq(H) amplitudes decrease rapidly with increasing latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude.The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.