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.     

The influence of geomagnetic activity on the upper mesosphere lower thermosphere in the auroral zone. II. Horizontal winds

A spaced antenna partial reflection radar located at Mawson, Antarctica (67°S, 63°E, invariant latitude 70°S), has been used to measure the horizontal wind field in the height range 70–110 km. Three years of data (1985–1987) from the radar have been analysed in order to investigate correlations between geomagnetic activity (determined from the local K-index) and the horizontal wind. Results are analysed using a randomization technique and show that larger winds are measured during geomagnetically active periods in both the raw (or unfiltered) wind values and in the medium-frequency (2–6 h period) and high-frequency (1–3 h period) components. The raw winds tend to be shifted towards the geographic NW to NE quadrant in the early morning hours during high K-times. The observed correlation is seen down to 86 km and shows a seasonal dependence. The mean r.m.s. velocity of the radar scatterers and the angular spread of the return echoes are also found to be correlated with geomagnetic activity. The medium- and high-frequency components of the wind are polarized in the magnetic zonal direction during all seasons of the year.     

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.     

Effects of composition and fountain effect on the annual variation of the day-time F-layer at low latitudes

The annual variation of the daytime F2-layer peak electron density (NmF2) is studied at two low latitude stations, Okinawa and Tahiti (geomagnetic latitudes ± 15°) for the sunspot maximum years 1979–1981. Observed values are compared with those calculated using the MSIS model and a simplified version of the continuity equation for day-time equilibrium conditions. Summer-winter differences imply an intensification of the fountain effect on the winter side of the equator at the expense of the summer side. This could be explained by a summer to winter neutral wind. Semi-annual variations, however, appear to be mainly due to changes in neutral composition.     

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).     

Faraday polarization fluctuations associated with amplitude scintillations at Lunping

The diurnal, seasonal and solar cycle variations of Faraday polarization fluctuations (FPF) associated with amplitude scintillations observed at Lunping, Taiwan (25.0°N, 121.2°E : geographic) during the period 1978–1981 are presented. The occurrence of polarization fluctuations is maximum in the premidnight hours. FPFs occur either simultaneously or with a time lag after the onset of amplitude scintillations. There is an increase in FPF activity with an increase in sunspot activity. Occurrence of FPF peaks in the equinoxes. There had been a moderate activity in summer while the winter occurrence is a minimum. The seasonal occurrence pattern compared with reports from other locations indicates a longitudinal control on FPF activity. The maximum probable duration of FPF ranges from 15 to 30 min. It was found that the association of FPF with range spread-F is much better than that with frequency spread-F. Large ambient ionization densities corresponding to plasma frequencies greater than 15 MHz appear to create a favourable environment for the occurrence of FPF.     

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.     

OI 630 nm imaging observations of equatorial plasma depletions at 16° S dip latitude

Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazil's National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.     

Sodium density and atmospheric temperature in the mesopause region in polar summer

Sodium lidar measurements have been performed during three summer seasons at a polar latitude (69°N), yielding profiles of sodium number density and temperature of the 85–100 km altitude region. Density measurements were performed during the months of June–August; temperature measurements only were made in August. The sodium layer was found to be both significantly weaker and more variable in summer than in winter. Measurements in summer 1987 yielded an average maximum Na density of about 900 atoms per cm³. The average maximum Na density during summer 1986 and 1988 was near 2600 atoms per cm³. The observed Na column density in summer varied from 3.10⁸ to 3.10⁹ atoms per cm². Temperature measurements were performed in August of 1986 and 1987. Mesopause temperatures of less than 125 K were observed in early August, rising rapidly throughout the month. The mean altitude of the mesopause was found to be about 87 km.     

Atmospheric X-rays at 11°S geomagnetic latitude

X-ray measurements at balloon altitudes were made at São José dos Campos, Brasil (23°12′S, 45°51′W geographic coordinates, ~11°S geomagnetic latitude) on 18 December 1981, using an omnidirectional NaI(T1) scintillation detector. Atmospheric X-rays, namely secondary X-radiation from cosmic rays, were measured for the energy interval 30–155 keV and up to an atmospheric depth of 5.5 g cm⁻². A comparison of the flux measured at the Pfotzer maximum during these measurements with those obtained previously by several research groups at other latitudes and with a similar technique has also been made. Finally, a comparison of the atmospheric component with that attributed to the diffuse component is also presented and it is concluded that both components are of about the same magnitude at ~ 5 g cm⁻² and at ~ 11°S geomagnetic latitude.     

A search for a solar cycle-temperature relationship in the middle atmosphere over Volgograd

The variation of temperature in the middle atmosphere (15–80 km) at Volgograd (49°N, 44°E) during an 11-year solar cycle (1971–1982) has been studied. The temperature of the stratosphere did not show any significant influence of the sunspot cycle, but the temperatures of the mesosphere showed a strong in-phase relationship with the solar cycle. Computed correlation and regression coefficients were positive and highly significant in this region. At 60 and 70km the temperature variations were almost linearly related to the sunspot number. Seasonal studies indicated that solar activity has a much stronger influence on temperature during the winter than during the summer.     

Behaviour of HILAT scintillation over spitsbergen

Results of the amplitude scintillation morphology of the HILAT satellite 137 MHz beacon transmission as measured at the Polish Polar Station at Hornsund, Spitsbergen (Δ = 73.4°) are presented. Seasonal, diurnal and latitudinal dependencies of scintillation intensity on magnetic activity were analyzed from over 2250 satellite passes recorded at solar minimum between April 1985 and March 1986. Regions with strong scintillation intensity appear to follow the auroral oval expansion and to move sunward with increasing level of magnetic activity. Maximum amplitude scintillation region coincides with the dayside cusp/cleft position during high magnetic activity. The dawn-dusk asymmetry in scintillation intensity is more distinct in winter than other months. The estimated summer/winter ratio of scintillation intensity is 1.4: 1. Numerical simulations compared with the observational results indicate that high latitude irregularities < 1 km are field-aligned and rod-like rather than sheet-like.     

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

Equatorial night-time spread-F characteristics observed from stations in the Western Pacific

A comparative study of the ionospheric spread-F observed at an equatorial station. Port Moresby (9°24.5′S, 147°9.9′E; magn. dip lat. 18°S). Papua New Guinea, with corresponding data from the East Asian stations during a period 27 May–19 June 1983, as well as the same periods for 1981 and 1982, is presented. The Port Moresby spread-F condition is particularly outstanding and persistent at night and the occurrence of spread-F between 23 LT and 05 LT near June solstice is very high, reaching 100 % of all observed nights. The latitudinal variation of nocturnal spread-F, its duration and connection with sunspot number are analyzed. The relationship between long-term variation of night-time spread-F occurrence and solar activity during June and December solstices from 1954 up to 1984 at a typical lower magnetic latitudinal station, Wuhan (30°32.7′N, 114°21.5′E; magn. dip lat. 26°N). is also investigated.     

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.     

Identification of the magnetospheric cusp and cleft using Pc1–2 ULF pulsations

Geomagnetic pulsations in the 0.1–2.5 Hz (Pc1–2) range recorded over 12 quiet summer days at six Antarctic stations between −62.3 and −80.6° invariant latitude were examined in order to map the spatial and temporal distribution of spectral characteristics. Ionospheric particle signatures associated with the magnetospheric cusp and boundary layer were deduced for three of these days using ground riometer, magnetometer and ionosonde measurements, and in-situ ionospheric particle data. Comparison with the magnetic pulsation data shows that specific Pc1–2 emissions are associated with these regions. Within the cusp, intense unstructured ULF noise in the 0.15−0.4 Hz range is observed. Less intense waves of this type are seen near the cusp location on mantle and plasma sheet boundary layer flux tubes. These emissions are quite distinct from the discrete, structured and narrowband emissions seen equatorward of the cusp. Whereas past discussions of cusp and cleft identification have usually focused on optical or satellite data, we conclude that ground-based observations of Pc1–2 pulsations can provide a more convenient, although less precise, monitor of high latitude features.     

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.     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

The influence of ionospheric absorption on mid-latitude whistler mode signal occurrence from VLF transmitters

Whistler mode signals from VLF transmitters received at Faraday, Antarctica (65° S, 64° W) during 1986–1991 show an annual variation in the number of hours over which signals are observed, with a maximum in June and a minimum in December. The variation was larger at solar minimum than at maximum and can be understood in terms of changes in absorption of VLF signals in the D-region, where the high geographic latitude of Faraday plays an important role in producing low attenuation levels during the austral winter. In contrast, very little such variation was observed at Dunedin, New Zealand (46° S, 171° E) in 1991. Nighttime whistler mode signals have start and end time trends that are consistent with the influence of F-region absorption. Increases in whistler mode occurrence appear to be associated with periods of high geomagnetic activity at solar maximum but not during solar minimum. A possible mechanism involving decreased F-region absorption is discussed.     

The effect of the mid-latitude ionospheric trough on whistler mode ducting during magnetic storms

Whistler-mode signals observed at Faraday, Antarctica (65° S, 64° W, Λ=50.8°) show anomalous changes in group delay and Doppler shift with time during the main phase of intense geomagnetic activity. These changes are interpreted as the effect of refracting signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough. The refraction region is observed to propagate equatorwards at velocities in the range 20–85 ms⁻¹ during periods of high geomagnetic activity (K_p ≥ 5), which is in good agreement with typical trough velocities. Model estimates of the time that the trough edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46° S, 171° E, Λ=52.5°) that have propagated at an average L-shell of 2.2 (Λ=47.6°) do not show such trough-related changes in group delay. These observations are consistent with a lower occurrence of the trough at lower invariant latitudes.