Seasonal variations of equatorial night-time thermospheric meridional winds

Using h'F data at two equatorial stations, night-time equatorial thermospheric meridional winds have been deduced for a period of two years to study their seasonal characteristics. It has been found that the thermospheric wind shows trans-equatorial flow from summer to winter hemisphere. During equinoxes the flow is mainly equatorward with a reversal to poleward direction around midnight hours. The abatement and reversal of equatorward wind which is weaker in summer compared to equinoxes is attributed to Midnight Temperature Maximum (MTM). The results of the present investigation are compared with those at other equatorial stations and also with the empirical model of Hedin et al. (1991).     

Effects of neutral temperature on meridional winds estimated from ionospheric data

Indirect determination of meridional winds using ground-based ionosonde data from low latitude regions, under the assumption that the thermosphere and the F-region of the ionosphere behave as a closely coupled system, has been critically examined. The significance of neutral temperature and its variations in the above estimates has been demonstrated through individual case studies after duly validating the procedure adopted. Since the measured neutral temperatures have shown large deviations from the existing atmospheric models on many occasions and more so during high solar activity periods, it has been shown that the neutral temperature effects on the F-region heights should be properly accounted for before one attempts to estimate meridional winds. However, it has also been shown that during low solar activity periods, use of atmospheric models may still provide a fairly reasonable average picture. Examples of these effects are presented and discussed.     

A study of the meridional structure of the equatorial red arcs in the night-time ionosphere from the ISIS-II satellite

Equatorial 6300 Å arcs observed by the ISIS—II satellite close to the magnetic equator over the African and Asian zones are studied for night-time conditions from 21:00 h to 02:00 h local time in the summer and spring of 1972–1974 and 1976, respectively. Case studies of the arcs have been made for quiet geomagnetic conditions and for minor storms. Sometimes very intense arcs with intensities of 1–2 kR are observed. Arcs of moderate intensities (300–400 R) are observed during geomagnetically disturbed periods. It is confirmed that these intensities can be fully accounted for theoretically by the dissociative recombination of molecular oxygen ions. Since the emission intensities are found to be sensitive to the geomagnetic activity, the influence of the latter has been taken into account and discussed.Equatorial spread-F (ESF)/bubble conditions are usually present at these local times. The data presented here show a correlation between the 6300 Å emission rate at one of the anomaly crests, the gradient in h (the lowest scaled real height from topside ionosonde trace) and the existence of ESF and gravity waves. This correlation is consistent with the scenario put forward by Maruyama and Matuura that the occurrence of ESF requires a symmetrical electron density distribution around the magnetic equator, so that a transequatorial wind causes an asymmetry and inhibits the formation of ESF.For the ISIS data we conclude that where strong transequatorial winds exist the 6300 Å emission rate at one of the anomaly crests is very large and there is a steep gradient in h. When these winds are weak, the 6300 Å emission is low and the gradient in h is also small. In the latter case, gravity waves of wavelength 200–400 km were present as well, which suggests that ESF is promoted by the existence of gravity waves. However, the magnetic disturbance level was higher during these orbits, which offers another source of gravity waves.     

Role of vertical winds on the Rayleigh-Taylor mode instabilities of the night-time equatorial ionosphere

In view of the recent observations on the presence of vertical winds in the equatorial ionosphere in the evening and night-time, the role of vertical winds in the Rayleigh-Taylor (R-T) mode instability has been re-examined. The mathematical treatment of Chiu and Straus, earlier developd for a case of horizontal winds, is extended to evaluate the role of vertical winds in causing the R-T mode instability. It is shown that the vertical (downward) winds of small magnitude have a very significant effect on the instability growth rate in the. F-region. A downward wind of l m s⁻¹ can cause the same growth rate as a 200 m s⁻¹ eastward wind at 260 km altitude. Furthermore, a downward wind of 16m s⁻¹ at 300 km can be as effective as that due to the gravitational drift itself. Similarly, an upward wind can inhibit the instability on the bottomside of the F-region. It appears that the polarity of the vertical winds (upward or downward) at the base of the F-layer plays an important role in the growth of the R-T mode plasma instability in the equatorial ionosphere.     

Synthesis of oblique ionograms from vertical ionograms using quasi-parabolic segment models of the ionosphere

This paper describes a method of synthesizing oblique ionograms from vertical ionograms based on representing the ionosphere by multiple quasi-parabolic segments (QPS). The advantage of this approach is that it allows analytical solutions to be obtained for several ray parameters when the Earth's magnetic field is neglected and the ionosphere is spherically stratified. The no-field results are representative of the ordinary mode. In addition, results for the extraordinary mode can be obtained by introducing a perturbation to the effective frequency and modifying the no-field results. The method is illustrated by comparing synthesized oblique ionograms with observed ionograms and with results obtained for the ordinary mode using the classical method.     

Spectral behaviour of TIDs detected from rapid sequence ionograms

The main purpose of this paper is to illustrate the usefulness of ionosonde observations in the study of the spectral evolution of travelling ionospheric disturbances (TIDs). An inversion method is introduced to yield the spatial and temporal variation of the ionospheric velocity from data of modern ionosondes or rapid-run ionosondes. Some inversion results of rapid sequence ionograms are spectrally analysed to obtain the height- and time-dependent power spectra of TIDs, and the evolution of these spectra is discussed in detail. It is found that spectral peaks shift regularly with time. As time increases, the peaks of the lower frequency components or those at the lower heights tend to shift towards lower frequency, while those of higher frequency or at higher altitudes lend to higher frequency. This property is explained by the current dispersion theory of atmospheric gravity waves. It is concluded that the spectral behaviour of TIDs can be well studied by using ground-based HF techniques, such as ionosonde observation.     

Electron density profiles over the Southern Ocean from oblique incidence ionograms

This paper presents a first attempt to use oblique incidence ionograms over the 4500 km path from Sanae, Antarctica, to Grahamstown, South Africa, to deduce information about the ionosphere in the intervening regions. It is shown that existing methods for the reduction of oblique incidence ionograms to N(h) profiles give reasonable results even over the two-hop path involved. By comparison with vertical incidence ionograms made from a research ship below the reflection regions it is shown that the maximum observed frequency is normally limited by conditions at the southernmost reflection point, though this may be modified by ionospheric tilts, sunrise and sunset.     

A method of determining horizontal structure of the ionosphere from backscatter ionograms

We have developed a technique lor oblique backscatter sounding (OBS) ionogram inversion as a diagnostic tool for the horizontally inhomogeneous structure of the ionosphere. Input data for the method include the leading edge of a backscalter ionogram that is measured through soundings in a given direction, and the vertical electron density profile measured over the sounding station or over some other site lying in the sounding direction. The method may be useful for reconstructing the two-dimensional electron density distribution in a vertical plane aligned with the direction of sounding. The inverse problem has been solved using the Newton Konlorovich method and the Tikhonov regularization method. The algorithm we have developed was tested against model data, that is, OBS ionograms synthesized using geometrical optics calculations for different models of the inhomogeneous ionosphere. Test results demonstrate that our method converges reliably, is stable to measurement errors and provides a good accuracy of reconstruction of inhomogeneous structures with scales of 100 2000 km. This indicates that this method shows promise as an operative remote diagnostic tool for ionospheric irregularities of natural and artificial origin.     

Inversion of oblique ionograms including the Earth's magnetic field

A technique for determining ionospheric electron distribution from oblique ionograms is presented, based on the inversion method of Reilly and Kolesar (Radio Sci. 24, 575, 1989). It makes use of an equivalent operating frequency and an additional term to account for magnetoionic effects associated with the Earth's magnetic field. The technique is demonstrated by application to synthetic oblique ionograms, and to an experimentally obtained ionogram.     

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.     

Starting models for the real height analysis of ionograms

Procedures are described for use in the real height analysis of ionogram data using the ordinary ray only, to allow for the presence of underlying (low density) ionisation. A controlled extrapolation of the virtual heights, with upper and lower limits, gives some useful correction under most conditions. For more accurate and consistent results a synoptic model is used to give a mean starting height at a fixed frequency of 0.5 MHz. Constraints are placed on the profile shape between 0.5 MHz and the lowest observed frequency ₁, to minimise the variations with different methods of analysis and different values of ₁. Suitable model starting heights are described for day and night conditions, and presented in tabular and graphical form. Equations are also given from which the model starting height can be calculated directly as a function of the local time, the month and the station latitude.     

The application of a new method for determining irregularity parameters from topside ionograms

A method is presented for determining the amplitude and vertical wavelength of ionospheric irregularities which produce distortions or perturbations in topside ionogram traces. By way of illustration we discuss the application of the method to several examples which give irregularity amplitudes ranging from 0.1% to 10% and vertical wavelengths from 40–300 km. From the examination of 2 years data obtained by the ISIS II topside sounder it was found that irregularities readily analyzed using this method were confined to the equatorial region. They are detected at approximately 1000 km altitude and above, however, they may exist below these altitudes, since it is shown that topside sounders are more sensitive to irregularities in regions where the ionospheric scale height is large. Their characteristics are consistent with their being part of the field-aligned or duct irregularity phenomenon and their occurrence is consistent with the production mechanism suggested by Cole.     

Vertical winds in the midlatitude thermosphere from Fabry-Perot interferometer measurements

Monostatic and bistatic measurements of thermospheric winds have been made with Fabry-Perot Interferometers at the Millstone Hill and Laurel Ridge Observatories. Synchronized observing sequences have been chosen to enable the determination of vertical winds from the measurements. The vertical winds are found to be significant on some nights with velocities of tens of m/s. For geomagnetically quiet nights, the averaged values for eight samples in the July–October 1992 period show little variation during the night, averaging ∼10 m/s downward. The average for three geomagnetically disturbed nights oscillates from a downward maximum of ∼50 m/s at ∼02 UT to an upward maximum of ∼20 m/s at ∼07 UT. Temporal variations in the vertical motions suggest oscillatory behavior with periods of ∼0.7, ∼1.8 and ∼3.4 h, respectively, on three quiet nights, possibly associated with gravity wave or tidal-harmonic effects.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.     

A study of meteor radar winds from two locations in the British Isles

Winds and tides have been measured by a two-station meteor radar system which has increased spatial resolution compared with single station radars used in the past. Narrow radar beams, pointing SW from Sheffield (53.5°N, 1.6°W) and 30°N of W from Shrivenham (51.5°N, 1.6°W), are arranged to converge over the U.K. MST radar site near Aberystwyth, thus defining a unique atmospheric volume in which meteor wind components are simultaneously measured from the two radar sites. The resultant ‘true’, or local, wind vector is compared with the spatially averaged vector obtained with the aid of beams pointing SW and NW from Sheffield only. It is found that the ‘true’ and averaged tidal winds are in good agreement, as expected from their large scale sizes, and that the main advantages of the dual station technique lie in the resolution of a small scale structure such as that related to internal atmospheric gravity waves. By the simultaneous deployment of two-station meteor radar, MST radar and LIDAR, such waves may now be studied through a large vertical section of the atmosphere in a geographically localized area.     

Seasonal mean structure of the night-time F2 region over Arecibo

Seasonal mean night-time variations of ion and electron temperatures, electron density, ion drift velocity, and light ion composition of the F2 region are derived from incoherent scatter observations at Arecibo based on 19 nights of observation over the latest sunspot minimum years 1974–1976. It is shown that the downward flux of ionization is sufficient to maintain the nocturnal F2 region against recombination at low latitudes. The difference in the electron density decay rate from summer to winter is consistent with the seasonal variation in magnitude of the ionization flux. The mean eastward electric field, which is responsible for any vertical component perpendicular to B, is very small throughout the night. However, the southward electric field, i.e. east-west ion drifts, shows a substantial systematic variation during the night, being southward (eastward ion drifts) before midnight and northward after midnight, with a mean amplitude of 1–2 mVm⁻¹. The H⁺ ion concentration shows a marked seasonal variation. The mean relative concentration of H⁺ ion to electron density at 500 km sometimes exceeds 50% before sunrise in winter. A strong anti-correlation of H⁺ ion concentration with magnetic activity is observed. The observed ion temperatures average about 20–30 K higher than the prediction of the Jacchia (1971) neutral model for the observed range of the 10.7 cm solar flux.     

Propagation characteristics of night-time whistlers in the region of equatorial anomaly

In this paper results of simultaneous multi-station observations of night-time whistlers obtained at very low latitudes for the last three years are presented. The propagation mechanism of these whistlers is also discussed. Some records can be shown to have the characteristics of ducted propagation by calculating the dispersion of the whistlers along a ducted path and by noticing the feature of strong intensity, as well as the existence of three-hop echoes. Whistlers in some other records, however, can be shown to be propagating along a non-ducted path by numerical ray-tracing, in which a non-dipole field (IGRF) is used and a negative horizontal gradient of ionization in the equatorial ionosphere is taken into account. The agreement of the results of calculation with observed facts seems to confirm the existence of a definite whistler path at very low latitudes. Finally, based on analysis of typical events, the effects of magnetic storms on the parameters of low latitude whistlers due to variations in the equatorial ionosphere are also discussed.     

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Model studies of the D-region negative-ion composition during day-time and night-time

A model for the negative-ion composition of the D-region is constructed, based on gas-phase reactions and photodissociation and photodetachment processes. The height distributions of negative-ion species, electrons and total positive ions for day-time and night-time are examined using time-dependent solutions of their continuity equations which incorporate simultaneous solutions for the minor neutral constituents involved. A distinction is drawn between the two isomers of NO₃⁻ and particular attention is paid to the influence of possible chemical and photodestruction of the more stable form of this ion.