Observations of the reflection coefficient of the sporadic E-layer at high latitudes

A method to measure the reflection coefficient of the sporadic E-layer using a conventional ionosonde is described, and some results obtained at Sodankylä, Finland (Φ=63.8°, φ= 120.0°), are presented. The observations are often found to be in agreement with the theoretical frequency dependence of the reflection coefficient of a thin horizontal layer in the presence of mode coupling. Some of the results can be interpreted as indicative of scattering from small-scale irregularities or reflections from larger inhomogeneities in the E_s layer.     

Amplitude statistics of signals reflected from the ionosphere

Statistical analysis methods used to define the amplitude distributions of signals returned from the ionosphere are discussed in this paper. Emphasis is placed on determining accurately the parameter B, which is the ratio of steady to random components present in a signal. Thus B > 1 if the signal is dominated by the steady component, and B < 1 when the random components dominate. This study investigates the characteristics of B for F-region and E-region ionospheric echoes, as well as some types of spread-F, observed at the southern mid-latitude station Beveridge (37.3 S and 144.6 E). The results indicate that amplitude measurements obtained in approximately 100 s are adequate for determining B. The results also illustrate some effects that the E-region can have on F-region echoes.It is found that frequency spreading, the most common type of spreading observed at Beveridge, displays strong specular reflections and some signal variation due to interference at the leading edge of the F-region echo (i.e. B > 2). Within the spread echo B fluctuates between 0 and about 1.5 but is typically less than 1. The autocorrelation function of signal amplitude has a relatively large coherence interval, suggesting that this type of spread-F is due to interference of specular reflections from coherent irregularity structures with horizontal scale sizes of tens of kilometres rather than scattering from small scale irregularities. A second form of spread-F which would generally be classified as frequency spreading on standard ionoerams is actually due to off-vertical reflections from patches ol irregularities which originate south (poleward) of Beveridge. Echoes within this oblique spread-F (OS-F) do not exhibit coherence indicating that the irregularities responsible are of a smaller scale than those producing normal frequency spread. Finally, the phenomenon of spreading occurring on the second hop, but not the first hop trace is studied. It is shown that the form of the second hop echoes can be reproduced using a simple geometric model of ground scatter. The interpretation is supported by the fact that B for spread second hop echoes is less than 1 whereas it is much greater than 1 for the corresponding first hop echoes.     

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.     

Analysis of Es traces from a calibrated oblique ionosonde

An analysis is performed on ionosonde data produced during five years of operation of an oblique sounder transmitting on a path from Darwin (12.4°S, 130.9°E) to Alice Springs (23.5°S, 133.7°E). It is found that the occurrence of sporadic-E (E_s) shows a relatively mild diurnal dependence, with a significant amount of E_s occurring in the early evening before midnight. It appears that, on average, nighttime E_s produces weaker reflections than daytime E_s.The power of the E_s reflections as a function of frequency is collated for all ionograms. The resulting power curve exhibits total and partial reflection sections. In trying to reproduce the partial reflection section of the curve it is shown that a layer without horizontal structure is required to be only 100 m thick. A second model involving a layer consisting of horizontally localised clouds of scatterers, with scale sizes ranging from 100 to 1000 metres, reproduces the partial reflection section of the curve quite well. The size, intensity and distribution of the clouds affects the curve shape on individual ionograms, resulting in the suggestion that nighttime layers are more irregular than daytime layers.     

Spread-F and ionization anomaly belt

The present investigation attempts to bring out the dynamics of the F-region at magnetic equatorial and low latitudes in the American zone. Data are examined for two sets of nights, one with strong range-type spread at Huancayo another with complete absence of spread-F. A prominent bulge of the F-region was observed within and below a latitude 10°N in the evening hours of the spread-F nights. Contours of electron distribution during post-sunset hours at the equatorial latitude, Huancayo (Dip 2°N); low latitude, Talara (dip 13°N); and a location near the anomaly crest location, Panama (dip 38°N), indicated a much steeper gradient in electron density at fixed heights on spread-F nights compared to a rather low gradient on the nonspread-F nights. Enhanced concentration of electrons at the anomaly crest location Panama, and a lower density at the equatorial location Huancayo, were observed on spread-F present nights. This is attributed to the phenomena of an evening plasma fountain in operation at equatorial latitudes on spread-F nights.     

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.     

Spread-Es structure producing apparent small scale structure in the F-region

When transmitting on 5.8 MHz the Bribie Island HF radar array synthesizes a beam that is 2.5 wide. The beam can be steered rapidly across the sky or left to dwell in any direction to observe the fading rates of echoes within a small cone of angles. With the beam held stationary, the time scale associated with deep fading of F-region echoes is usually more than 5 min. This is consistent with the focusing and defocusing effects caused by the passage of ever-present medium-scale travelling ionospheric disturbances (TIDs). On occasion the time scale for deep fading is much shorter, of the order of tens of seconds or less, and this is thought to be due to the interference of many echoes from within the beam of the radar. It is shown that the echoes are not due to scatter from fine structure in the F-region, but rather due to the creation of multiple F-region paths with differing phase lengths by small, refracting irregularities in underlying, transparent spread sporadic-E, (Spread-E_s). The natural drift of the Spread-E_s causes the phase paths of the different echoes to change in different ways causing the interference.Two methods are used to investigate the rapidly fading F-region signals. Doppler sorting of the refracted F-region signal does not resolve echoes in angle of arrival suggesting that many echoes exist within a Fresnel zone [Whitehead and Monro (1975), J. atmos. terr. Phys. 37, 1427]. Statistical analysis of F-region amplitude data indicates that when the range spread in E_s is severe on ionograms, then a modified Rayleigh distribution caused by the combination of 10 or so echoes is most appropriate. Using knowledge of the refracting process the scale of E_s structure is deduced from these results. Both methods find a Spread-E_s irregularity size of the order of 1 km or less. It is proposed that the Rayleigh type F-region signals seen by Jacobsonet al. [(1991b), J. atmos. terr. Phys. 53, 63] are F-region signals refracted by spread-E_s.     

Effect of the large-scale convection electric field structure on the formation of thin ionization layers at high latitudes

A three-dimensional simulation of the high-latitude ionosphere was applied to investigate the geographical distribution of E-region thin ionization layers which may be formed by the action of the convection electric field. The simulation model computes the ion densities (O⁺, O⁺₂, N⁺, N⁺₂, NO⁺, Fe⁺), and temperatures as a function of altitude, latitude, and longitude. The stationary state momentum and continuity equations are solved for each ion species, then the energy equation is solved for electrons, neutrals, and a generic ion having the mean ion mass and velocity. The various electric field patterns of the Heppner and Maynard [(1987) J. geophys. Res.92, 4467–4489] convection electric field model were applied and the ionization density pattern was examined after a time sufficient for the formation of thin layers (≈2000 s). It was found that large areas of thin ionization layers were formed for each of the electric field patterns examined. Southward IMF B_z conditions resulted in thin layers forming in the pre-midnight sector in the latitude range north of about 70° to about 80°, and after midnight between 60 and 70°. For northward B_z conditions, the layers were mainly in the pre-midnight sector and covered a latitude range from about 60 to 80°.     

Three solar cycles of day-time southern hemisphere Es activity

The hourly values of the ionosonde parameters f_oE_s and f_bE_s for times near mid-day have been examined over three solar cycles 1947–82. The daily parameters were studied for two southern hemisphere stations: Christchurch (temperate zone) (−43°.6′ geographic, −48°.1′ geomagnetic) and Rarotonga (subtropical) (−21°.2′ geographic, −20°.7′ geomagnetic). The data represent the longest such analyzed sporadi-E record in the literature. The seasonal variations of per cent occurrence of f₀E_s ⩾ 5 MHz and f_bE_s ⩾ 4 MHz for both stations show no sunspot dependence, strong activity in local summer and, in the case of Christchurch, minor enhancements in local winter. Long-term f_bE_s occurrence is closely associated with Zurich sunspot number R_z (correlation coefficients 0.93 for Christchurch and 0.76 for Rarotonga) with all seasons, showing the same in-phase R_z control for both stations. Long-term f_oE_s occurrence for both stations and for all seasons exhibits no dependence on R_z. There is evidence of a 6 year period in seasonal f_oE_s activity for Rarotonga. The data show a large and unexplained systematic decrease in f_oE_s occurrence for both stations and for all seasons by a factor of about 3 from 1947 to 1982.     

Some effects of geomagnetic activity and ionospheric height rises on mid-latitude spread-F occurrence

The analysis investigates the base heights of the ionosphere (h′F) when spread-F is recorded at Brisbane, Australia, for 2 separate periods, namely July and August 1966 when spread-F occurs frequently and September and October 1966 when the activity is much lower. It is found that for July and August there is little tendency for spread-F to occur preferentially when the base height is above the average for the period. However, for the September–October period, spread-F occurs more often (by a factor of 2 or 3 depending on the geomagnetic activity) when the base height is above rather than below the average height. Also, this analysis shows that the overall spread-F occurrence (for both periods investigated) decreased to some extent following increased geomagnetic activity. This suppressed activity in the hours following geomagnetic activity is confirmed by superposed-epoch analyses using K indices (for Macquarie Island) as controls. It is suggested that the results of all the analyses might be explained by invoking a transition height in the ionosphere (controlled by the neutral-particle density of the upper atmosphere). Ionospheric off-vertical reflections from above this height would be recorded as spread-F traces in this model. This transition height would be low in July and August when the neutral-particle density is low and higher in September and October. It is further proposed that changes in the neutral-particle density could also be associated with reduced spread-F activity following increased geomagnetic activity, as well as influencing the diurnal, annual and sunspot-cycle variations of spread-F occurrence in mid-latitudes.     

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index (S_10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread-F (ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.     

Auroral riometer absorptions and the F-region disturbances observed over a wide range of latitudes

Standard riometer data from a southern auroral station were compared with ionograms obtained at five stations positioned from sub-auroral to equatorial latitudes. The rapid onset in riometer absorption, during intense substorm activities in an equinoctial period, was associated with a sequential propagation of ionospheric disturbances deduced from the F-region parameters h′F and range spread-F. The time shift between absorption maxima and extrapolated commencement times of the disturbances was consistent with the presence of large-scale travelling ionospheric disturbances (TIDs), propagating equatorwards with velocities lying typically in the range 600–900 m s⁻¹, and with a median velocity of 720 m s⁻¹. It is suggested that the onset of TIDs is associated with high-energy particle precipitation, manifested by the occurrence of auroral absorption events. Similarity of absorption increases at the southern and northern conjugate points, found from a previous riometer study, would indicate that large-scale TIDs are simultaneously generated in both hemispheres.     

Ionospheric trends in mid-latitudes as a possible indicator of the atmospheric greenhouse effect

Using long-term ionosonde measurements in mid-latitudes (Juliusruh: 54.6°N, 13.4°E; 1957–1990), the first experimental hints of a decrease of the peak height of the ionospheric F2-layer were found. In contrast to that the long-term variations of the peak electron densities in the F2-layer, as well as the E-layer, are small. These results qualitatively agree with the predictions of Rishbeth [(1990) Planet. Space Sci.38, 945] who expected a lowering of the E- and F2-layer caused by a global cooling of the strato, meso- and thermosphere due to the increasing greenhouse effect.     

Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill

Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0° magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34cm volume backscatter cross-section, σ_v, less than ∼5 × 10⁻¹³ m⁻¹, the phase speed is at or below the usual ion acoustic speed in the E-region (350m/s), and increases only slowly with the observed backscattered power amplitude (∼50 m/s per 10dB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/s at times, and increases more rapidly with backscattered power (∼200 m/s per 10dB). Phase velocity/time maps observed over a 3° span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations.     

The role of electric field and neutral wind direction in the formation of sporadic E-layers

The effect of an electric field and a homogeneous neutral wind on the vertical ion motion in the ionospheric E-region is investigated. An electric field pointing, in the northern hemisphere, in the quadrant between geomagnetic north and west is found to he capable of driving plasma towards a certain height from both above and below. A homogeneous neutral wind blowing in a direction between east and north has a similar effect. Unlike in the wind shear model, the resulting plasma sheet may be created within a quite limited height interval only. It seems possible that the midnight occurrence maximum of mid-latitude type E_s-layers, observed at high latitudes, is caused by electric fields in the Harang discontinuity region. It is also suggested that the flat type E_s-layers often observed before a substorm onset are caused by electric fields. The wind shear theory is investigated using a screw-like neutral wind profile. The effects of right- and left-handed wind screws are compared and rules are derived which define the conditions leading to convergent and divergent nulls in the vertical ion velocity. In the northern hemisphere, a right-handed screw is found to be more effective than a left-handed one with equal pitch in compressing plasma into thin sheets.     

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.     

Sudden sodium layers in polar latitudes

We discuss in this paper sudden sodium layers (SSLs), which we observe with a sodium lidar instrument at Andenes, Norway (69°N). We speak of a SSL if, in a narrow altitude range (typically less than 2km), the Na density increases over the normal Na density by a factor of at least 2 within 5 min. Between December 1985 and November 1987, we have observed 42 such layers in 378 h of lidar measurements. This number increases to 75 if we only require an increase of a factor of 1.5 within 8 min. At our observation site, SSLs have the following properties: (a) they develop between 90 and 110 km altitude, (b) they develop between 20 and 02 LT, (c) their appearance shows a strong, positive correlation with that of ƒ-type E_s layers, and (d) their appearance does not show a strong correlation with either riometer absorption or meteor showers. We discuss a number of potential processes for SSL formation. SSLs above 100km can be formed in ƒ-type E_s layers by the conversion of Na⁺ ions into neutral Na. The development of SSLs below 95 km requires the presence of an additional reservoir of Na, such as Na-bearing molecules, ions, and/or ‘smoke’ particles. We also evaluate the proposal that SSLs are the outcome of single meteoroids entering the upper atmosphere, a proposition for which we find little observational support.     

A comparison of three methods of measuring tidal oscillations in the lower thermosphere using EISCAT common programmes

The EISCAT Common Programme can be used in three ways to monitor tidal oscillations in the lower thermosphere. In Common Programme One (CPI) tristatic observations provide measurements of the ion-velocity vector at several heights in the E-region and one height in the F-region. In Common Programme Two (CP2) monostatic measurements give profiles of ion velocity in the E-region while tristatic measurements give continuous measurements of ion velocity in the F-region. From the ion velocities and the ion-neutral collision frequency, the vector of the E-region neutral wind can be determined and both east-west and north-south components of the diurnal, semi-diurnal and ter-diurnal oscillations can be identified. CP1 and CP2 also provide profiles of the field-aligned ion velocity, and these can be used to calculate the north-south component of the neutral wind without knowing the ion-neutral collision frequency, but the result is affected by any vertical component of neutral velocity. The three methods are compared and the advantages of CP2 demonstrated.     

Field aligned airglow observations of transequatorial bubbles in the tropical F-region

It is possible to form images of the tropical F-region ionization structures, variously labelled as ‘bubbles’, ‘plumes’, or ‘depletions’, in a plane perpendicular to the magnetic field by observing the airglow emissions associated with them in a field aligned direction. Structures which are present at altitudes from 250 km to more than 700 km above the dip equator map down to the 250–350 km region, where recombination and associated airglow emissions occur, ranging from the equator to dip latitudes of 15° or more. The structures can be viewed in a field aligned direction from sites in the range 17°–23° dip latitude. Measurements with high angular resolution (as small as 0.1° in the meridian) could show structures as small as 2 km. It is possible to make simultaneous measurements in both 6300 and 7774 Å recombination emissions, from which the height h_max of the peak plasma concentration n(e)_max on the field line can be estimated from a ratio of the emission rates. It is possible to make maps of n(e)_max and h_max either by raster scanning the sky in the two emissions or by imaging them onto an imaging detector. Useful data can be obtained from one site over a range of 20° in dip latitude and 10° in dip longitude. Observations in the same magnetic meridian as a backscatter radar system are desirable, as also are observations from near magnetic conjugate points. Imaging characteristics for the observation sites in the range of dip latitude 17°–23° have been calculated.     

A comparison of mid-latitude and equatorial-latitude spread-F characteristics

Experimental evidence using a fast-swept-gain technique on an ionosonde is presented to support the idea that mid-latitude spread-F irregularities are large-scale wave-like structures. Also,diurnal and annual distributions of spread-F occurrence at an equatorial station at times of low sunspot activity are shown to be similar to those found for mid-latitude stations. The sunspot-cycle variation of post-midnight spread-F occurrence is also found to be similar in the two latitude regions. The similarity of certain spread-F characteristics at both mid- and equatorial-latitude regions is discussed. An attempt is made to reconcile current spread-F models for these two latitude regions by proposing that the primary spread-F structures for equatorial regions are large-scale wave-like structures. It is further proposed that the small-scale plasma instabilities have a role of modifying the traces resulting from specular reflections from the large-scale structures.