Reconstruction of horizontally-inhomogeneous ionospheric structure from oblique-incidence backscatter experiments

Fridman and Fridman [(1994) J. atmos. terr. Phys. 56, 115] suggested a method of reconstructing the horizontally-inhomogeneous ionospheric structure using vertical- and oblique-incidence backscatter sounding (OBS) ionograms measured at a single location. In the present paper this technique has been used to analyze experimental data and tested against independent vertical sounding (VS) measurements. By using the OBS and VS ionograms measured at Irkutsk as source data for the method we reconstructed ionization profiles over Tomsk (1050 km to the west of Irkutsk). We found that the reconstructed profiles are in reasonable agreement with the profiles obtained from VS measurements at Tomsk.     

A comparison of the height of the maximum electron density of the F2-layer from real height analysis and estimates based on M(3000)F2

Estimates of the height of the F-layer peak based on formulations using the ionospheric transmission factor M(3000)F2 are compared with hmF2 derived from the real height analysis of digital ionograms acquired at a mid-latitude station. Based on the analysis of 27 hours of quiet data, our result shows that the M(3000)F2 methods are highly accurate and that the formulation developed by Bradley P. A. and Dudeney J. R., (1973, J. atmos. terr. Phys. 35, 2131) is most accurate.     

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.     

On VLF radio wave reflection from distant mountain ranges—theory

We present an analysis for the reflection or scattering of VLF radio waves by gently sloping parallel ridges that may be well displaced from the great circle path between the transmitter and receiver antennae. It is shown that, under certain conditions, a spatial harmonic of the transverse profile of the terrain will produce a secondary signal that could be not far below the direct or primary signal. Such a resonant scattering mechanism may explain some of Thomson's [(1989), J. atmos. terr. Phys. 51, 339] observations in Dunedin, New ZeaLond.     

Mid-latitude frequency spread and its association with small scale ionospheric stratifications

A variety of ground based radio techniques have provided new information relating to the nature of mid-latitude F-region irregularities responsible for frequency spreading on ionograms. Firstly, an analysis of ionograms covering a restricted frequency band indicates that frequency spreading is primarily caused by duplicate traces which are often unresolved in group path on standard ionograms. Furthermore, where angle of arrival information is available, the duplicate traces are shown to represent reflections from markedly different directions and the spread in critical frequencies is therefore indicative of a horizontal gradient in the peak electron density over a scale size of the order of many tens of kilometres. Secondly, the individual duplicate traces themselves are shown to comprise quasi-horizontal trace (QHT) segments which are unresolved on conventional ionograms and contribute to the diffuse appearance of spread-F traces on those ionograms. Difficulties in attributing these observations to the widely held view that scattering from small scale structures is the causative mechanism are discussed.     

MF and HF ducting within equatorial bubbles

MF and HF conjugate ducting is often observed while satellite sounders are within equatorial bubbles. This paper examines two possible forms of this propagation. The first is guiding by the bubble itself, the second is ducting along irregularities, of small cross-section, embedded in the bubble. One bubble model, based on observations by Dyson and Benson [Geophys. Res. Lett. 9, 795 (1978)], gives some results at variance with observation. Nevertheless it is considered that slight changes to the model, such as asymmetries between the conjugate ionospheres, should remove the discrepancies. The results show that bubbles themselves will definitely produce conjugate ducting on occasions. The alternative explanation requires ducts of small cross-section, distributed throughout the bubble in order to produce conjugate ducting on successive ionograms. Good matches between calculated and observed echo traces for conjugate echoes were obtained using this model. It is likely that both forms of propagation occur.     

On significant quantities of negative ions observed around the mesopause

Significant amounts of negative ions are sometimes observed at altitudes as high as the mesopause region (Kopp E. and Hermann L., 1984, Annales Geophysical2, 83; Ganguly S., 1984, J. atmos. terr. Phys. 46, 633). Using a suitable ion-chemical scheme in which water clustering to negative ions is also considered, the effect of change in temperature, T, and concentrations of NO, O, O₃ and H₂O on the abundance of negative ions in the mesopause region has been examined. It is shown that a drastic increase in the concentration of O₃ or a decrease in neutral temperature by about 20 K. around 85km could make the concentration of total negative ion comparable to that of electron.     

Radar measurement of the seasonal variation in the velocity of the sunrise terminator

The HF phased-array pencil beam radar at Bribie Island, Australia, used to measure horizontal movements of the ionosphere, has been calibrated using the known velocity of the sunrise terminator. The seasonal variation in the velocity of the terminator has been resolved, both in magnitude and direction.The technique uses single-station ionospheric sounding, and requires the angle of arrival and Doppler shift of ionospheric echoes to be measured as the terminator passes overhead. Pfister's theorem [(1971), J. atmos. terr. Phys. 33, 999] then allows calculation of the velocity of the reflecting surface. The difference between theory and experiment is less than 3% in speed and 2° in direction on average.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

Tidal winds at mesopause altitudes over Arecibo (18°N, 67°W), 5–11 April 1989 (AIDA '89)

An imaging Doppler interferometer (IDI) radar was operated during the three AIDA '89 campaigns in Puerto Rico over the period March–May of 1989. The output of the IDI analysis characterizes radar scattering in terms of a number of discrete ‘scattering points,’ also referred to as ‘multiple scattering centers,’ IDI/MSC for short. For each of these points the three-dimensional location, radial velocity and amplitude and phase are determined, similar to the output of meteor radars. We have applied the conventional Groves [(1959) J. atmos. terr. Phys. 16, 344–356] meteor wind radar analysis to the scattering points to produce the mean apparent motions over the height range from 70 to 110 km which are presented here. The mean apparent motion of the scattering centers is the quantity that would correspond to the neutral atmosphere wind or bulk motion if the scattering points are physical entities (such as turbulent eddies) whose motions are determined solely by advection. This is the quantity which is treated as the ‘wind’ in the analysis which follows and which should be compared to the wind measurements as deduced from the other methods employed during this campaign. There is, however, a caveat which supports the contention of Hineset al. [(1993) J. atmos. terr. Phys. 55, 241–287] that extreme care must be used in interpreting the velocities measured by partial reflection radars as winds. The current application of the Groves method of analysis has revealed motions from which one would infer a typical equatorial easterly circulation, with mean meridional circulation becoming significant only above 96 km. A periodogram analysis of the complete data interval (5–11 April) has shown the diurnal tide to be the most significant feature of the wind field at these altitudes, with zonal amplitudes up to some 50 m/s and meridional amplitudes approximately half this value. The 12 and 6 h tides become as significant as the diurnal above 100 km. The two day (48 ± 5 h) wave is the next most significant feature, with zonal amplitude increasing with height up to 30 m/s at 110km. The semidiurnal tide is not at all well developed below 100 km. However, analysis on a day by day basis reveals a significant semidiurnal component which is not phase coherent over the total interval. Mean vertical velocities are of the order of tens of centimeters per second and are considered to be more realistic than the meters per second velocities usually inferred from analyses of meteor trail drifts.     

VHF transequatorial propagation via three dimensional waveguides

The three dimensional model of VHF propagation along ionospheric waveguides or ducts developed by Platt I. G. and Dyson P. L., 1989 [J. atmos. terr. Phys. 51, 897–910] has been used to study transequatorial propagation (TEP) along equatorial bubble irregularities aligned along the earth's magnetic field. Bubbles with circular and elliptical cross-sections and different apex heights have been used to determine the region illuminated in the conjugate hemisphere by TEP and the power of the signals. The results show that the three dimensional waveguide model can explain many features of TEP, including those not well explained by the simpler waveguide models used previously. The calculations therefore confirm that TEP can be caused by guided propagation along equatorial bubble irregularities aligned along the magnetic field. The results show that a bubble located on the transmitter's longitude can illuminate a wide area of the conjugate hemisphere. A well defined maximum central power region occurs which in most cases is centred at a latitude slightly lower than the conjugate of the transmitter. Bubbles with circular cross-section can illuminate a region more than ten degrees in latitudinal extent and five degrees in longitude. Bubbles which are vertically elongated at their apex illuminate a much narrower region in longitude. TEP between points with relatively large separation in longitude can result from either leaky rays or ducts tilted out of the magnetic meridian plane. This explains some observations of TEP previously thought to be inconsistent with waveguide propagation. The variation of power loss with frequency depends on a number of factors such as bubble shape and height. Co-ordinated experiments involving a transmitter and several receivers in the conjugate hemisphere should enable properties of bubbles to be determined.     

Coherent-array HF Doppler sounding of traveling ionospheric disturbances: I. Basic technique

We present an introduction to the use of phase-coherent, multi-receiver HF Doppier sounding arrays for measuring the horizontal velocity of traveling ionospheric disturbances (TID's). The point of departure is the theorem of Pfister (1971, J. atmos. terr. Phys. 33, 999) relating ray Doppler to ray zenith angle for a monostatic full reflection sounder. Retaining the simple model of a specular, smooth ionospheric reflector which is deformed by a propagating undulation, we first generalize the theorem to bistatic sounding geometry and then include the effects of amplitude in addition to phase. Next, these results are cast into an algorithm for treating multi-receiver phase sounders containing many diverse baselines, in order to obtain an accurate and unambiguous solution in the plane of wave slowness (inverse of velocity). The point spread function of this solution is controlled by process bandwidth and by array geometry. We illustrate the coherent-array approach using data from an eight receiver array during passage of a TID.     

Monthly simulations of the solar semidiurnal tide in the mesosphere and lower thermosphere

Monthly simulations of the solar semidiurnal tide in the 80–100 km height regime are presented. These calculations benefit from the recent heating rates provided by Groves G. V. (1982a,b) (J. atmos. terr. Phys. 44, 111; 44, 281), the zonally-averaged wind, temperature and pressure fields developed for the new COSPAR international reference atmosphere [Labitzke K., Barnett J. J. and Edwards B. (1985) Handbook for MAP 16, 318], and eddy diffusivities determined from gravity wave saturation climatologies and used by Garcia R. R. and Solomon S. (1985) (J. geophys. Res. 90, 3850) to simulate oxygen photochemistry and transport in the mesosphere and lower thermosphere. Some of the main characteristics of the observed semidiurnal tide at middle and high latitudes are reproduced in our simulations: larger amplitudes in winter months than in summer months, and the bi-modal behavior of the phase with summer-like and winter-like months separated by a quick transition around the two equinoxes. The phase transition is also more rapid in the spring, consistent with observations. The wavelengths are also longer in summer than in winter, at least below 95 km (whereas in July and August the simulations exhibit some discrepancies above this altitude), similar to the observational data. Semidiurnal amplitudes are generally smaller and the phases more seasonally symmetric at middle and low latitudes, as compared with the tidal structures above about 50° latitude. In addition, hemispheric differences in the mean zonal wind result in marked asymmetries in tidal behavior between the Arctic and Antarctic regions, and suggest that a comparative study of tide, gravity wave and mean flow interactions in the Arctic and Antarctic mesosphere and lower thermosphere would be fruitful.     

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations.The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [Forbes and Vial (1989), J. atmos. lerr. Phys. 51, 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed.The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [Forbes. S and Hagan (1988), Planet. Space Sci. 36, 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.     

Results of the new and old Umkehr algorithm compared with ozone soundings

Vertical ozone distributions from regular Umkehr observations at Arosa, Switzerland, from 1956 to 1990 retrieved with the newly developed algorithm of mateer and Deluisi (1992, J. atmos. terr. Phys. 54, 537), using Bass-Paur absorption coefficients and described in this issue, are compared with the corresponding results obtained with the old routine [Mateer and Duetsch (1964), NCAR, Boulder, Colorado, Part I, 105 pp.], officially in use at the World Ozone Data Center at Toronto. For the period 1967–1989 they are also compared with the sounding data obtained at Payerne, Switzerland with the Brewer-Mast electrochemical instrument.The annual mean values calculated from Umkehr observations increased, using the new algorithm, in layer 4 and to a lesser extent in layers 3 and 1. They decreased strongly in layer 2 and also 6 and became smaller, too, in layers 7 and 8. The new Umkehr yields annual mean values which are much closer to the sounding results than those of the old routine. The seasonal variation shows somewhat larger differences. There are big discrepancies between the trends obtained from both Umkehr algorithms and those calculated from the soundings in the region of the ozone maximum and in the troposphere. The former discrepancies may be due to the changes in the relation between total ozone and vertical distribution which occurred during the past 20 years and which are not taken into account in the definition of the a priori profiles in the new routine. It seems that useful trends can only be obtained in layer 6 and above using Umkehr observations.     

Mesospheric wind studies during AIDA Act '89: morphology and comparison of various techniques

The Arecibo Initiative in Dynamics of the Atmosphere (AIDA) '89 was a multi-instrument campaign designed to compare various mesospheric wind measurement techniques. Our emphasis here is the comparison of the incoherent scatter radar (ISR) measurements with those of a 3.175 MHz radar operating a s an imaging Doppler interferometer (1131). We have performed further analyses in order to justify the interpretation of the long term IDI measurements in terms of prevailing winds and tides. Initial comparison of 14 profiles by Hines et al., 1993, J. atmos. terr. Phys. 55, 241–288, showed good agreement between the ISR and IDI measurements up to about 80 km, with fair to poor agreement above that altitude. We have compiled statistics from 208 profiles which show that the prevailing wind and diurnal and semidiurnal tides deduced from the IDI data provide a background wind about which both the IDI and ISR winds are normally distributed over the height range from 70 to 97 km. The 3.175 MHz radar data have also been processed using an interferometry (INT) technique [Van Baelen and Richmond 1991, Radio Sts. 26, 1209–1218] and two spaced antenna (SA) techniques [Meek, 1980, J. atmos. terr. Phys. 42, 837–839; Briggs. 1984, MAP Handbook, Vol. 13, pp. 166–186] to determine the three dimensional wind vector. These are then compared with the IDI results. Tidal amplitudes and phases were calculated using the generalized analysis of Groves, 1959, S. atmos. terr. Phys. 16, 344–356, historically used on meteor wind radar data. Results show a predominance of the diurnal S₁¹ tidal mode in the altitude range 70–110 km, reaching a maximum amplitude 45 ms⁻¹ at 95 km, with semidiurnal amplitudes being about 10–15 ms⁻¹ throughout the height range considered. There is evidence of the two day wave in data from 86–120 km, with amplitudes on the order of 20 ms⁻¹.     

Night-time equatorial F-region zonal plasma drifts from VHF backscatter radar observations

Night-time equatorial F-region plasma drifts are deduced from VHF backscatter radar observations of F-region irregularities. The zonal drifts reveal large vertical shears. It is found that the irregularity polarization electric field (though small compared to the ambient field) is significant in affecting the observed zonal drifts.     

On Pfister's theorem and its generalization

The relation between angle of arrival and Doppler shift that has been called Pfister's theorem is shown to be a special case of a result that applies to ionospherically transmitted signals between fixed ground sites, whenever the ionosphere moves without change of shape. This general result does not require the existence of an effective mirror reflector or any special symmetry of the ionosphere, but depends only on the validity of ray theory. The formula obtained by Pfister [(1971) J. atmos. terr. Phys.33, 999] is shown to apply, provided the ray returning from the ionosphere follows the same path as the incident ray.     

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.