An assessment of the effect of gravity waves on the width of radar Doppler spectra

Radars can be used to obtain turbulence parameters by measuring the width of the Doppler spectrum of returned power. Other effects can contribute to the Doppler-spectrum width, including short-period gravity waves, making turbulence parameter measurements more difficult. In this paper, an experimental study of the effect of gravity waves on the width of the Doppler spectrum is presented. The data were obtained using the Poker Flat 50 MHz VHP radar. A parameter Y_i is used to describe the discrepancy between the width of spectra sampled over a long period of time and that taken over a shorter time. If Y_i is not significantly different from 1.0, no contamination is present. The experimental data considered were of a time resolution that allowed the period and amplitude of the contaminating gravity wave to be determined. A theoretical expression for gravity wave contamination proposed by Hocking [(1988) J. geophys. Res.93, 2475–2491] was tested and found to agree with measurements. It was also found that non-unity values of Y_i occurred in some cases. This suggests that, at commonly used sampling times, short-period gravity waves can contaminate spectral width estimates.     

Fractal analysis of gravity wave spectra in the middle atmosphere

Wind fluctuations in the middle atmosphere behave like colored noise processes. They have a continuum of scales without dominant features and a power spectrum density (PSD) that often decays with frequency ƒ as ƒ^−β. Spectral index β is generally obtained through least-square fit to PSD estimated by Fourier methods. Graphs of colored noise have fractal plane-filling properties depending on β. An efficient method for finding β using the fractal dimension (D), based on analysis of 1/ƒ noise in galactic X-ray luminosities by McHardy I. and Czerny B., (1987, Nature325, 696), is described. An empirical relation is found between D and β and its validity is confirmed in limiting cases. Then D is obtained from power-law dependence of a length metric L(μ) on scale μ. The method is applied to middle-atmospheric velocity data from the Poker Flat radar in Alaska. Variations of D follow those in β, from an earlier analysis by Bemraet al., (1986, Handbook for MAP20, 216), but show an offset of 0.1–0.2 even after corrections for outliers, gaps, and additive noise. Usefulness of this method for screening data as an aid to spectral analysis is examined.     

Ionospheric modelling in support of single station location of long range transmitters

Position estimates derived from a large data base of bearing and elevation angles of signals from distant HF transmitters have been analysed, with a view to comparing the validity of available ionospheric models and to examining ionospheric limitations to the accuracy of single station location of such transmitters. In general, the accuracy of the position estimates is almost entirely controlled by a limited ability to model in sufficiently accurate detail the ionospheric effects on the signal propagation. Median miss distances for those cases with a reliable identification of the propagation mode were about 7% for both E and F2 propagation for all models considered. Difficulties were encountered with the International Reference Ionosphere, which failed to support the observed propagation in half the F2 propagation cases. Standard deviations of the bearing errors were about 0.5° for E modes and 0.7° for F2 modes and were largely attributable to the effects of the ionosphere and not to instrumental errors     

The investigation of long-distance HF propagation on the basis of a chirp sounder

Experimental data on round-the-world HF radio signals near the terminator are given. The critical frequency of the ionospheric waveguide is found to be F_c ∼ 16–17 MHz. At frequencies F < F_c the group delay has a negative dispersion $\text{τ}\text{dot}{}_{\mathrm{<mtext>g</mtext>}}\text{= ∂τ/∂F ⋍ −100}$ μs/MHz and $\text{τ}\text{dot}{}_{\mathrm{<mtext>g</mtext>}}\text{⋍ 80}$ μs/MHz for frequencies f > f_c. Ray-tracing calculations are carried out. It is found that the low frequency branch of round-the-world signals (F < F_c) is formed mainly by waveguide modes and the high frequency branch (F > F_c) by 0 ricochet and hop modes.Experiments on waveguide modes escaping from the ionospheric channel due to field-aligned scattering by artificial ionospheric turbulence are carried out. The conditions for trapping of radio waves in the ionospheric waveguide are investigated. It is shown that if the gradient of the critical frequency F₀F2 is less than minus 2 × 10⁻² MHz/100 km radio wave trapping takes place in the ionospheric waveguide at frequencies exceeding by 1–2 MHz the maximum observed frequency of the hop mode. The frequency time characteristics of the mode and the geophysical conditions for the effective control of radio waves escaping from the waveguide are defined.     

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.     

Re-radiation of VLF radio waves from mountain ranges

VLF signals transmitted from Hawaii, Japan and Australia (NPM, NDT, NWC) and scattered from large mountain ranges such as the Andes or Rockies are monitored in New Zealand in the presence of the much larger direct signals. Measurements of the amplitudes of these indirect signals are reported in the range 0.l-2.0μVm⁻¹.Sudden changes in the refractive index along the path, such as those caused by abrupt changes in ground conductivity, are shown to give rise to reflected amplitudes much smaller than those measured. However, the measured amplitudes are found to be comparable with reasonable estimates of the re-radiation from typical surface currents induced by the incident wave in mountains of suitable height, ground conductivity and steepness.     

Magnetometer and incoherent scatter observations of an intense Ps 6 pulsation event

In the morning sector of 21 April 1985, during the recovery phase of a geomagnetic storm, a Ps 6 pulsation event was recorded by the EISCAT magnetometer cross in northern Scandinavia. Simultaneously, the EISCAT incoherent scatter radar measured E- and F-region plasma parameters with a latitudinal scanning program. Electric fields and height-integrated Hall and Pedersen conductivities are derived. Two-dimensional patterns of these quantities are constructed for one Ps 6 period. The conductance patterns closely resemble the typical auroral forms of eastward drifting Ω bands with low and high conductances at the northern and southern edges of the scanned area, respectively. From the equatorward region a tongue of high ionization extends poleward into the dark area. The location of the maximum southward current is slightly displaced towards the west from the centre of the conductance tongue. The east-west disturbance electric field points towards the tongue; the north-south fields are enhanced outside and reduced inside the high conductance region. As has been previously suggested, the observations can be explained with a model which superposes currents caused by conductance variations and electric fields. Both effects need to be taken into account for this event. The current structures move within a few degrees in the direction of the background E×B drift, but their speed is about 15% lower than the average F-region plasma drift.     

Simultaneous ground-based observations of polar cap arcs and spacecraft measurements of particle precipitation

In order to investigate the particles which produce the polar cap aurora at the Vostok station in Antarctica, charged particle data obtained by the DMSP satellites for some days in a period from April to August 1985 were surveyed. Due to the satellite orbit the local time range in which the data were available was the morning sector. For all the events when sun-aligned arcs were observed on the ground the simultaneous DMSP measurements on almost the same field line showed an increased integral number flux J. > 10⁸ (cm⁸/s/sr)⁻¹ of the precipitating electrons with energy E_e > 200 eV. The electron spectra with double peaks are typical of intense electron precipitation in the polar cap arcs. The most noticeable feature of ion spectra in the polar cap arcs is the prominent minimum in ion flux in the energy range 0.1 < E_i < 1 keV in contrast with the oval precipitation ; this feature gives the possibility to separate the polar arcs from the aurora in the oval. In some events the satellite crossed the system of two widely separated arcs ; one of them was a sun-aligned arc whereas the other was circular at constant latitude according to the Vostok data. The analysis of the DMSP electron and ion precipitation data has shown that in these events the latitude-oriented arcs are located in the polar cap and not in the auroral oval.     

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.     

Calculation of auroral Bahner volume emission height profiles in the upper atmosphere

Energetic protons entering the atmosphere will either travel as auroral protons or as neutral hydrogen atoms due to charge-exchange and excitation interactions with atmospheric constituents. Our objective is to develop a simple procedure to evaluate the Balmer excitation rates of H_α and H_β, and produce the corresponding volume emission rates vs height, using semi-empirical range relations in air, starting from proton spectra observed from rockets above the main collision region as measured by Reasoneret al. [(1968) J. geophys. Res.73, 4185] and Søbraaset al. [(1974) J. geophys. Res.79, 1851]. The main assumptions are that the geomagnetic field is parallel and vertical, and that the pitch angle of the proton/hydrogen atom is preserved in collisions with atmospheric constituents before being thermalized. Calculations show that the largest energy losses occur in the height interval between 100 and 125 km, and the corresponding volume emission rate vs height profiles have maximum values in this height interval. The calculted volume emission rate height profile of H_β compares favorably with that measured with a rocket-borne photometer.     

Fair weather electric field in Port Moresby

Measurement of the fair weather electric field at Port Moresby during the dry season in July–August 1988 shows no clear effect of the world-wide thunderstorm activity predicted by Mauchlyy S. J. (1923, J. geophys. Res. 28, 61). The observed field is generally low—approximately, 60V/m—during daytime but remains relatively higher—about 150 V/m—at night. The field strength increases slowly after sunset but decreases sharply after sunrise. The levels observed are typical of isolated sites and the frequent temperature inversion observed during the period is suggested as the likely explanation of the observed behaviour of the field.     

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.     

The impact of gravity waves on nitric oxide in the lower thermosphere

Observations of nitric oxide (NO) by the Solar Mesosphere Explorer (SME) during equinox indicate a lower-thermosphere equatorial minimum which is at variance with theoretical predictions. To address this discrepancy a zonally averaged model of the thermosphere and upper mesosphere is used to evaluate the influence of a latitude variation in turbulence. Five numerical simulations were performed with different latitude structures of eddy diffusion (K_T), ranging from uniform in latitude, peaks at low, mid-, or high-latitude, to a hemispherically asymmetric distribution. A local increase in eddy diffusion causes the lower thermosphere to cool and induces a latitude pressure gradient that drives horizontal and vertical winds. The circulation, turbulent transport and temperature dependent chemistry act to change the distribution of species. Comparison of the model predictions of NO with SME data, and simulated wind and temperature structure with empirical climatology, indicates a preference for a midlatitude peak in K_T.     

Antarctic polar cap total electron content observations from Casey Station

In early 1990 a modified JMR-1 satellite receiver system was installed at Casey Station, Antarctica (g.g. 66.28°S, 110.54° E, -80.4°A, magnetic midnight 1816UT, L = 37.8), in order to monitor the differential phase between the 150 and 400 MHz signals from polar orbiting NNSS satellites. Total electron content (TEC) was calculated using the differential phase and Casey ionosonde foF2 data, and is presented here for near sunspot maximum in August 1990 and exactly one year later. The data are used to investigate long-lived ionization enhancements at invariant latitudes polewards of − 80° A, and the ‘polar hole’, a region from −70 to − 80° A on the nightside of the polar cap where reduced electron densitiy exists because of the long transport time of plasma from the dayside across the polar cap. A comparison is made between the Casey TEC data and the Utah State University Time Dependent Ionospheric Model (TDIM) which uses as variables the solar index (F 10.7), season (summer, winter or equinox), global magnetic index (K_p), IMF B_y direction, and universal time (UT) [sojkaet al. (1991) Adv. Space Res.11(10), 39].     

Modelling of transequatorial propagation of HF radio waves

In the geometrical optics approximation, a synthesis oblique ionogram of ionospheric and magnetospheric HF radio wave signals propagating between magnetic conjugate points has been carried out. The magnetospheric HF propagation is considered for a model of the waveguide formed by field-aligned irregularities with depleted electron density. The characteristic peculiarities of the magnetospheric mode have been determined: (i) strong disperion of the group delay with a frequency at 14–18 MHz, from − 1.4 to 0.6 ms/MHz for magnetically conjugate points at geomagnetic latitudes φ = 30°, 40° and 50°, respectively, (ii) spreading ∼ 1–2 ms, and (iii) a possibility of propagation between magnetic conjugates points at moderately low geomagnetic latitudes φ₀ ∼ 30–40° at frequencies exceeding 1.5 times the maximum usable frequency (MUF) of multi-hop ionospheric propagation.     

Inertio-gravity waves and subtropical multiple tropopauses: Vertical wavenumber spectra of wind and temperature observed by the MU radar,radiosondes and operational rawinsonde network

We have carried out continuous observations of the tropopause region over Japan for three weeks during the Baiu (early summer rain “in Japan”) season in 1991, by using a VHF Doppler radar (the MU radar), radiosondes launched at the radar site and operational rawinsondes at five meteorological stations. Based on these observations, we try to examine the hypothesis that the multiple tropopauses and the dominant inertio-gravity waves are one and the same feature, and obtain some interesting results that are not inconsistent with this hypothesis. First, vertical wavenumber spectra and hodographs analyzed from the radar wind data in the tropopause region suggest that inertio-gravity waves with vertical wavelengths of ∼ 2 km are quasi-monochromatically dominant (with 2–3 day scale variabilities of 10–20%), and are in accordance with activities of the subtropical jet stream and mesoscale cyclone-front system activities observed by the operational network. Second, striking (potential) temperature fluctuations are detected simultaneously by the radiosondes and rawinsondes, which appear as multiple tropopauses in meridional cross-section analysis. Third, vertical wavenumber spectra analyzed from the radiosonde temperature data are consistent with the radar wind spectrum, if we assume that both wind and temperature fluctuations are mainly induced by the dominant inertio-gravity waves. Finally, we confirm that the dominant interio-gravity waves can be barely detected also from routine rawinsonde (1.5-km running-mean) wind data if the amplitude is larger than 1.5 m/s. However, the monochromatic wave structures are generally quite localized in space and time.     

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.     

Further evidence for a 6-h tide above Arecibo

Since the publication of results suggesting the existence of a 6-h tide in the E region above Arecibo [(Tonget al., 1988) J. geophys. Res.93, 10047–10051], much more data has been collected and analyzed. In particular, the time-height trajectories of middle and upper E region Tidal Ion Layers (TILs) for early January 1989 closely resemble those from early January 1981, which first revealed the presence of a 6-h quasi-periodic intermediate layer structure. Further, the January 1989 observations form an ‘overture’ to the March–May 1989 AIDA (Arecibo Initiative in Dynamics of the Atmosphere) campaign, which yielded a total of 28 days of additional data regarding TIL motion. Interestingly, the AIDA data set is dominated, above about 120 km altitude, by sporadic intermediate layers [(mathewset al., 1993) J. atmos. terr. Phys.55, 447–457] and certainly does not show the consistent 6-h period TIL feature seen in the two January data sets. In reviewing all data collected over the past 10 yr and the extensive 1989 observations in particular, we conclude that the basic TIL structure is controlled by two separate tidal wind patterns. We refer to these as the normal pattern and the ‘deep-winter’ pattern. The normal pattern includes the combination of diurnal and semidiurnal tides, while the deep-winter pattern has an additional 6-h tidal component. The deep winter pattern remains unexplained, but we suggest that the 6-h periodicity, which appears to be phase locked with the semidiurnal tide, is generated via in situ non-linear frequency doubling of the semidiurnal tide. The January 1989 results also manifest a TIL structure, below 100 km altitude, which has not been previously reported.