The height distribution of radio meteors: observations at 6 MHz

As part of a program aimed at deriving the true influx to the Earth of small meteoroids we have measured the height distribution of radio meteors to limiting magnitude + 6 (mass ~ 1.0 mg) at a frequency of 6 MHz for the combined Daytime Arietid and Zeta Perseid showers, and also for the Eta Aquarids; these showers have widely different velocities but no substantial dependence upon velocity is apparent. The distributions peak at ~ 105 km, 10 km above the peak found using conventional VHP meteor radars and in line with observations previously made by us at 2 MHz. Instrumental limitations confine the span of heights to 84 <h <116 km, but it is notable that within this range the 6 MHz height distributions appear to be symmetric, with a swift drop-off above 105 km; this contrasts with the asymmetric 2 MHz distribution which showed a slow drop-off with many meteors to 140 km. The origin of this difference is probably due to diffusion and the finite-velocity effect, which will be considered in a subsequent paper.     

Response of high frequency radar to meteor backscatter

The response of a High Frequency (HF) Radar System to echoes backscattered from underdense meteor trails is calculated. Three propagation modes are identified according to whether the echo is received along the direct (line-of-slight) path, or along two possible paths from beyond the horizon involving ionospheric reflection. The system response contours in terms of meteor radiant position on the sky are presented in the altitude-azimuth and celestial ecliptic coordinate systems. Diurnal echo rate curves are deduced for point radiants, which correspond to meteor showers and for a density distribution of radiants which is appropriate to sporadic meteors.The calculations are compared with observations of integrated meteor echo power from sporadic meteors made with an experimental radar system at frequencies throughout the HF band. Satisfactory agreement is reached between predictions and observations as functions of time of day, radar frequency and range. The extension of observations to include ionospherically propagated echoes permits meteor echo rates to be simultaneously monitored over an area of the Earth's surface of the order of 10⁶ km² with a single radar system.A greater than normally accepted echo rate is required to explain our observations. However, we believe that this enhanced rate is consistent with the true echo height distribution and the attenuating effects of trail initial radius and diffusion, which are particularly severe at the radar frequencies normally used for meteor detection near and beyond the top of the HF band. Our echo rate is consistent with the meteoroid cumulative mass distribution which may be inferred from a simple interpolation between satellite and visual measurements.     

The height distribution of radio meteors: comparison of observations at different frequencies on the basis of standard echo theory

A comparison is made between the observed height distributions of underdense radio meteor echoes measured at frequencies of 2, 6, 26 and 54 MHz and a simple model based on standard theory. This theory takes account of the initial radius of the train, the finite formation time due to the meteor velocity, and diffusion in the time between radar sweeps. The main features of the measured VHF (26 and 54 MHz) height distributions are predicted by this model, with peaks below 100 km and few meteors detected above 105 km. The main features of the measured HF (2 and 6 MHz) distributions are also successfully predicted, with peaks at ~105km. The model indicates few 6 MHz echoes from above 115km, in line with the observational data, although for the data used there is an instrumental cut-off just above this height. It is suggested that even at 2 MHz perhaps 50% of all meteor trains above 105km remain undetected. A comparison of the model and the measured 2 MHz distribution, which displays many meteors to at least 130 km, reveals that the true height distribution continues to rise above 105 km and would peak above 110 km were it not for the limited detectability of such high altitude meteors, even at such low frequencies.     

Meteor observations by the Arecibo 430 MHz incoherent scatter radar—I. Results from time-integrated observations

The paper discusses an incoherent scatter radar (ISR) power profile interference detection and meteor filtering technique and presents the meteor interference statistics derived from the Arecibo 430 MHz ISR. The statistics obtained from the Arecibo ISR are comparable with the meteor statistics obtained by other radar and optical techniques. Using the hourly rate of meteor flux, the average visual magnitude of the meteors i s estimated to be as faint as + 14, probably, the faintest ever reported. For low echo meteors, the mean ablation height exhibits a logarithmic relationship with the returned power below 105 km. However, once the echo power exceeds a certain threshold, the mean height remains constant at 107 km for all meteors irrespective of their returned power. The unique aspect of the meteor trails reported here is that they are observed in the radial direction of the radar beam. The lack of aspect sensitivity and the high meteor rate detected by the Arecibo 430 MHz system seem to suggest that the scattering mechanism in the UHF range of frequencies may be different from the mechanism operating at VHF. Some future prospects of exploiting; meteor trails to study the neutral wind and other aspects of the meteor region are also proposed.     

Mean winds at 60–90 km observed with the MU radar (35°N)

Mean winds at 60–90 km altitudes observed with the MU radar (35°N, 136°E) in 1985–1989 are presented in this paper. The zonal wind at 70 km became westward and eastward in summer and winter, respectively, with a maximum amplitude of 45 m s⁻¹ westward in early July and 80 m s⁻¹ eastward at the end of November. The meridional wind below 85 km was generally northward with the amplitudes less than 10 m s⁻¹. In September to November, the meridional wind at 75–80 km becomes as large as 20–30 m s⁻¹. Those zonal wind profiles below 90 km show good coincidence with the CIRA 1986 model, except for the latter half of winter, from January to March, when the observational result showed a much weaker eastward wind than the CIRA model. The height of the reversal of the summer wind from westward to eastward was determined as being 83–84 km, which is close to the CIRA 1986 model of 85 km. The difference between the previous meteor radar results at 35–40°N, which showed the reversal height below 80 km, could be due to interannual variations or the difference in wind measurement technique. In order to clarify that point, careful comparative observations would be necessary. These mean winds were compared with Adelaide MF radar observations, and showed good symmetry between the hemispheres, including the summer reversal height, except for the short period of eastward winds above Kyoto and the long period over Adelaide.     

Mean winds observed by the Kyoto meteor radar in 1983–1985

Mean winds at 82–106 km altitude have been almost continuously monitored by the Kyoto meteor radar over the period from May 1983 to December 1985. The mean zonal wind becomes eastward with amplitudes as large as 30 m s⁻¹ in the summer months (May–August), maximizing early in July at 95 km altitude, while it is less than 10 m s⁻¹ at all the observed altitudes during the equinoxes. It is normally eastward in winter at low altitudes, although it sometimes becomes westward during sudden stratospheric warmings. The mean meridional wind is usually equatorward and is weaker than the zonal component. A southward wind exceeding 10 m s⁻¹ is detected in July and August. The observed mean winds are compared with the CIRA 1972 model and coincidences with sudden warmings of changes in zonal wind direction are pointed out.     

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.     

Large amplitude quasi-periodic fluctuations associated with a mid-latitude sporadic E layer

For the first time a sounding rocket has been launched into a mid-latitude sporadic E event which was shown to be the source of VHF radar echoes. The layer had a very high peak electron density (∼10⁶ cm⁻³) and was thicker (∼5 km) than most events previously studied by rockets and incoherent scatter radars. The layer was modulated in a remarkable quasi-periodic manner which has not been reported earlier. Twenty cycles of these structures were detected and they seem to be oriented horizontally rather than vertically with periods in the rocket frame in the rage 6–10 s. There is also some evidence that the modulation was detected below as well as above the peak in the electron density, although the bulk of the flight was above the peak. Although the VHF radar echoes were decaying at the time and place where the rocket traversed the E layer, one burst of high amplitude short wavelength fluctuations was detected by the space-borne instruments and had a power spectrum similar to that of a secondary gradient drift mode. This burst occurred at the peak of one of the periodic electron density fluctuations. We discuss two possible sources for the dominant fluctuations: large-scale gradient drift waves and atmospheric acoustic waves. The latter seem most consistent with the data.     

Climatologies of semi-diurnal and diurnal tides in the middle atmosphere (70–110 km) at middle latitudes (40–55°)

The radars utilized are meteor (2), medium-frequency (2) and the new low-frequency (1) systems: analysis techniques have been exhaustively studied internally and comparatively and are not thought to affect the results. Emphasis is placed upon the new height-time contours of 24, 12 h tidal amplitudes and phases which best display height and seasonal structures; where possible high resolution (10 d) is used (Saskatoon) but all stations provide monthly mean resolution. At these latitudes the semi-diurnal tide is generally larger than the diurnal (10–30 m s⁻¹ vs. < 10 ms⁻¹), and displays less month to month variability. The semi-diurnal tide does show significant regular seasonal structure; wavelengths are generally small (⩽50 km) in winter, large in summer (≲ 100 km), and these states are separated by rapid equinoctial transitions. There is some evidence for less regularity toward 40°C. Coupling with mean winds is apparent. The diurnal tide has weaker seasonal variations; however there is a tendency for vertical wavelengths and amplitudes to be larger during summer months. On occasions in winter and fall wavelengths may be less than 50 km. Again the seasonal transitions are in phase with reversals of the zonal wind. Agreement with new numerical models is to be shown encouraging.     

Observations of lunar tides in upper atmosphere winds at Poker Flat,Alaska

The lunar semidiurnal tide is extracted from hourly values of winds in the 75–105 km region measured by the Poker Flat Alaska MST radar used in the meteor mode. Since year-to-year variations are apparent, detailed results for 1983 and 1984 are presented. Inferred vertical wavelengths range from 17 km in March 1983 to 46–55 km in September of 1983 and 1984. The height progression of the phase is frequently too irregular to derive a vertical wavelength. Amplitudes of 3 m s⁻¹ are common and range up to 8 m s⁻¹. Amplitudes generally are largest at the equinoxes, especially in September, with another maximum in winter sometimes occurring. Reasonable agreement is found with lunar tidal measurements at Saskatoon, and some points of similarity are found with the solar semidiurnal tide at Poker Flat.     

Recent work on mid-latitude and equatorial sporadic-E

Theoretical and experimental work since 1970 is summarized. Mid-latitude sporadic-E is most likely due to a vertical shear in the horizontal east-west wind and this theory accounts for the detailed observations of the wind and electron density profiles. Preferred heights of sporadic-E are separated by about 6km and descending layers are often seen moving down with velocities in the range 0.6–4 ms⁻. Sometimes sporadic-E layers are very flat and uniform, and at other times form clouds of electrons 2–100km in size moving horizontally at 20–130 ms⁻¹. Sporadic-E is probably not correlated with meteor showers; this is a rather surprising result since the ions are meteor debris.The major problems with windshear theory are to account for the dramatic seasonal variation and, to a lesser extent, for the geographical and diurnal distributions.The Q-type equatorial sporadic-E appears to be due to the gradient instability. There is a very much smaller amount of new experimental data available in this area.     

On the role of dust in the summer mesopause

We propose that dust formed at the cool summer mesopause may have optical properties very different from that measured for bulk material of ice. The smallness of the dust and possible surface impurities may lead to high photoelectric yields and low workfunctions. For such reasons the dust in the summer mesopause may, at least occasionally, be charged to substantial positive surface potentials while pure ice, with its high photoelectric workfunction, would be charged to low and negative potentials by collisions with plasma particles. The presence of ‘dressed’ dust particles, with surface potentials of some volts, can lead to enhanced radar backscatter. We also suggest that the apparent reductions in electron density (‘bite-out’), which have been observed in the radar backscatter region, can be caused by the inability of an electrostatic probe to deflect the massive dust particles.The dust density which is required by our model to explain radar backscatter and electron bite-outs is of the order of 10 cm⁻³ for dust of radius above 5 × 10⁻⁶ cm.     

Schumann resonance effects of electrical conductivity perturbations in an exponential atmospheric/ionospheric profile

Eigenmode solutions are computed for the n = 1 … 3 Schumann resonances in a perturbed, unmagnetized vertical atmospheric conductivity profile σ = 10⁻¹⁶ exp (z/3.1) mho m⁻¹ for z ⩽ 100 km and σ = 10⁻² mho m⁻¹ for z > 100 km. For the unperturbed exponential profile the radial electric field E_r is nearly constant z ≲ 40 km, and decreases rapidly above 50 km. The tangential field E_ϑ > E_r for z ≳ 65 km. The Joule dissipation profile in this case has an absolute maximum at about 50 km and a smaller relative maximum at 90 km with a deep relative minimum at 65 km. The maximum dissipation thus occurs in the middle atmosphere, making the Schumann resonances particularly susceptible to conductivity perturbations in this region. The perturbations of this study comprise Gaussian-shaped enhancements or depressions of FWHM ≈ 10 km impressed on the unperturbed profile. Eigenfrequencies and Q-values are computed for the full range of perturbation amplitudes 10⁻³−10³ and altitudes 30–90 km. The perturbations induce overall eigenfrequency variations of ± 1.0, ±1.5, and ±2.5 Hz in the n = 1, 2, and 3 modes, respectively, and Q-values spanning the range 3.5–11.0. The results of this calculation extend those of previous works investigating the Schumann resonance response to atmospheric conductivity perturbations, and may be useful for interpreting experimental observations in terms of external ionization source intensities of GCR, Lyman-α, or solar cosmic or X-rays, or variations in middle atmospheric chemical constituents.     

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.     

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.     

Observations of winds in the Antarctic summer mesosphere using the spaced antenna technique

Observations of winds in the 60–100 km height range were made at Mawson (68°S, 63°E) during December 1981 and January 1982 with the MF spaced antenna technique. The prevailing winds are in accord with other recent observations made at high latitudes and show a peak in the zonal wind near 80 km with westward winds of 30 m s ⁻¹. The meridional winds maximize near 90 km with an equatorward flow of 10 m s⁻¹. The diurnal tidal components are in reasonable agreement with recent model predictions, especially in phase. The amplitudes tend to be larger than the model values. The semidiurnal tide is not as stable as the diurnal tide and shows evidence for interference effects between different modes.     

Measurement of O2(a1Δg) emission in a total solar eclipse

The altitude distribution of the oxygen infrared atmospheric bands at 1.27 μm was measured during the total solar eclipse of 26 February 1979. The ozone concentration profile has been derived from these airglow measurements and indicates that at 85 km the concentration at totality was 7 × 1.7 cm⁻³, with no well defined upper layer. This reduced concentration, which is typical of summertime conditions, was probably due to perturbations in the mesospheric chemistry and transport induced by a winter warming event that was in progress at the time of the eclipse. At 60 km the ozone concentration, 2.7 × 10¹⁰ cm⁻³, was enhanced above that normally measured. This increase may also have been caused by the stratospheric warming event but the effects of a particle precipitation event, which was also in progress during the eclipse, may be important.     

Balloon measurements of stratospheric ion conductivities over the tropics

Conductivity measurements of negative and positive ions were made from about 20 to 35 km by two identical balloon-borne spherical probes at Hyderabad (17.5°N, 78.6°E), India on 22 April 1989 and 22 December 1990. One balloon was launched at 0158 h IST (Indian Standard Time) which reached its ceiling around 0330 h IST. After that time, it floated for about 3 h, 1.5 h before sunrise and 1.5 h after sunrise. Thus it gave data for both day- and night-time conditions at float altitude. The other balloon was launched at 0535 h IST. It gave data for daytime only. Several interesting results have been obtained at the float altitudes. During the night, in the flight of 22 April 1989 the conductivity values of positive ions were found to be about 1.5 times those of negative ions at the float altitude. During the day, in the flight of 22 April 1989, the positive ion conductivity values were found to increase with the increase of solar elevation angle at around 37.5 km altitude. The negative ion conductivity values, however, did not show any day-night variation. In the flight of 22 December 1990, these features were not seen. Instead, a pocket was found where conductivity values were very high (of the order of 10⁻¹¹ mho m⁻¹) at an altitude of about 32.5 km. Also in this flight, the positive ion conductivity was always found to be approximately equal to that of the negative ion conductivity.     

E-region nitric oxide concentrations inferred from the 26 February 1979 eclipse expedition

Nitric oxide (NO) concentrations have been determined from the analysis of positive ion composition data obtained by AFGL for eclipse and post-eclipse conditions near Red Lake, Canada. Values of about 3 × 10⁸ cm⁻³ for 105–110 km and about 3 × 107 cm⁻³ for 90 km have been established. A residual ion-pair production rate of about 50 cm⁻³ s⁻¹ is estimated for eclipse totality at 110 km. A factor of two uncertainty is thought to be appropriate for all deduced values. The calculated NO concentrations appear to be within the range of typical variations for this season (late winter) and latitude (51°N).     

Ozone and the duration of overdense radio meteors

This paper describes a study of the duration distribution of overdense radio meteor echoes using combined radio and visual data gathered over many years. We have paid particular attention to the wellknown change of slope which has previously been attributed to electron attachment to the atmospheric molecules. Using visual and photographic meteor data to estimate the heights, we have determined the height variation of the time constant associated with the electron loss process and we find that it corresponds closely to that of a reaction of meteoric ions with ozone to form oxide ions which quickly recombine dissociatively with the free electrons. We suggest that the duration distribution of overdense radio meteors could constitute a valuable diagnostic for monitoring the ozone concentration in the 80–100 km height range.