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.     

A narrow auroral arc observed with EISCAT

On the evening of 13 January 1983 we made simultaneous observations of optical and radar aurora using low light television cameras together with the EISCAT radar system. At 19 h 16 m 06 s UT an extremely bright auroral arc moved rapidly (about 2 km s⁻¹) through the EISCAT radar beam. The associated rapid rise and fall in the E-region electron density indicates that there was an intense narrow electron beam associated with the optical arc. We estimate that the ionisation rate in the E-region increased at least 20-fold (from 1 × 10¹⁰ m⁻³ s⁻¹ to >2 x 10¹¹ m⁻³ s⁻¹) for 1 or 2 s as the arc passed by. In addition, there was a brief (<4 s) increase of 130% in the signal returned from 250 km altitude which coincided with the arc crossing the radar beam at that height. In view of this coincidence, we find that a possible explanation is that the increase arose from short-lived molecular ions, for example vibrationally excited N⁺₂ ions, produced in the F-region by soft precipitation associated with the arc.     

Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere regions—II. The semidiurnal tide

The semidiurnal tidal dynamics of the Antarctic and Arctic mesopause regions (95 ± 15 km) are investigated through comparative analyses of monthly mean tidal wind fields determined from radar measurements at the Scott Base (78°S), Molodezhnaya (68°S), and Mawson (67°S) stations in the Antarctic, and the near-conjugate stations of Heiss I. (81°N) and Poker Flat (65°) in the Arctic region. The main feature common to all stations is the fall equinoctial maximum in amplitude (10–20 m s⁻¹), which is also reproduced by the most recent numerical tidal model. However, the wintertime amplitude growth with height and the shorter vertical wavelengths characterizing the model are features not reflected in the data. There is also a spring equinoctial maximum in the Antarctic data which the model does not reproduce.Examination of interannual variability reveals characteristics similar to those noted in Part I for the mean zonal wind; namely, some degree of year-to-year variability superimposed on apparent long-term decreases of order 0.3–0.5 m s⁻¹ yr⁻¹ (depending on month) in the Southern Hemisphere semidiurnal tidal amplitudes. Numerical simulations presented herein indicate that changes of this magnitude cannot even be induced (via mode coupling) by a change in the mean zonal wind field of order 30%, and are more plausibly explained by a secular change in the tidal forcing by ozone insolation absorption. However, contrary to Part I, the annual mean tidal amplitude is not characterized by any significant secular trend, remaining within the 10.0 ± 2.5 m s⁻¹ range throughout the 1970–1986 period. Analyses of other data sets are required to ascertain confidence in the apparent trend reported here.     

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.     

Radar and optical observations of mesospheric wave activity during the lunar eclipse of 6 July 1982

Simultaneous measurements were made using a 2.66 MHz interferometer radar, infrared photometers, and imaging systems during the total lunar eclipse of 6 July 1982. The radar data showed that a series of six discrete scatterers passed overhead at 103 km with an average spacing of 54 min, and two passed overhead at 88 km, also 54 min apart. The 88 km events were approximately 27 min out of phase with those at 103 km. One of the 88 km events was examined in detail; the radar returns appeared to come from a single scatterer or a few clustered scatterers, with a velocity of 135 m s⁻¹ almost due south, at 6° below the horizontal. The speed and period give a horizontal wavelength of 440 km, and the phase shift between 88 and 103 km activity suggests a 30 km vertical wavelength, in agreement with values for typical medium-scale traveling ionospheric disturbances (TIDs). Infrared images were made in the near infrared, and photometric measurements were made on and off the 8−3 band of OH. These observations, made from one site near the radar and a second site 575 km south, showed wavelike structures appearing first over the radar, then further south until they filled most of the sky. The speed of development of the infrared structure pattern in the sky is consistent with the 135 m s⁻¹ southward wave speed observed by the radar, but the structures themselves appeared in place, then drifted slowly northward at 10 m s⁻¹. The photographically determined wavelengths were 30–60 km, considerably shorter than the 440 km determined with the radar.     

Tidal wind observations with incoherent scatter radar and meteorological rockets during MAP/WINE

Measurements of winds in the mesosphere and lower thermosphere were carried out during the main phase of the MAP/WINE project in January and February 1984 with the EISCAT UHF incoherent scatter radar near Tromsö, Norway, and with meteorological rockets launched from the Andøya Rocket Range, Norway. The radar measurements yield wind profiles between the altitudes of about 80 km and 105 km and the rockets between about 60 km and 90 km. Results from both techniques are combined to yield mean profiles which are particularly evaluated in terms of tidal variations. It is found that the semidiurnal tide constitutes an essential wind contribution between 85 km and 105 km. Whereas the tidal amplitudes are below 5 m s⁻¹ at about 80 km, they increase to 20–30 m s⁻¹ at 100 km. The average vertical wavelength of 35 km points to the S₄² mode, but coupling and superposition of different modes cannot be excluded.     

Simultaneous observations of the quasi 2-day wave at Mawson,Antarctica, and Adelaide,South Australia

Observations of the Austral quasi 2-day wave at Mawson, Antarctica (67°S, 63°E) are presented and compared with those from Adelaide (35°S, 138°E). The data were obtained from partial-reflection radars which have been measuring winds continuously since mid-1984, and the results presented here are the first to record the 2-day wave in middle atmosphere winds from Mawson. They show that 2-day period oscillations of 10–15 m s⁻¹ are a regular feature of the high latitude southern hemisphere summer. The wide longitude and latitude separation of the radar stations permits estimates of propagation velocity and latitude phase structure, and results are consistent with the passage of a westward travelling Rossby-gravity (3, 3) wave.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

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.     

A tidal wind model for 85–120 km based on CIRA-86: a feasibility study

This paper examines the feasibility of deriving a climatology of the diurnal variations of the wind in the 85–120 km region from the tidal components of temperature, density, and composition contained in the new COSPAR International Reference Atmosphere, CIRA-1986, Part I: Thermosphere Models [(1988), Adv. Space Res.8, 9]. To derive the wind field, we used the zonal and meridional momentum equations which have been modified from the characteristic scales of the tidal components observed in the 85–120 km region. The CIRA temperature and density model was used to derive the eastward (westerly) and northward (southerly) pressure gradient forces which serve as the forcing functions in the coupled momentum equations. Ground-based wind data from the Mesosphere-Lower Thermosphere (MLT) radar network is used as an independent data set to check the accuracy of the derived tidal wind model. At midlatitudes, the model reproduces some of the general features observed in the radar tidal data, such as the dominant semidiurnal tide with increasing amplitude with height and clockwise (counterclockwise) rotation of the velocity vector observed in the northern (southern) hemisphere. The model overestimates the semidiurnal amplitudes observed by radar by 50–75% during most seasons with the best agreement found during the equinoctial months. The model exhibits little phase variation with height or season, whereas the radar data exhibit a downward phase progression during most seasons (other than summer) characteristic of upward propagating tidal waves, and large seasonal phase variations associated with seasonal changes in vertical wavelengths. The diurnal tidal amplitudes, which are generally 5–20 m s⁻¹ at mid-latitude radar stations and are dominant over the semidiurnal amplitudes at lower latitudes, are less than 5 m s⁻¹ at all latitudes in the model.     

Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements

DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47-01.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15–30 m s⁻¹ exists. Simultaneously, the F-region plasma was moving downward at a speed of 30–50 m s⁻¹ These events appear in the local time sector of 23.002&#x0304;23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25–50 m s⁻¹ are possible at times near midnight in the equatorial F-region.     

Direct radar observations of TIDs in the D- and E-regions

We have observed Traveling Ionospheric Disturbances (TIDs) in the night-time D- and E-regions using a 2.66 MHz imaging Doppler interferometer radar. TIDs were observed in two distinct ways. In the first, the TID was manifested as discrete traveling surges, with average spacings of 54 min. The D-region surges were so well defined that they could be tracked as they passed close to overhead by using the phase differences across the antenna arrays. A velocity of 135 m s⁻¹ to the south was measured, giving a horizontal wavelength of 440 km typical of medium scale TIDs. The direction of phase travel relative to the horizontal was −6° (i.e. downwards). These observations were made during a night of extraordinary OH infrared mesopause structure activity made visible by the presence of a total lunar eclipse. In the second type of TID observation, we show the Doppler interferometer analysis of ripples on the under surface of sporadic-E layers taken on two nights of significant OH infrared and OI 5577 Å wave activity. The TIDs were observed to propagate at speeds of 120–300 m s⁻¹, with directions predominately toward the southwest, again typical of medium scale TIDs. These results show definite wave effects on MF radar returns and thus suggest that the measurement of mesospheric bulk winds with MF radars should be approached with some caution. Comparison of the TID characteristics with the OH structure characteristics show that the TIDs travel faster than the OH structures, have longer apparent horizontal wavelengths and generally travel in the opposite direction.     

First results with the Adelaide VHF radar: spaced antenna studies of tropospheric winds

The first results from a VHF radar of the ST type located at Buckland Park near Adelaide, Australia (35°S, 138°E), are presented. The radar is designed to be versatile and can be used to measure velocities in the lower atmosphere using both the spaced antenna (SA) and Doppler beam-swinging (DBS) techniques. Here studies of irregularities and motions made with the spaced antenna technique are discussed. It is shown that the scale of the diffraction pattern formed by the backscattered radiation varies with altitude, with the mean pattern scale being smaller in the troposphere than in the stratosphere. The observations are consistent with the backscattered energy decreasing as a function of off-vertical angle by 1.5 dB per degree in the troposphere and by about 2.8 dB per degree in the lower stratosphere. An intercomparison of zonal velocities measured with the SA and DBS methods shows good agreement. In May and August 1984 an extensive comparison was made between the velocities measured by the SA method and winds determined from over 80 balloon-borne radiosondes released from Adelaide Airport, situated some 36 km to the south of the radar. The velocities were compared on a statistical basis and showed excellent agreement, although the SA speeds tended to be 1–2 m s⁻¹ smaller in magnitude than the radiosonde velocities. Overall, the rms differences between the two sets of measurements was only 3–4ms⁻¹ throughout the troposphere, a result which is consistent with the random errors inherent in each technique, as well as the spatial separation between the radar and balloon observations. The utility of the SA method for meteorological observations is illustrated by a study of both the horizontal and vertical wind fields during the passage of a cold front made in November 1984. The high time resolution available with the radar allows detailed studies of the development of the pre-frontal jet, the wind convergence into the front and associated vertical motions.     

Gravity-wave motions in the mesosphere and lower thermosphere observed at Mawson,Antarctica

We present the results of MF radar observations of mean winds and waves in the height range 78–108 km at Mawson (67°S, 63°E), Antarctica. The measurements were made in the period from 1984 to 1990. Climatologies of the prevailing zonal and meridional circulations made with a 12-day time resolution show that the mean circulation remained relatively stable over the 6 yr of observation. Climatologies of gravity-wave motions in the 1–24 h period range were also generated. These reveal that the r.m.s. amplitudes of horizontal wave motions near the mesopause (~90 km) are about 30 m s⁻¹, and that there is some anisotropy in the motions, especially at heights below 90 km. Meridional amplitudes are larger than zonal amplitudes, which suggests a preference for wave propagation in the north-south direction. Comparisons with MST radar wind observations made near the summer solstice at Poker Flat, Alaska (65°N) and at Andøya, Norway (69°N) show similarities with the Mawson observations, but the wave amplitudes and mean motions are larger in magnitude at the northern sites. This suggests hemispheric differences in wave activity that require further study.     

Shadow bands during the total solar eclipse of 3 November 1994

We recorded shadow bands just before and just after the total phase of the solar eclipse of 3 November 1994. The recordings were made using two broad-band silicon photodiodes separated by 100 mm. They were mounted on a plate that faced the eclipsed Sun, which was at an altitude of 32.4° as seen from our observing site 4500 m above sea level between Putre and Lake Chungara in northern Chile. The irradiance fluctuations associated with the shadow bands were around 0.008 W m⁻² rms on a background of about 2–8 W m⁻². The cross-correlation function indicates that the shadow bands were moving at a speed of about 1.8 m s⁻¹ perpendicular to their extent. The power spectral density functions are in accord with the shadow band theory of Codona (1986). We carried out a similar experiment in Baja California during the eclipse of 11 July 1991. In spite of teh considerable differences between the two circumstances, our results on the two occasions are broadly similar.     

The influence of geomagnetic activity on the upper mesosphere/ lower thermosphere in the auroral zone. I. Vertical winds

A scanning Fabry-Perot spectrometer (FPS), located at Mawson station, Antarctica (672S, 63°E, invariant latitude 70°S) was used to obtain vertical wind, temperature, and emission intensity measurements from the λ558 nm emission of atomic oxygen. The measured temperature is used to assign an approximate emission height to the observations. A spaced-antenna partial-reflection radar was run concurrently with the FPS from which the presence of enhanced ionization in the D-region could be inferred from the return heights and strengths of the echoes. Large upwards winds of approximately 30 m s⁻¹, at altitudes less than 110 km, appear to be a direct response of the neutral atmosphere to intense auroral events. It is suggested that the observed upwelling is a result of particle heating at heights below the principal emission height. At higher altitudes, vertical winds of a similar magnitude are also measured during geomagnetically disturbed conditions, although here they do not appear to be associated with particular auroral events. In this case it is suggested that upwelling is produced by a combination of Joule and particle heating.     

Effective electron collision frequency measurements in the E- and F-regions

The collision frequency v in the ionosphere has often been determined by measuring differences in the amplitude and group path of two closely spaced signals reflected in the region of high group retardation. In this paper we describe a method of measuring v using a CW double-side-band modulated signal reflected obliquely in the ionosphere. This allows v to be determined on a continuous basis and it is found that the value of v obtained is 1–5 × 10⁴ s⁻¹ for the E-region and ~ 10³ s⁻¹ for the F-region. It is shown that measurements made just after sunset, when the E-region is still present, are more representative of E-region values than F-region.     

Estimates of gravity wave momentum fluxes in the winter and summer high mesosphere over northern Scandinavia

As part of the MAP/WINE campaign (winter 1983–1984) and the MAC/SINE campaign (summer 1987) high resolution wind profiles were obtained in the upper mesosphere using the foil cloud technique. Vertical winds were derived from the fall rate of the foil clouds and are used for estimating the momentum fluxes associated with vertical wavelengths shorter than about 10 km. From the ensemble average of 15 observations over an altitude range of 74–89 km we calculate a zonal net momentum flux of +12.6 ± 4.5 m²s⁻² in summer. The average of 14 measurements in winter between 73 and 85 km indicates a zonal net momentum flux of −3.7 ± 2.4 m²2 s⁻².     

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.     

Can the high latitude ionosphere support large field-aligned ion drifts?

Ion velocities perpendicular and parallel to the geomagnetic field have recently been deduced by Smith et al. from bistatic measurements at 71° geomagnetic latitude in the afternoon sector. The results of this experiment include large (>400 m s⁻¹) downward ion velocities parallel to the magnetic field that persist for hours, small (100 m s⁻¹) ion velocities perpendicular to the magnetic field and electron density profiles with extremely narrow full-width at half-maximum. The explanation of these results was that the ionospheric flux tubes observed were near the terminator, and thus, sunlit at the top and in darkness at the bottom. The difference in production between the top and bottom of the flux tube creates an excess of ions at the top, which rapidly diffuse downwards. A three-dimensional, time-dependent model of the ionosphere has been used to test this explanation. Numerical experiments were performed to determine upper limits for the downward ion velocity. Assuming reasonable vertically-induced ion drifts due to either neutral winds or plasma convection, these upper limits were substantially smaller than the measurements. The location of the terminator was found to contribute a maximum of about 60 m s⁻¹ to the vertical ion velocity due to diffusion in a partially illuminated flux tube. In an attempt to explain the narrow density profiles without invoking an additional ionization source, the downward force in the model was arbitrarily increased, as would occur due to parallel electric fields in the ionosphere. Since the interpretation of these measurements as large field-aligned flows seems untenable by a model thought to be consistent with the currently accepted physics of the atmosphere, an alternate hypothesis is presented. If the common volume measurement is made in a region of O⁺ precipitation, then the line profile would not be Doppler shifted when viewed off-zenith. Therefore, the field-aligned velocities would be small, and the narrow width of the profiles would be due to enhanced electron densities in an O⁺ arc.