Small scale structure and turbulence in the mesosphere and lower thermosphere at high latitudes in winter

The MAP/WINE campaign has yielded information on small scale structure and turbulence in the winter mesosphere and lower thermosphere by a number of very different remote and in situ techniques. We have assimilated the data from the various sources and thus attempted to present a coherent picture of the small scale dynamics of the atmosphere between 60 and 100 km. We review physical mechanisms which could be responsible for the observed effects, such as ion density fluctuations, radar echoes and wind corners. Evidence has been found for the existence of dynamic structures extending over distances of the order of 100 km; these may be turbulent or non-turbulent. The results indicate that gravity wave saturation is a plausible mechanism for the creation of turbulence and that laminar flows, sharply defined in height and widespread horizontally, may exist.     

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.     

Mesospheric studies using the EISCAT UHF and VHF radars: a review of principles and experimental results

Studies of the mesosphere and ionospheric D-region are carried out with the incoherent scatter as well as with the MST radar technique. We briefly describe the principal differences between these techniques and the corresponding scattering mechanisms. Particular emphasis is placed on the observation of the coherent echoes from the summer mesosphere, which the EISCAT VHF and UHF radars have recently also detected. We present an overview of the historical development of EISCAT observations and results in these areas, and suggest possible directions of progress which would make the EISCAT radar systems even more suitable for mesosphere measurements.     

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.     

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.     

First low-power VHF radar observations of tropospheric,stratospheric and mesospheric winds and turbulence at the Arecibo Observatory

First VHF radar measurements with height resolution of 300 m and angular resolution of 1.7° were carried out in low latitudes at the Arecibo Observatory, Puerto Rico. A short outline is given of the experimental set-up which consisted of a 160W average power radar-transceiver and a self-contained digital radar control and data acquisition unit. The new VHF feed system of the Arecibo dish is described shortly. Reliable radar echoes were detected from the troposphere, lower stratosphere and from some heights in the mesosphere, indicating that the described VHF radar is capable of proper investigations of dynamical processes in the low latitude middle atmosphere. The angular dependence of aspect sensitive tropospheric and stratospheric turbulence structures was measured to be 1.5–2.5 dB degree⁻¹. Echoes from the mesosphere indicate a patchy structure of turbulence. The analysis of the signal-to-noise ratio shows considerably high reflectivity in the upper troposphere, which can be caused by high-reaching tropical cumulus convection. Wind profiles measured with the VHF radar between 7.5 and 19.5 km with a height resolution of 300m are very similar to radiosonde wind profiles. Mesospheric VHF radar winds are roughly consistent in amplitude with tidal winds.     

Investigations of the winter anomaly in ionospheric absorption in January 1981

Ground-based and rocket-borne investigations were carried out in January 1981 in the Volgograd region to study space-time peculiarities of the winter anomaly in ionospheric radio wave absorption (WA). Electron-density altitude profiles N_e(h) were measured with rockets, by the coherent frequency method and by using electrostatic probes; temperature profiles T(h) were measured by a resistance thermometer: wind velocity and direction were measured by radio-observations of a chaff cloud and of the payload parachute drift. At the same time, ionospheric radio wave absorption was measured in Volgograd at two frequencies, 2.2 and 2.7 MHz, by the A1 method. The condition of the lower ionosphere could be determined from absorption data and from f min parameter data obtained from vertical sounding ionograms. “Salvo” launchings of the rockets were performed on 14 January, when absorption was anomalously large, and on 21 and 28 January, which were days of normal winter absorption.Data analysis has shown that N_e values on the day with excessive absorption exceeded the same values on a normal day at altitudes from 72 to 95 km; on 21 January N_c values exceeded those of 29 February 1980 (without WA) at all altitudes below ~ 90 km. The absorption at Volgograd on 28 January was somewhat higher than on 21 January and than at stations at higher latitude, which may be due to a stable local increase of N_e values in the altitude range 80–90 km. The temperature in the region of the N_e-enhanced values (up to the limit altitude of measurements, about 80 km) was below the standard temperature (COSPAR, 72), both on 14 January and on the normal days. Measurements carried out at night have shown that winter N_c values considerably exceeded those during the autumn. The zonal and meridional wind profiles (up to about 80 km) at Volgograd exhibit a stable eastward flux, both in the stratsophere and in the mesosphere. The value of the wind velocity meridional component on 21 January is close to zero at all altitudes. On 14 and 28 January the wind profiles show an irregular structure with large velocity gradients at all altitudes above about 50–60 km.The absorption data and f min data from a number of stations, viz. from Juliusruh to Yakutsk (in longitude) and from Arkhangel'sk to Rostov-on-Don (in latitude), show that anomalously excessive absorption occurred over a vast distance exceeding 100° of longitude at ~ 55° latitude and that, based on the dates of absorption maxima (f min), one may conclude that the source of the disturbance was moving from west to east. Data on the motion of the air as shown by rocket and radiometeoric observations, indicate the same wind direction in the stratosphere as in the mesosphere. These data and the constant pressure charts point to the conclusion that the enhanced radio absorption values at mid-latitudes may be explained by a transport of dry air rich in nitric oxide from the auroral zone towards lower latitudes. The transport is provided by a stable circumpolar vortex existing in winter time. This mechanism may explain both the normal and anomalous winter absorption, as well as the post-storm effect.     

The EISCAT mesospheric measurements during the CAMP campaign

A brief outline is given of the experimental technique used during the Cold Arctic Mesopause Project to record the first D-region ion line spectra with the EISCAT incoherent scatter radar. The data analysis shows that echoes from mesospheric heights between about 70 km and 90 km can be detected during disturbed periods of enhanced electron density during particle precipitation events. Electron density profiles were determined which show a fairly high density, up to 5 × 10¹⁰ m⁻³ in the upper D-region. The measured meridional winds were lower than 10 m s⁻¹. A fit of the measured height profile of spectral width to temperature and neutral density models yielded a measured temperature profile in good agreement with simultaneous rocket data. The mesopause temperature was determined to be as low as 130 K. This detailed analysis of the spectral width profile indicates that below about 77–80 km the ratio of negative ions to electrons exceeded unity. Finally, some discussions are added on the limitations and significance of these first mesosphere observations.     

Consistency of rocket and radar electron density observations : implication about the anisotropy of mesospheric turbulence

We report about a quantitative comparison of rocket observations of electron density fluctuations and simultaneous 53.5 MHz radar measurements that were obtained during the MAC/SINE campaign in northern Norway in summer 1987. Out of three rockets launched during the Tur-bulence/Gravity Wave salvo on 14 July 1987, two were flown during conditions that allowed a detailed investigation. For a large part of the data from these rocket flights it is found that the radar reflectivity is about 10 dB, enhanced over what would be expected from the rocket observations in the case of isotropic electron density fluctuations. The observations can be reconciled under the assumption of an anisotropic turbulence. Assuming a simple model spectrum for the electron density fluctuations, we derive a relation between the rocket and radar observations that covers the whole range from isotropic turbulent scatter to Fresnel scatter at horizontal density stratifications. For the observed dataset, an anisotropy which typically corresponds to a ratio of the horizontal to the vertical coherence length of about 10 is consistent with the comparison of rocket and radar observations. A similar anisotropy is found also from the observed aspect sensitivity of the radar echoes. The variation of the anisotropy with height and time shows an anticorrelation with the turbulence level of the mesosphere as deduced from the spectral width of the radar echoes. The anisotropy is found to maximize in heights where the electron density displays deep ‘bite-outs’. These depletions in the electron density were independently observed by a Langmuir and an admittance probe on board two of the rockets.     

Poker Flat MST radar observations of high latitude neutral winds at the mesopause during and after solar proton events

Observations with the Poker Flat, Alaska, MST radar during and after solar proton events in 1982 and 1984 suggest that winds in the altitude range of ~ 80–90 km were altered as a consequence of the influx of energetic charged particles and large electric fields at high latitudes. The atmospheric changes accompanying these events appear to result in a reduction of the semidiurnal tide and an enhancement in the diurnal tide. It is suggested that these changes could result from the alteration of the local tidal heating distribution produced by the particle precipitation, either through changes in the local ozone distribution or as a result of mesospheric Joule heating.     

The dynamics of ion layer generation in the 80–150 km altitude region at low and mid-latitudes

The association of sporadic ion and sporadic sodium layers in the low-latitude, 90–100 km altitude region suggests that we must look beyond the windshear theory for details of the formation mechanism of sporadic layers in the 80–150 km altitude region. We present evidence, including specific 85–105 km results from the AIDA-89 and the ALOHA-90 campaigns, that 80–150 km altitude sporadic layers—including sporadic sodium layers—are generated in a complex interplay of tidal and acoustic-gravity wave (AGW) dynamics with temperature-dependent chemistry where wave-produced temperature variations are both adiabatic and dissipative or turbulent (non-reversible) in origin. We suggest that layering processes are best studied with an instrument cluster that includes sodium and iron lidars, MST radar (turbulence), incoherent scatter radar (electron concentration and winds), meteor radar techniques (winds), passive optical/IR imaging techniques, and appropriate rocket payloads to study a significant volume of the 80–150 km altitude region. We introduce the concept of volumtric radar and lidar techniques.     

Dominant vertical scales of gravity waves in the middle atmosphere observed with the MU radar and rocketsondes

We have simultaneously observed wind motions in the altitude range of 5–90 km by means of the MU radar, rocketsondes and radiosondes. Dominant vertical scales of wind fluctuations due to gravity waves were 2–5 km in the lower stratosphere, about 5–15 km in the upper stratosphere and longer than 15 km in the mesosphere. The increase in the vertical scale with altitude is interpreted in terms of the saturation of upward propagating gravity waves. In the stratosphere, the observed vertical wavenumber spectra showed smaller amplitudes and more gradual slopes than the model values. Furthermore, the wind velocity variance in the stratosphere increases exponentially with an e-folding height of about 9 km, implying that the gravity waves were not fully saturated. On the other hand, the spectra in the upper stratosphere and mesosphere agreed fairly well with the model spectra. The variance in the mesosphere seems to cease increase of the wave amplitudes and agrees reasonably well with the model value.     

MST radar studies of wind and turbulence in the middle atmosphere

A survey is presented of recent developments in the observation of wind and turbulence in the stratosphere and mesosphere using MST radars. One of the highlights of these developments is the growing recognition that the MST/ST radar is a valuable tool for routine monitoring of the atmospheric wind field. Furthermore, preliminary observations have shown the feasibility of monitoring atmospheric turbulence as well. Recent observations of mesospheric turbulence support theoretical models that emphasize the role of propagating waves in coupling the lower and middle atmospheres. Scientific groups in several countries are now planning or constructing MST radars so that within a few years observations should be available from diverse geographical locations spanning the globe.     

Intense turbulence in the polar mesosphere : rocket and radar measurements

Simultaneous observations of polar mesospheric summer echoes (PMSE) have been made with two different frequency radars during the launch of a sounding rocket designed to measure the fluctuations in the electron density in the same height range. The cross-section for radar backscatter deduced from the rocket probe data under the assumption of isotropic turbulence is in reasonable agreement with the measured signals at both 53.5 MHz with the mobile SOUSY radar and 224 MHz with the EISCAT VHF radar, which correspond to backscatter wavelengths of about 3 and 0.75 m, respectively. Some controversy exists over the relative roles of turbulent scatter vs specular reflections in PMSE. A number of characteristics of the data obtained in this experiment are consistent with nearly isotropic, intense meter-scale turbulence on this particular day. Since equally compelling arguments for the importance of an anisotropic-type mechanism have been presented by other experimenters studying PMSE, we conclude that both isotropic and anisotropic mechanisms must operate. We have found the inner scale for the electron fluctuation spectrum, which corresponds to the diffusive subrange for that fluid, and have compared it to the inner scale for the neutral gas. The latter was found from the Kolmogorov microscale, which in turn depends on the energy dissipation rate in the gas. We found the dissipation rate from the spectral width of the 53.5 MHz backscatter signal and from the rocket electron density fluctuation data. The diffusive subrange was found to occur at a wavelength a factor of about 10 times smaller than the viscous subrange. This corresponds to a Schmidt number of about 100. High Schmidt numbers have been reported in recent measurements of the diffusion coefficient of the electrons in this height range made with the EISCAT incoherent scatter radar. About 15 min after the rocket flight an extremely high radar reflectivity was found with the SOUSY system. We have been able to reproduce this high level theoretically by scaling the rocket data with an increase in the neutral turbulence energy dissipation rate by a factor of 14 as deduced from the SOUSY spectral width, an increase in the electron density which is consistent with riometer data, and a 33% decrease in the electron density gradient scale length which is hypothesized. We also estimate the radar reflectivity at 933 MHz and conclude that signals in excess of thermal scatter levels would have occurred at the peak of the event studied, provided that the electron fluctuation spectrum decreases as k⁻⁷ in the viscous subrange. If the spectrum has an exponential form, however, a turbulent source cannot explain the enhanced 933 MHz echoes reported by EISCAT.     

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.     

The E-region Rocket/Radar Instability Study (ERRRIS): scientific objectives and campaign overview

The E-region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude (E-region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation (δn/n) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.     

Heights of MF radar scatter (1986/87) and the wind field (55–95 km): Saskatoon,Canada

The Saskatoon MF radar (2.2 MHz) at 52°N, 107°W, has been used to measure the heights of occurrence of radar scatter during four seasons, and twelve months of 1986/87. Mean winds, and gravity waves are also available, by the spaced antenna method and from the same radar echoes. Certain heights, called elsewhere ‘preferred heights’, are identified near 60km, 70km, 75km in summer, and 80–86 km. Several layers have seasonal and diurnal variations. Associations with electron density gradients (rocket data), mean wind shear in summer, and gravity wave amplitude-minima in the equinoxes are effectively demonstrated. Case studies, involving 3 h data sets of radar scatter and wind elaborate the comparison: gravity waves of long period (τ > 6 h) are shown to modulate the scattering process.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.