Studies of high latitude mesospheric turbulence by radar and rocket 1: energy deposition and wave structure

During early spring, 1985, the MAE-3 (Middle Atmospheric Electrodynamics) Program was conducted at Poker Flat Research Range, Alaska to study the origin of wintertime mesospheric echoes observed with the Poker Flat MST radar there, by probing the mesosphere with in situ rocket measurements when such echoes occurred. Pre-launch criteria required the appearance of echoes exhibiting some wave structure on the MST radar display; these could be met even under weak precipitation conditions with riometer absorption near or above 1.0 dB. Two morning rockets were launched under such conditions, the first (31.048) on 29 March 1985, at 1703 UT and the second (31.047) on 1 April 1985, at 1657 UT. Both payloads were deployed on a high altitude parachute near a 95 km apogee to provide a stable platform for data acquisition within the mesosphere (below 80 km). Each payload carried a solid state detector to measure energetic electrons between 0.1 and 1.0 MeV and an NaI crystal detector to measure x-rays from >5 to >80 keV. Payload 31.048 also carried a positive ion ‘turbulence’ probe which measured ion density changes (ΔN_i/N_i) during payload descent, whereas 31.047 carried a nose tip ‘turbulence’ probe designed to measure electron density changes (ΔN_e/N_e) during upleg ram conditions plus a Gerdien condenser for the measurement of bulk ion properties during downleg. The energy deposition curves for each event exhibited peak deposition rates between 75 and 80 km with a half width of 16–18 km, almost exclusively induced by precipitating relativistic electrons. They also showed a maximum bottomside gradient between 65 and 75 km. Radar echoes and atmospheric turbulence were observed in the same altitude domain, consistent with the anticipated need for adequate free thermal electron gradients to make such phenomena visible on the radar. The vertical wave structure from radar echoes was found to be consistent with that observed in horizontal wind and temperature profiles measured by Datasondes flown shortly after each large rocket. An analysis of the wave structure from radar data has shown that although large scale waves (λ_z ~ 7 km) were found to be present, a higher frequency shorter wavelength (∼ 1–3 km) component probably played a more significant role in modulating the signal-to-noise structure of the radar echoes.     

Equatorial spread-F: recent results and outstanding problems

Recent results and outstanding problems in the field of equatorial spread-F are reviewed. The discussion is organized about four distinct wavelength regimes: long (≳ 20 km). intermediate (20 km–100 m). transitional (100 m–10 m) and short (≲ 10 m). The intermediate waves are best understood, although the enhanced turbulent power near 1 km wavelength needs explanation, as does the saturation mechanism itself. The role of shear flow and irregularity seeding by ‘geophysical noise’ in the neutral atmosphere form important future research topics at the longest scales. Identification of drift waves in the transitional wavelength range has been a major new step in explaining the full spread-F phenomenon. This drift wave turbulence in the transition regime will be actively studied in the near future, particularly with regard to its role in determining the saturated amplitude of the intermediate waves, as well as in the anomalous diffusion of plasma at high altitudes. Although waves as short as 11 cm have been unambiguously detected and linear theories exist, the origin and amplitude spectra of these short wavelength waves inside topside plumes remain in some doubt and should also be examined in more detail.     

A simple model of gravity-wave momentum and energy fluxes transferred through the middle atmosphere to the upper atmosphere

Vertical fluxes of momentum and energy through the middle atmosphere are calculated by using a simple semi-empirical model of quasi-monochromatic internal gravity waves with dominant vertical wavenumbers. In this model those dominant gravity waves are assumed to saturate and break at each observational altitude by an effective critical-layer mechanism. The dominant value of the vertical wave-number is expressed by an exponential function of altitude, decreasing upward with a scale height of 34 km. This expression gives the momentum and energy flux densities decreasing upward with scale heights of 12 and 18 km, respectively, and typical values at 100 km altitude are estimated as 4 × 10⁻⁵ Pa and 4 × 10⁻³ W/m². A heat flux induced by wavebreaking turbulence also has an order of magnitude similar to that of the wave energy flux. Variabilities around these values and comparisons with other momentum and heat inputs to the upper atmosphere are only briefly discussed.     

EDIA-EISCAT comparison between small scale F-region irregularities and large scale electron density structures at sub-auroral latitudes

Small scale sub-auroral F-region irregularities were observed on 6–7 February 1984 by the two HF radars of the EDIA experiment while the EISCAT UHF system was scanning the ionosphere between 57° and 66° invariant latitude at a slightly different longitude. The bistatic EDIA system was mainly designed to detect the F-region irregularities at sub-auroral latitudes and to measure their perpendicular velocities. This paper is devoted to an examination of the morphology of the irregularity regions detected by the HF radars and of their production mechanisms, by comparison with the horizontal and vertical electron density profiles measured by EISCAT. It is shown that decametric irregularities observed at about 360–430 km height are not associated with any large scale horizontal density gradients in the F-region (350km). However, a strong north-south gradient observed at lower altitudes (150–200km), which is likely to indicate the southern boundary of the high energy particle precipitation zone, is well correlated with the strong scattering regions observed by the HF radars. The EISCAT electron temperature measurements at 350km height also show horizontal gradients which are well correlated with the small scale F-region irregularities. We discuss implications of these observations on the mechanisms of production of irregularities in the sub-auroral F-region.     

Dynamo action in sporadic-E formation

A theoretical study of the effects of background winds on wind shear-produced sporadic-E layers requires an account of the dynamo electric fields which result from the plasma motion produced by these winds. When a sporadic-E layer is carried downward by a descending wind shear the final height of the layer may vary by some 10 km depending on the background wind. Due to a loading effect on the dynamo, for a given background wind, the final height may vary by about 6 km depending on the degree of ionization in the E-layer in the magnetically conjugate hemisphere. The time scale for dispersal of a sporadic-E layer by drifts in wind-induced electric fields may be as little as 2 h.     

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.     

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.     

Propagation of turbulence structures detected by VHF radar

In the high latitude wintertime mesosphere VHF radar measurements usually reveal several turbulence layers at heights between 65 and 85 km which are closely related to strong vertical wind shear. The turbulence layers are superposed by turbulence bursts, which often form sequences with periods similar to those of simultaneously observed velocity oscillations. The horizontal propagation velocity of the resulting turbulence structures can be obtained by cross-correlating the signal power time series measured at three antenna beam positions. A statistical study using a total of 71 events shows that there is a significant correlation between the propagation velocity of turbulence structures and the mean wind, being consistent with the assumption that turbulence is advected by large scale motions. It is suggested that the observed turbulence bursts are due to secondary static instabilities, which for their part are generated by primary Kelvin-Helmholtz instabilities in regions of strong wind shear.     

Sudden sodium layers in polar latitudes

We discuss in this paper sudden sodium layers (SSLs), which we observe with a sodium lidar instrument at Andenes, Norway (69°N). We speak of a SSL if, in a narrow altitude range (typically less than 2km), the Na density increases over the normal Na density by a factor of at least 2 within 5 min. Between December 1985 and November 1987, we have observed 42 such layers in 378 h of lidar measurements. This number increases to 75 if we only require an increase of a factor of 1.5 within 8 min. At our observation site, SSLs have the following properties: (a) they develop between 90 and 110 km altitude, (b) they develop between 20 and 02 LT, (c) their appearance shows a strong, positive correlation with that of ƒ-type E_s layers, and (d) their appearance does not show a strong correlation with either riometer absorption or meteor showers. We discuss a number of potential processes for SSL formation. SSLs above 100km can be formed in ƒ-type E_s layers by the conversion of Na⁺ ions into neutral Na. The development of SSLs below 95 km requires the presence of an additional reservoir of Na, such as Na-bearing molecules, ions, and/or ‘smoke’ particles. We also evaluate the proposal that SSLs are the outcome of single meteoroids entering the upper atmosphere, a proposition for which we find little observational support.     

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.     

The height of electron content changes in the ionosphere from ATS6 beacon data

A technique is described which uses relative changes in Faraday rotation and modulation phase of satellite radio signals to determine the median height of the enhancement (or depletion) in the electron density of the ionosphere. During the post sunrise formation of the F mlayer the incremental layers have a median height of around 210 km (±40) and in the afternoon the decremental median is above the peak at 340 km (±40) on a winter day. A winter night-time enhancement just after midnight appears as a thick layer extending upwards from the peak, with a median height at about 730 km. The method applies to large scale irregularities but not to small, dense, scintillation-causing irregularities for which Faraday and modulation phases do not represent the total electron content.     

Observations of thermospheric neutral winds by the UCL Doppler imaging system at Kiruna in northern Scandinavia

Since the 1982/1983 winter, the UCL group, in collaboration with the Swedish Institute for Space Physics (previously Kiruna Geophysical Institute), has operated a Doppler imaging system at the high latitude station of Kiruna (67°N, 22°E). The Doppler imaging system is an imaging Fabry-Perot interferometer of 13.2 cm aperture. This instrument has been operated on a ‘campaign’ basis for mapping thermospheric winds using the OI emission at 630 nm (240 km altitude) from a region up to about 400 km radius about Kiruna. In November 1986, the performance of this wide-field Doppler imaging system was augmented by improvements to the detector and all-sky optics. We present data from December 1986, obtained during periods with both clear skies and active auroral and geomagnetic conditions. Maps of the neutral wind flow within the auororal oval during disturbed conditions and near magnetic midnight show continuous and rapid changes of thermospheric winds. The typical scale sizes of eddies observed within the mean flow around magnetic midnight are 100–300 km, with fluctuations at all time scales resolved by the 10 min between successive Doppler images. The local and short period fluctuations appear to be a filtered response of the thermosphere to rapid local variations of the convection and precipitation patterns, within a background of global scale changes     

A model of the lower ionosphere above Red Lake,Canada, during the 26 February 1979 solar eclipse

Measurements of ionization sources, ionization profiles and minor atmospheric constituents were conducted during the 26 February 1979 solar eclipse above Red Lake, Canada. A model of the lower thermosphere has developed to describe the D- and E-regions of the ionosphere for this case with the model being guided by the measurements. During the eclipse a rather intense particle precipitation event was in progress. For this reason, an auroral deposition code was coupled to a chemical-kinetics code to calculate degraded primary and secondary electron fluxes, ionization rates, positive ion and electron densities. The model was calibrated with the experimental measurements of electron flux below 100 km and electron density between 70 and 150 km. This calculation not only satisfactorily described the ionization in the E-region but also the gross electron density characteristics of the D-region. Bursts in the observed electron flux were also simulated with the model to give electron density profiles that were remarkably consistent with small perturbations seen in the electron density measurements.     

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.     

Characteristics of ionospheric irregularities capable of producing quasi-horizontal traces on ionograms

This paper presents simulated ionograms calculated for a parabolic ionospheric layer containing irregularities in the form of small amplitude waves. With small amplitudes, perturbation techniques can be used enabling results for the irregular ionospheres to be calculated from the results for smooth ionospheres. This approach is relatively straightforward and avoids having to ray trace new paths each time the irregularity parameters are changed. It is, however, restricted to irregularities which do not cause multiple echoes. Irregularities with vertical wavelengths of up to a few kilometres can produce significant changes in the ionosphere over height intervals smaller than those involved in reflecting a single pulse. Consequently, in the simulation procedure, it is essential to consider not just the carrier frequency but the complete frequency spectrum of the pulse. Irregularities with vertical wavelengths of the order of 10 km or more can produce ripples in an ionogram trace. These will, of course, be more evident on ionograms with high frequency resolution. Irregularities with vertical wavelengths of up to several kilometres and amplitudes up to a few per cent can produce significant pulse spreading and splitting. The actual effects depend not just on the irregularity properties but also on the ionosonde pulse width, gain and frequency and height resolutions. Some simulations show trace splitting and quasi-horizontal traces similar in many respects to effects observed by Bowman (1987, J. atmos. terr. Phys. 49, 1007) and Bowmanet al. (1988, J. atmos. terr. Phys. 50, 797). Consequently it is suggested that, at least in some cases, small amplitude (≤3%) and small scale (≤4 km) irregularities produce the spread-ifF reported by these authors.     

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.     

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.