Showing papers in "Annales Geophysicae in 1993"

On explaining the local time variation of ionospheric storm effects

Abstract: This study attempts to explain the local time variation of ionospheric storms at middle latitudes using a model which is based on a close coupling between thermospheric and ionospheric disturbance effects. Daytime positive storms are attributed to traveling atmospheric-ionospheric disturbances and subsequent changes in the global wind circulation. Negative storm effects are attributed to neutral gas composition changes which are advected toward middle latitudes during night and which subsequently rotate into the day sector. This disturbance scenario is supported by the results of a data analysis

EFEDA - European field experiment in a desertification-threatened area

Abstract: During June 1991 more than 30 scientific teams worked in Castilla-La Mancha, Spain, studying the energy and water transfer processes between soil, vegetation, and the atmosphere in semiarid conditions within the coordinated European research project EFEDA (European Field Experiment in Desertification-threatened Areas). Measurements were made from the microscale (e.g., measurements on single plants) up to a scale compatible with the grid size of global models. For this purpose three sites were selected 70 km apart and heavily instrumented at a scale in the order of 30 sq km. Aircraft missions, satellite data, and movable equipment were deployed to provide a bridge to the larger scale. This paper gives a description of the experimental design along with some of the preliminary results of this successful experiment.

A statistical cloud scheme for use in an AGCM

Abstract: We have designed a diagnostic subgrid scale cloud scheme that brings some coherence to the various representations of clouds within the GCM. It takes into account interactions between cloud fraction coverage, liquid water content and turbulence (for stratiform clouds). The scheme has been developed on the basis of a statistical cloud scheme and a level 2 subgrid scale turbulence scheme (Yamada and Mellor) in which we introduce the effects of water vapor and liquid water. The scheme has been implemented in the T42-20 level version of the French spectral GCM used for climate studies. Simultaneously, large-scale precipitation has been modified according to Smith's scheme (with different parameterizations for liquid and ice water). The GCM has been run with and without the new parameterization. We have studied the impact of the implementation of two new parameterisations on the results of the French Emeraude GCM in January. The model simulations are validated with satellite data from the ERB experiment (total cloudiness field) and from the ERBE experiment (visible and IR radiation fields at the top of the atmosphere)

Observation by the Intercosmos-24 satellite of ELF-VLF electromagnetic emissions associated with earthquakes

Abstract: The measurements of emissions in the frequency range 8 Hz−20 kHz during 180 orbits of the Intercosmos-24 satellite from November 16, 1989 to December 31, 1989, are analyzed Twenty-eight rather strong earthquakes (52

Structure functions and intermittency of velocity fluctuations in the inner solar wind

Abstract: In this paper we provide evidence for the intermittent nature of the fluctuations of the flow velocity and Alfven velocity in the inner solar wind between 0.3 and 1.0 AU. We analyse the p-th order structure functions in the time domain between 40.5 seconds (instrumental resolution period) and 24 hours, and investigate their scaling properties in the Alvenic range from fractions of minutes to several hours. Generally, the fast and slow wind regimes have to be discriminated, because the scaling features of the velocity differences with time lag r vary systematically with the wind speed. Radial trends are established by comparison of Helios data obtained near 0.3 AU and 1.0 AU. The second-order structure function for p=2, which is equivalent to the power spectrum, shows essentially an exponent s(p)=α−1, as is predicted from self-similar scaling for a spectral index α. Yet there appears substantial departure from simple self-similarity, and also from homogeneous intermittency (uniform fractality) at higher-order p's for all types of flows analysed, which is clearly evident in the normalized dimensionless structure functions

AMPTE/CCE CHEM observations of the energetic ion population at geosynchronous altitudes

Abstract: The paper presents results of a statistical study of average characteristics of the energetic ion population at geosynchronous altitudes, using energetic-ion (1-300 keV/e) measurements from the CHEM spectrometer aboard the AMPTE Charge Composition Explorer between January 1985 and June 1987. Data were sorted into four MLT groups and two extreme geomagnetic activity levels ('very quiet' for AE less than 30 nT and 'very active' for AE greater than 700 nT). A clear quiet-time dayside feature found in the measurements was a dip in H(+) and He(2+) spectra, at 6.6 keV/e in the prenoon sector and at 3.5 keV/e in the postnoon sector. During active times, the ion fluxes increased (except for He(+)), and the O(+) contribution to the total energy density increased dramatically. The pitch angle distributions were normal during quiet times and isotropic or field-aligned during active times.

A modelling study of the equatorial topside ionosphere

Abstract: A fully time-dependent mathematical model of the Earth's ionosphere and plasmasphere is presented. In the model the ions are O + , H + , He + , N 2 + , O 2 + and NO + , and the Earth's magnetic field is represented by an eccentric dipole. Using meridional and zonal neutral air wind velocities from the HWM90 thermospheric wind model it is found that the modelled electron concentration values are in reasonable accord with observations made during the daytime at Jicamarca, whilst the modelled values at night are much too large. Close agreement in the modelled and observed electron concentration values is obtained during night-time when the neutral air wind velocities are taken to be zero during the period 20:00-04:00 LT. It is concluded that there are significant differences in the night-time neutral air wind velocities given by the HWM90 thermospheric wind model and those occurring at equatorial latitudes in the South American longitude zone

Interpretation of HF radar ionospheric Doppler spectra by collective wave scattering theory

Abstract: According to the theory of collective wave scattering by a non-uniform plasma, the fluid motion spectrum can be either the Gaussian Doppler spectrum due to velocity dispersion, or a Lorentzian spectrum when the scattering wavelength becomes longer than the correlation length of velocity fluctuations. These two types of Doppler spectra are present in data from the SHERPA HF coherent radar, which detects 8 to 20-m wavelength density fluctuations in the high-latitude ionosphere. Most autocorrelation functions are best fitted to an exponential shape, corresponding to a Lorentzian Doppler spectrum. Nevertheless, the fit is sometimes better for a Gaussian shape. Each of the two spectral shapes is related to values of the local ionospheric parameters. We present preliminary measurements of the correlation time and correlation length of the waves, and concluding remarks on the significance of spectral widths for geophysical studies

The Atmospheric Momentum Budget over a Major Mountain Range: First Results of the PYREX Field Program

Abstract: The PYREX program is a major field study of the dynamical influence of the Pyrenees mountains (on the border between France and Spain) on the atmospheric circulation. In October and November 1990 a large number of experimental means had been deployed in the area, including additional sounding systems, automated stations at high elevation sites, wind profilers, sodars, constant level balloons, and four research airplanes. The main focus was on the quantification of the retardation of the cross-mountain flow by the range, but a number of related mesoscale phenomena had been captured, e.g. the formation of lee waves, lee eddies, sheltering areas, and local surface winds. The data interpretation was supported by an extensive effort in meso-scale numerical modelling, and it is argued that only numerical models, conveniently qualified by observations, can document the momentum budget in a consistent way. Improvements in the field of mountain waves and mountain roughness representation in the large-scale atmospheric models are expected from the PYREX results, as well as improvements in the performance of mesoscale models for the operational forecasts. The present paper presents a broad introduction to the experiment. It reviews the experimental set-up and the available data sets. Finally, two more specific topics are discussed in depth, as examples of the available data and modelling strategy: the lee wave event of 15 October 1990 is presented with both experimental and model results. Some of the findings concerning the Tramontana wind are also discussed

Experimental ionospheric tomography with ionosonde input and EISCAT verification

Abstract: Results are presented of tomographic images of ionospheric electron density obtained from experimental measurements of total electron content at a linear chain of stations. Comparison with independent measurements made using a special scan or the EISCAT incoherent scatter radar show broad agreement except under extreme ionospheric conditions. The large-scale horizontal features of the electron density are replicated well in the tomographic image. The vertical profile is dependent on the ionospheric model used to create the background ionosphere formed as an initial condition in the reconstruction process. The inclusion of additional independent information in the form of the electron density and height of the layer peak obtained from ionosondes, has been incorporated into the imaging procedure. The resultant tomographic images show improved agreement with the EISCAT measurements in the vicinity of the layer maximum

Some recent studies of probable connections between solar and atmospheric variability

Abstract: The paper briefly reviews some recent investigations of possible low-frequency effects of solar variations on the earth's surface temperature and on the occurrence of El Ninos along the coast of Peru. It draws attention to the stable, strong correlations between the height of constant pressure levels in the stratosphere and the sunsport cycles, and to a theory that attributes the observed decadal oscillation in the lower stratosphere to interaction between UV radiation and ozone in the upper stratosphere, with the dynamic effect of this radiative interaction reaching into the lower stratosphere and upper troposphere. We not, too, that the satellite measurements, which began in 1979, show a change in total solar irradiance that is in-phase with the sunspot cycle; and also a change in the earth's surface temperature, determined through the outgoing long-wave radiation, which parallels the sunspot cycle. The temperature change was at least five times larger thant predicted by theory for the size of the observed irradiance change

Estimating observation error statistics for atmospheric data assimilation

Abstract: All advanced data assimilation systems require reasonable estimates of the observation error statistics for each observation system used in the assimilation. Without such estimates, no assimilation system can extract all the available information from the observations. Observation error has two components - instrument error and the error of representativeness. This latter error can be associated with the error of the forward interpolation operator. This connection is examined in a simple model context and the spatial and temporal characteristics (and correlation with the signal) of the forward interpolation error are explored. Observation error statistics are determined for radiosonde networks using conventional innovation covariance techniques, which are applicable because it can be assumed that radiosonde observation errors are neither horizontally correlated nor correlated with forecast errors. For satellite observation systems, these assumptions are not always appropriate, and conventional innovation techniques cannot be used. A generalized innovation crosscovariance method for determining observation error statistics for satellite systems is proposed. In principle, this technique can determine spatial correlations and correlations between forecast and observation error. It is successfully tested in a simple model context

A comparative study of the time-dependent standard 8-, 13- and 16-moment transport formulations of the polar wind

Abstract: The ionosphere, composed of O(+), H(+), and electrons is modeled with four different transport formulations. The equations corresponding to the standard set, the 8-, 13-, and 16-moment approximations are presented, and the collision terms are expressed. Using a time-dependent technique, the ionosphere is studied between altitudes of 200 and 8600 km. The production of electrons and O(+) ions is described by a neutral atmosphere simplified photoionization scheme, while the energy deposition is supported by a downward electron heat flow of -0.005 erg/sq cm per s imposed at the topside boundary. When the models reach a steady state equilibrium, the electron solutions show differences due to the introduction of temperature anisotropies and heat flows between the components parallel and perpendicular to the magnetic field. As a corollary, the ions show structures depending on the level of approximation. A depletion of a factor of 10 is then applied to the ion densities above a certain altitude, and the development of the perturbation is followed for 1000 s for all the models.

Cooling of the Upper Atmosphere due to CO2 Increases: A Model Study

Abstract: A radiation module has been developed for a global 3-d mechanistic model of the middle atmosphere (0-150 km). This paper describes the algorithm in detail, especially the non-local thermal equilibrium (non-LTE) approach in the terrestrial spectral region (15-μm-band) of CO 2 , which is necessary for a correct calculation of the cooling rates above a height of 80 km. Initial experiments using the model are presented and discussed in this work. Most attention has been given to an experiment which tries to assess the dynamical effect of doubling the amount of CO 2 , not only in the stratosphere but also in the mesosphere and lower thermosphere. The results show that an increase of CO 2 procedures a general cooling of the whole middle atmosphere with the strongest values in the lower thermosphere (∼110 km), where a cooling trend of 2 K per decade is produced. Since there are no long time series of measurements in the latter height region, and due to the large natural variability of the temperature at heights above 80 km, this model can help to examine and to estimate possible trends caused by anthropogenic changes of greenhouse gases

The response of the large scale ionospheric convection pattern to changes in the IMF and substorms - Results from the SUNDIAL 1987 campaign

Abstract: This paper reports multi-point observations of ionospheric convection made during the SUNDIAL 1987 campaign. Two specific intervals of varying interplanetary magnetic field, which also include several substorms, have been identified for detailed study. The two intervals differed considerably in both the input of energy from the solar wind to the magnetosphere prior to the substorm expansion phase onset and the response during the expansion phase. One substorm can be represented by the classical picture of growth phase, expansion phase and recovery phase. The other, which occurred during an interval of weak energy input from the solar wind to the magnetosphere, was more complex, in particular during the expansion phase. Observations of the earth's magnetic field in the midnight sector provide the timing of the expansion phase onset of substorms which allow the separation of ionospheric effects due to changes in IMF and substorms. The primary ionospheric effect of a change in the IMF from positive B z to negative B z is an enhancement in the plasma flow magnitude. The response time of the ionospheric convection to each southward turning varies from 15 minutes near 1800 MLT to 30 minutes near 2100 MLT and closer to an hour near midnight. In both instances, a growth phase can be identified as an increase in ionospheric convection following a southward turning of the IMF. The overall length of each growth phase was similar, about 90 minutes, despite the different prevailing solar wind and IMF conditions. Furthermore this time is somewhat longer than previous observations. During one of the substorms, which consisted of several intensifications, there is evidence that the nightside flow reversal moved progressively to earlier local times in response to each substorm intensification

Tomographic imaging of the ionospheric mid-latitude trough

Abstract: The paper describes an experimental campaign carried out in the United Kingdom in December 1990 during which simultaneous observations were made, using transmissions from the Navy Navigational Satellite System (NNSS) satellites, at four stations covering a latitudinal range of some 8°. The resulting measurements of total electron content have been used in a reconstruction algorithm to image the electron density on a two-dimensional grid for each of the satellite passes monitored. Results are presented showing the development of the mid-latitude trough throughout one night. The potential usefulness of tomographic techniques in ionospheric sensing is discussed in the light of the results obtained

Study on thermal internal boundary layer structure over Thumba, India

Abstract: The paper describes the observed characteristics and structure of the thermal internal boundary layer (TIBL) at a coastal station, Thumba, during onshore flow using data obtained from monostatic Doppler SODAR, tower-based instruments, radiosonde and pilot balloon ascents. The study shows that during a well-developed TIBL the surface layer stability parameter −z/L is in the range −0.2 to −0.05, indicating dominance of mechanical turbulence. The depth of the TIBL at Thumba, obtained from temperature profiles matches very well with that deduced from the SODAR C T 2 height profiles and from horizontal wind spectra. The TIBL height computed using some of the empirical relations suggested in the literature are less than that obtained from the temperature profiles suggesting the need for modification of empirical relations. Comparison of the turbulent parameters in the TIBL with those of the convective boundary layer (CBL), σ w /w * versus z/z i and C T 2 z i 2/3 /θ * 2 versus z/z i shows reasonable agreement only up to 0.3 to 0.4 z/z i , beyond which the parameters have lesser values compared to that for the CBL and this aspect is mainly attributed to the observed wind structure

A new approach to calculate the excitation of atmospheric gases by auroral electron impact

Abstract: The altitude profiles of the ratio of auroral electron energy spent on the excitation of the concrete atmospheric gas (N 2 , O 2 , O) to the production rate of the concrete excited state of this gas have been calculated by the Monte-Carlo simulation of electron transport in the Earth's atmosphere. We call this ratio the excitation energy cost. Calculation results show that excitation energy costs are independent of the initial energy as well as of the penetration depth of the electron flux. These properties of the energetic costs provide the basis for a new method of calculating the production rate of ionospheric species excited by auroral electrons. This method allows the solution of the electron transport equation to be avoided and may be used for real time modeling of various auroral phenomena. A comparison is made between the results obtained by the new method and by the more accurate Monte-Carlo method

Statistical studies of Pc 3-5 pulsations and their relevance for possible source mechanisms of ULF waves

Abstract: A number of statistical studies using spacecraft data have been made of ULF waves in the magnetosphere. These studies provide an overview of ULF pulsation activity for r = 5-15 R(E) and allow an assessment of likely source mechanisms. In this review pulsations are categorized into five general types: compressional Pc 5, poloidal Pc 4, toroidal harmonics, toroidal Pc 5 (fundamental mode), and incoherent noise. The occurrence distributions and/or distributions of wave power of the different types suggest that compressional Pc 5 and poloidal Pc 4 derive their energy locally, most likely from energetic protons. The toroidal pulsations, both harmonic and fundamental mode, appear to be driven by an energy source outside the magnetopause - directly upstream in the sheath and solar wind for harmonics and the flanks for fundamentals. Incoherent pulsations are a prominent pulsation type but from their occurrence distribution alone it is unclear what their dominant energy source may be.

Influence of the dipolar magnetic field on transport of proton-H atom fluxes in the atmosphere

Abstract: Transport of proton-H atom fluxes in a nitrogen atmosphere has been simulated by the Monte-Carlo method and collision-by-collision algorithm The model, which includes a dipolar magnetic field, was compared to one without a field The influence of the dipole magnetic field on the energy deposition function, albedo, radial beam spreading and charge-state fractions was investigated for beams with an isotropic initial angle distribution and for monodirectional beams; the initial proton energy was in the range 1-100 keV Two mechanisms of field influence were considered: (i) the adiabatic relation for protons, and (ii) the convergence of the magnetic field lines for H-atoms It was found that both mechanisms are important in calculating the albedo and the charge fractions in the albedo fluxes It is shown that the radius of the p-H beam depends strongly on the altitude of the maximum energy deposition

Observations of electron density irregularities in the plasmasphere using the VLA radio interferometer.

Abstract: Transverse irregularities in the line-of-sight total electron content have been studied with the Very Large Array radio-interferometer. Unresolved cosmic radio sources are used to back-illuminate the geoplasma. The baseline-differenced electrical phase time series are Fourier analyzed and then used to fit the constants (amplitude and trace wavevector) for a plane-wave model at each frequency. At frequencies > 0.003 Hz, i.e. above the spectral region dominated by atmospheric gravity waves, the VLA database reveals a distinct class of irregularities with apparent propagation azimuths within ±15° of magnetic east and with trace propagation speeds in the range 0.1-1.5 km s −1 . It is shown that these magnetic eastward directed (MED) irregularities cannot be explained as any perturbation generated by compressional waves at ionospheric heights. An alternative model is proposed, wherein they are geomagnetically-aligned irregularities essentially frozen in the plasmasphere, corotating with the Earth. The relative motion of the radio lines-of-sight past the corotating irregularities produces exactly the class of high-frequency disturbances seen at > 0.003 Hz. The frequency, and the trace speed, are artifacts of the relative motion of the line-of-sight. The trace speed can be used to infer where, along the line-of-sight, the irregularity occurs. This tool allows statistics of the location, shape, and incidence of the irregularities to be studied in some detail

Water vapor feedback and the ice age snowline record

Abstract: Average tropical sea surface temperatures during the last major glaciation may have been no more than about 1 C cooler than at present, while snow lines on high peaks may have descended 1 km, suggesting a drop in temperature of about 5 C in the mid-troposphere. This feature is likely to be characteristic, since the tropical atmosphere cannot sustain large horizontal temperature gradients. This implies that during the last glaciation the lapse rate in the lower half of the tropical troposphere was about 20 percent greater than at present. A reduction in the radiative cooling rate at these levels is required for such a reduction in static stability. A radiative-convective model with a physical parameterization for tropical convection is used to determine that only a significant increase in relative humidity in the middle and upper troposphere can cause the necessary increase in the lapse rate in the lower troposphere.

Wave and stability properties of multi-ion plasmas with application to winds and flows

Abstract: In many space plasma phenomena heavy ions are present in abundances which are not neglibible relative to protons so that the various components of the plasma are mutually coupled through the Lorentz force and the quasi-charge neutrality constraint. Some of the novel and interesting effects which this coupling gives rise to are discussed here within the context of wave and stability properties of the system. In particular we show how differential streaming between the protons and a havier species (such as alpha particles in the solar wind) can give rise to low frequency compressional instabilities. The effects of Coulomb collisions, finite Debye length and kinetic effects are also discussed and it is emphasized that collisionless Landau damping can quench such instabilities except when the ions are sufficiently cold relative to the electrons. In connection with steady flows (e.g. solar and polar winds) it is shown how the idea of critical points must be generalized and interpreted within the framework of the stationary waves of the system including differential streaming, and how these points are intimately connected with nonlinear mutual regulation of the ion fluxes

Correlations between the fluctuations of pressure, density, temperature and magnetic field in the solar wind

Abstract: Compressive fluctuations and the pressure balance in the solar wind are investigated with Helios data obtained in 1975-1976 from measurements in the inner heliosphere between 03 and 10 AU Broad-band (6×10 −3 Hz-6×10 −6 Hz) correlation spectra and hourly correlation coefficients for any pair of solar wind parameters like speed, density, temperature, magnetic field magnitude, thermal and magnetic pressure, and the total pressure of the plasma are calcultated and analysed We find that the nature and intensity of the compressive fluctuations strongly depend on the flow speed and systematically vary with the stream structure

The unified scaling model of atmospheric dynamics and systematic analysis of scale invariance in cloud radiances : Nonlinear processes in geophysics

Abstract: The unified scaling model of the atmosphere links the large ans small scale dynamics by a single scaling but anisotropic regime, rather than distinct isotropic two- and three-dimensional turbulent regimes as posited in the standard model. We argue that the study of mesoscale clouds is a particularly stringent test for the standard model and we present (perhaps the first) systematic analysis of the scaling of energy spectra of satellite radiances over five wavelength channels and spanning the range of scales from 160 m to 4000 km (the entire mesoscale). The study mostly involved 15 consecutive scenes of AVHRR data (1.1 km resolution, 512×512 pixels) taken over the same location at the same local time in February 1986

The solar cycle variability of lunar and solar daily geomagnetic variations

Abstract: Hourly mean values of the geomagnetic field from observatories between 6° dip latitude and 60° geomagnetic latitude are analysed for solar and lunar daily variations. Three sets of data are used from 18 observatories over 20 years (1958-77), from 5 observatories over 75 years (1910-1984) and from about 100 observatories over 4.5 years. The the harmonics of the horizontal components of S and L for each year are expanded into series of spherical harmonics. External and internal parts are separated by use of a spherically symmetric model of the earth's electric conductivity. A linear regression e n m (R)=A n m (1+M n m R) is evaluated to express the dependence of the external potential coefficients e n m of S and L on the sunspot number R. By taking A n m and the Wolf ratio M n m as complex numbers, the phase shifts between S, L and the sunspot number is considered. The R-dependence of S is found to be about 2.5 times that of L, and as expected, the amplitudes |e n m | increase with R for both S and L. When the sun is active, the foci of the equivalent currents are about 1/2 to 1 hour later in local time for S, but earlier for L, compared to their positions during quiet years. This is a consistent result for both data sets. Theoretical investigations show that the R-dependence or the ionospheric conductivity leads to a phase shift as observed for the lunar variations. An explanation for the different behaviour of S and L is still lacking

Comparison of different error growth formulas and predictability estimation in numerical extended-range forecasts

Abstract: Different analytical formulas of temporal evolution are compared for the deviation between twin integrations carried out with the same general circulation model, starting from neighbouring initial conditions. We propose a generalization of the error growth formulas of Lorenz (1982) and Dalcher and Kalnay (1987), and study a limit case corresponding to the Gompertz growth law. The use of the generalized formula allows us to show that the growth rate of small errors and the saturation rate are different characteristics. We compare the error growth parameters given by the different formulas in the spectral domain. The stability of the results is tested by examining the influence of different lengths of the series and of different weightings in the least squares fit. We provide an upper bound for dynamical predictability and propose a measure for the characteristic time interval during which most of the error growth occurs

An implicit time-stepping scheme for chemical species in a global atmospheric circulation model

Abstract: An implicit time-stepping scheme is proposed for the incorporation of a complex chemistry in a three-dimensional general circulation model (3D GCM). The scheme has been tested in a single computational box and in the UGAMP (United Kingdom Global Atmospheric Modelling Programme) 3D GCM. The implicit scheme, which is unconditionally stable, has been shown to provide a computationally more efficient solution than the previously employed explicit scheme, while maintaining an acceptable level of accuracy. The saving achieved is determined by the length of the time step and tests have been carried out with both constant and variable time steps for the chemistry. An implicit solution with constant time steps, that takes 15% of the computing time of an explicit solution with shorter steps, has good accuracy for all chemical species except those that change rapidly on short time scales. With variable chemical time steps, an implicit solution, with a 50% saving in computing time over an explicit solution with shorter variable steps, is as accurate as the explicit scheme for the longer-lived species but is more accurate for the fastest changing species

Gravity wave parameters measured with EISCAT and Dynasonde

Abstract: Gravity wave activity in the F-region over Tromso has been studied in detail using measurements from the EISCAT incoherent scatter radar, and the NOAA digital ionospheric sounder (Dynasonde). It has been shown that the field-aligned EISCAT measurement of electron density contains variations which indicate the presence of upward propagating gravity waves with downward progressing wavefronts. The spectral components of these waves have a harmonic relationship, which could suggest a non-linear generation mechanism. The different frequency components have been separated using digital filtering, and the observed frequencies and field-aligned wavelengths, together with the background neutral wind (which has been determined from the average field-aligned ion velocity using an appropriate diffusion correction), have been incorporated into the Hines dispersion equation to obtain the horizontal and vertical wavelengths. The periods and horizontal wavelengths are compared with those measured by the Dynasonde and found to be in good agreement

Observations of the F-region horizontal and vertical winds in the auroral zone

Abstract: The MICADO instrument consisting of a Michelson interferometer has been designed to observe winds and temperature in the mesosphere and thermosphere. Three winter campaigns were organized at Sodankyla (Finland) and Tromso (Norway) from 1988 to 1991. This paper is mainly dedicated to wind observations in the thermosphere by use of the O( 1 D) oxygen emission line. As a function of magnetic activity and universal time, a mean model of F-region meridional, zonal and vertical neutral wind components is presented, discussed and compared with recent empirical models. Magnetic activity effects are shown on lines intensily, temperature and wind, particularly the numerical relationship of its vertical component with the amplitude of the magnetic perturbation