Neutral Air Motions Research Articles

Simultaneous and colocated observation of winds and tides by MF and meteor radars over London, Canada (43°N, 81°W), during 1994–1996

Simultaneous and colocated comparisons of winds and tides by MF and meteor VHF radars have been made in the 85–94 km height range from July 1994 to June 1996 over London, Ontario, Canada. Results have been obtained for every month of the year. From these comparisons it is concluded that, in general, the MF spaced antenna technique and the meteor method both provide reliable means for synoptic studies of neutral air motions in the height range 85–94 km, at timescales of greater than 12 hours, and therefore are valuable tools in middle atmospheric research. However, we do find that some parameters seem to be estimated with greater precision than others. In particular, the measurements of the zonal long‐term wind variations, and the semidiurnal tide in both the zonal and meridional directions, seem particularly robust and reliable, while there is somewhat less consistency between measurements of the zonal diurnal tide and the meridional monthly mean winds. The former problem is very likely to be an artifact of the strong diurnal variation in meteor count rates. However, we cannot claim that our meridional monthly mean wind agreement is always good, and these discrepancies deserve further investigation.

Origin of flux ropes in Venus' ionosphere

The joule dissipation inside flux ropes in Venus' ionosphere is so great that they must be formed near, and maintained at, the place where they are observed. Thus ropes are not formed by a Kelvin‐Helmholtz instability of the ionopause. The hypothesis that ropes may be formed by the dynamo action of internal gravity waves in Venus' thermosphere (Luhmann and Elphic, 1985; Cole, 1993) is strengthened by discussion of a magnetic evolution equation which includes neutral air motion. However, the dynamo process would work only at altitudes at which νin ≥ ωi. At altitudes or parts of a rope where νin ≪ ωi, the process does not work. A solar wind dynamo is therefore examined to account for the ropes. Thereby a major new heat source for ions of the Venus ionosphere associated with the ropes is uncovered.

Gravity wave generation of simultaneous auroral sporadic- E layers and sudden neutral sodium layers

Measurements from the EISCAT incoherent scatter radar are used to show that the action of a gravity wave, generated by auroral Joule heating, could be the mechanism responsible for the simultaneous occurrence of sporadic- E layers and sudden neutral sodium layers. The radar measurements were obtained during an event previously reported on the basis of ionosonde and lidar observations ( Von Zahn and Hansen, 1988, J. atmos. terr. Phys. 50, 93). The EISCAT observations indicate that downward neutral air motions, probably associated with gravity waves, correlate with the appearance of sporadic- E layers. Computer simulations are used to demonstrate that the observed vertical winds can produce thin layers in a pre-existing broad layer of minor constituents, such as metallic atoms or ions, resulting in sudden or sporadic- E layers, respectively.

Studies of high latitude mesospheric turbulence by radar and rocket 2: measurements of small scale turbulence

Measurements of mesospheric small scale turbulence and associated larger scale wave structures were obtained during equinox by rocket probes flown in coordination with the nearby Poker Flat MST Radar. One rocket series (29 March 1985) included a parachute-borne, electrostatic probe for measuring relative fluctuations in positive ion density, while the other salvo (1 April 1985) included a fixed bias nose tip probe to measure electron current and associated small scale density irregularities. The measured small scale plasma density variations are indicative of neutral air motions. A − 5 3 value for the spectral index, characteristic of turbulence in the inertial subrange, was generally observed for plasma density fluctuations in the region of strong MST radar returns. A comparison of the variance calculated from the power density spectra of the nose tip probe's current with the MST radar's S/N ratio demonstrated consistent measurements of electron density fluctuations. Also, both measurements indicated the presence of saturating waves. The occurrence of 1–3 km wavelike perturbations, superposed on a larger wave (~7 km) in the wind velocity field, was identified by the MST radar. Consistency between MST radar, meteorological and probe measurements of the wave structures is observed in the region where turbulence was detected. The 1–3 km waves are believed to be more important in the transport of energy and momentum and also in the production of turbulence in the lower mesosphere.

Gradient-drift instability of nighttime mid-latitude Es-layers

We have previously reported observations (Tanaka and Venkateswaran, 1982a,b) which suggest that nighttime E-region irregularities in mid-latitudes are often associated with E s -layers and might arise as a consequence of the cross-field or gradient-drift instability below the layer peak. In this paper we investigate how the ambient ionospheric electric field, the vertical shear (which is assumed to be linear) of the east-west component and the vertical velocity (which is assumed to be constant) of the wind or neutral air motion control the location of the instability, and the apparent drift velocity of the resulting irregularities measurable by ground-based radar.

A numerical study of the lunar tide in the mid-latitude F2-region of the ionosphere—II. Oscillations of the ion and neutral gas velocities

The drift due to the lunar electric field which is communicated from the ionospheric E-region causes a re-distribution of the electron density and a neutral air motion in the F2-region. The lunar tide in the electron density of the F2-region due to the lunar electric field has been obtained and discussed in Part I. The lunar tides in the ion and neutral velocities which are obtained in the same way are dealt with in this paper. It is found that the lunar tidal oscillations in the drifts and neutral winds markedly depend on the solar time, and that the amplitude and phase in the x-component of the drift averaged over the solar times are 10.1 m s −1 and 2.6 h and those in the x-component of the neutral wind are 4.6 m s −1 and 4.3 h, respectively, at 45.0° dip.

Thermospheric winds and exospheric temperatures from incoherent scatter radar measurements in four seasons

The time dependent equations of neutral air motion are solved subject to three constraints: two of them are the usual upper and lower boundary conditions and the third is the value of the wind induced ion drift at any given height. Using incoherent radar data, this procedure leads to a fast, direct numerical integration of the two coupled differential equations describing the horizontal wind components and yields time dependent wind profiles and meridional exospheric neutral temperature gradients. The diurnal behavior of the neutral wind system and of the exospheric temperature is presented for two solstice and two equinox days. The data used were obtained by the St. Santin and the Millstone Hill incoherent scatter radars. The derived geographic distributions of the exospheric temperatures are compared with those predicted by the OGO-6 empirical thermospheric model.

Concluding remarks, session 4: Atmospheric dynamics

Meteorologists used to criticize those who described themselves as aeronomers or ionospheric physicists, pointing out their neglect of neutral air motions in calculating ionospheric behavior. This situation has changed dramatically in the last few years. At this meeting, the effects of neutral air motions were considered either implicitly or explicitly in over one third of the papers presented. Some of the topics that explicitly touched on neutral atmosphere dynamics are described in the following paragraphs.

Neutral air motions deduced from barium releases experiments—I. Vertical winds

Neutral air motions have been deduced from the triangulation of the neutral component of barium point cloud releases carried out in the altitude range between 140 and 260 km in low and high geomagnetic latitudes. Special attention had been given to vertical motions. The accuracy of the measurements was typically around ±5 m sec −1. 48 clouds were triangulated. The motion of 17 clouds indicated that neutral vertical winds were present upward or downward. The order of magnitude of these winds is 15 m sec −1, peak values are around 40 m sec −1. During morning twilight we find much less cases of vertical winds than during evening twilight.

An analysis of the upper atmospheric wind observed by LOGACS

Wind velocities at 140–200 km altitude were observed by a Low-G Accelerometer Calibration System (LOGACS) flown on an Agena satellite during a geomagnetic storm. An interesting wind reversal observed by the satellite at auroral latitudes is satisfactorily explained by the neutral air motion caused by the E × B drift deduced from the ground-based geomagnetic data recorded at stations near the meridian of the satellite orbit.

The diffusive motion of an initially spherically symmetric cloud of ionization in the Earth's upper atmosphere

The diffusive motion of an initially spherically symmetric ion cloud in the presence of a magnetic field (the geomagnetic field) is investigated. The electric field is assumed to be simply the space-charge electric field and possible effects of neutral air motion are ignored. Data appropriate to an altitude of 97.5 km are employed and the computed results are compared with an exact (similarity-type) solution.

Observation of F Region Vertical Velocities at Millstone Hill, 1, Evidence for Drifts Due to Expansion, Contraction, and Winds

The theory for the drift of ions in the F region is briefly reviewed, and it is shown that, for the restricted case of no external electric fields or neutral air motion, the velocity of the ions along the magnetic field lines in the upper part of the F2 region can be derived from the continuity equation. Experimental results for the electron density, electron and ion temperatures, and O+ vertical velocity obtained by the incoherent (Thomson) scatter technique at Millstone Hill are presented for two 24‐hour periods taken a year apart in March 1969 and 1970. The largest vertical velocities encountered occur around sunrise and sunset and give rise to vertical fluxes of the order of 5 × 108 ions/cm2/sec through 600‐km altitude. It is shown that these fluxes are a result primarily of the growth or decay of the layer, though in the case of sunset the thermal contraction resulting from the decrease in the electron temperature with time assists in establishing the observed fluxes. The vertical velocity observed near 300 km is compared with the expected ambipolar diffusion velocity that should result from the observed pressure gradients in the plasma and from acceleration due to gravity. The difference between the observed and computed velocities is attributed to thermospheric neutral winds which have diurnal behavior similar to those determined by Vasseur (1969a, b) from incoherent scatter observations in France.

Distributions of Nighttime F‐Region Molecular Ion Concentrations and 6300‐Å Nightglow Morphology

A numerical technique is employed in solving the coupled, nonlinear system of equations for the O+, NO+, and O2+ number densities in the nighttime F region, including the effects of diffusion, E×B drift, and neutral air motions. The best fits to the ‘sunset’ [Johnson, 1967] and ‘midnight’ [Holmes et al., 1965] rocket observations of the ion profiles obtained at White Sands, New Mexico, are achieved by iteratively varying the neutral wind, the electrodynamic drift, the neutral atomic nitrogen and nitric oxide concentrations, and the ion‐atom interchange and dissociative recombination rate coefficients. The best fit results for both sets of profiles with a Jacchia model atmosphere are achieved with the rate coefficients (cm3 sec−1) at 300°K given by γN2 = 1.1×10−12, γO2 = 2.0×10−11, αNO+ = 3.5×10−7, and αO2+ = 3.1×10−7, each with an assumed inverse temperature dependence. In addition, the ‘sunset’ best fit solutions require that the chemical time constant τ for reaction of O2+ with N and NO be 500 seconds at 200 km and that the net vertical plasma transport velocity wz due to E×B drift and a neutral wind be downward with a speed of 14 m sec−1. The midnight best fit solutions are achieved with τ = 2.5×103 at 200 km and wz = 10 m sec−1 upward. The 6300‐Å nightglow morphology in the inter‐tropical region is computed directly from the ion concentrations calculated for assumed drift and neutral wind models representative of equinoctial, sunspot‐minimum conditions. Isophote contours of zenith intensity are presented to illustrate the nightglow morphology associated with the decay of the Appleton anomaly. Calculated results are compared with the observations of Barbier [1964] to demonstrate the influence of electrodynamic drift on the morphology of nightglow enhancements. The results show that the electrodynamic drifts required to produce the observed enhancements are consistent with the drifts observed at Jicamarca.

The interpretation of ionospheric radio drift measurements—II Kinesonde observations of microstructure and vertical motion in sporadic E

Dynamic processes in the E-region, resulting mainly in neutral air motions, appear as the direct causes of certain phenomena (e.g. sporadic E) and are also responsible for much of the less obvious variability of the region. Since it is the time variation of the vertical structure which is usually observed, and since the vertical ionization gradients are large, vertical motions are of greatest interest. We show here that the vertical component of motion of the neutral air, although of relatively small magnitude, cannot be neglected in estimating vertical ion motions. It is therefore important that the total motion of the air, and of the ionization, be subject to measurement. We describe a new “kinesonde” radio sounding system with which three-dimensional structure and motions in the ionsphere may be observed at the ground. The kinesonde is a much-elaborated version of the well-known spaced-antenna drift experiment. It obtains digital magnetic tape recordings of the amplitude and RF phase variations of selected echoes on six freely chosen frequencies and on four spaced antennas, simultaneously. The observations are analyzed by a statistically rigorous method in which many of the cross-correlations between pairs of these 48 channels are used to determine a four-dimensional (x, y, z, t) “correloid” in a moving (Vx, Vy, Vz) coordinate system. The velocity V is identified with motions at the ionospheric reflection level and other information describes the random motion, spatial scale, and anisotropic properties of the measurements. In the present paper, we assume that the calculated velocities approximate those of the neutral wind, and infer therefrom the vertical ion motions which these air motions would induce. Measurements performed on sporadic E layers give vertical ion velocities ranging from 1–15 m/s, often upward. In the vicinity of a daytime Es layer, the upward velocity is found to decrease with altitude implying a convergence of ionization toward the sporadic E-layer. Kinesonde measurements are also compared with the time and height variations of several sporadic E-layer sequences obtained by N(h) analysis of ionograms, and are found to account for much of the variation observed.

The upper atmosphere as a multiply refractive medium for neutral air motions

Under the influence of gravity and heat conduction four plane characteristic waves obliquely incident on a horizontally stratified atmosphere can propagate, two of them upward and the two other downward. The two pairs of characteristic waves are the well known acoustic gravity waves and the heat conduction waves. Molecular viscosity generates two further pairs of characteristic waves, the ordinary and the extraordinary viscosity waves. Ion drag and Coriolis force make the atmosphere anisotropic with respect to the characteristic waves. Their propagation characteristics for east to west and north to south propagation differ from each other. Some analytical solutions of the eigenvalues of these eight characteristic waves are given in this paper. Numerical calculations of the eigenvalues depending on the parameters of the thermosphere, on frequency, azimuth and on angle of incidence are presented and discussed in some detail.

Movements of ionospheric irregularities and atmospheric winds

The various phenomena which are associated with motions or apparent motions in the ionosphere are outlined, and the ways in which the atmosphere may be divided into regions are briefly examined, particularly in relation to the interaction between the neutral atmosphere and the ionisation. The sources of movements in these regions are discussed, and the irregularities in the ionosphere and their origin are also examined. The various methods for measuring movements or apparent movements in the ionosphere are summarized. The experimental results so far available are sketched out with emphasis placed upon establishing what reliable evidence there is for real mass movement of both plasma and neutral atmosphere throughout the ionosphere. In the lower ionosphere there is a considerable amount of direct experimental data on neutral wind motions, and radio techniques appear to give values in agreement with these measurements. The significance of the radio data for the F-region is examined in some detail with respect to the little information which is available on neutral air motion in this region. Strong evidence is presented that the major cause of the movement of small irregularities in this region is electrodynamic.

An idealized problem with F2-layer applications

An idealized problem is solved which illustrates the coupling between neutral air motion and the transport of plasma in a model F2-layer. The magnetic field lines are assumed to be horizontal and the effects of both vertically upwards and vertically downwards Hall drifts are considered. Very slow circulating motion of the neutral air is assumed to take place in a vertical plane parallel to the magnetic field lines. This motion is assumed to be coupled with the diffusive and Hall motions of the plasma. The problem first is studied in detail, and exactly, by assuming the electron density N to be a given function of position. The streamlines of the air motion are drawn and also a parameter b which is the ratio of the horizontal component of neutral air velocity to the horizontal velocity of the ions. The parameter b is found to lie within the range in which ion-drag might be expected to interfere seriously with the usual form of the F2-layer diffusion equation. It is shown secondly that if only the production and loss functions for electrons are known a non-linear diffusion equation is produced. It is suggested that in future theoretical. F2-layer calculations it may be necessary to construct a more complicated diffusion equation of this type, allowing for curvature of the field lines. The results indicate that for calculations of the magnitude of neutral air winds affected by ion-drag it may be a good approximation to assume that the horizontal ion velocity is the same as the horizontal neutral air velocity.

The influence of horizontal air motions on F2-region ionization

The effect of large scale neutral air motions in the F2-layer is studied theoretically for special models by numerical methods (with production and loss of electrons) and by approximate methods (when production and loss are absent). The results show that generally neutral air transport towards the equator enchances the electron density at the peak, in agreement with theoretical results obtained at moderate latitudes. The work is also of interest in connexion with the theories of ion drag and of electrodynamic lifting of the F2-layer.