A review of theoretical and observational results describing atmospheric gravity wave (AGW)/traveling ionospheric disturbance (TID) phenomena at high latitudes is presented. Some recent experimental studies of AGW's using the Chatanika incoherent scatter radar and other geophysical sensors are reported. Specifically, the following features are described in detail: (1) cause/effect relations between aurorally generated AGW's and TID's detected at mid-latitudes, including probable ‘source signature’ identification, (2) AGW source phenomenology, particularly a semiquantitative assessment of the relative importance of Joule heating, Lorentz forces, intense particle precipitation, and other mechanisms in generating AGW's, and (3) detection of TID's in the auroral ionosphere. Several instances of F region electron density, temperature, and plasma periodicities accompanied by horizontal plasma velocities which were consistent with theoretical AGW/TID models are documented.

Atmospheric gravity waves generated in the high‐latitude ionosphere: A review

The most intense, F region irregularities in the high-latitude ionosphere appear to be produced by convective plasma processes and in particular, by the fluid (gradient drift) interchange instability. Such irregularities are produced by convectively mixing plasma across a mean plasma density gradient with the transport of higher-density plasma into regions of lower-density plasma (and vice versa) leading to the development of an irregularity spectrum that extends in scale from about 10 km down to the ion gyroradius. The mean plasma density gradient that must be present to allow irregularity production by this interchange process appears to be associated with larger-scale (>10 km) plasma structure produced by other means. Because much of the recent progress on this research topic stems from this recognition, a significant portion of this review is dedicated to a description of the characteristics and processes of 10-km plasma structure and their relationships to those of smaller-scale irregularities. From this review, we synthesize a descriptive model of plasma structures in the high-latitude F layer that unifies most of the diverse and independent observations. For the large-scale plasma processes, the model includes (1) the formation of 1000-km-scale “patches” in the polar cap from solar-produced plasma that is transported poleward from lower latitudes; (2) the reconfiguration of patches as they convect into the auroral region and become the latitudinally confined, but longitudinally extended, plasma density enhancements near the equatorward auroral boundary; and (3) the production of localized enhancements and depletions along the poleward auroral boundary by soft-particle precipitation and large but localized electric fields. In the model, the most intense, smaller-scale irregularities are in spatial proximity to these large-scale plasma features, the implication being that the presence of the latter allows formation of the former. The irregularity characteristics are consistent with production by the instability and a morphology controlled by (1) a “slip” velocity (i.e., plasma drift relative to the neutral gas) that is moderately small except in regions of nonuniform plasma convection or under time-varying conditions (e.g., substorms, pulsation events) and (2) a highly conducting auroral E layer that damps irregularity growth and enhances decay. The final irregularity spectrum appears to be produced by (1) global convective processes acting on solar-produced plasma at the largest scales (>50 km), (2) particle precipitation at scales greater than 10 km, (3) perhaps some form of wave activity around 10 km, and (4) the instability at the smaller scales (<10 km).

High-Latitude F-Region Irregularities: A Review and Synthesis

Global propagation of atmospheric gravity waves: A review

In this paper we review the theory of acoustic-gravity waves, the interaction of such waves with the ionosphere, the experimental support for the existence of such waves in the upper atmosphere, and the role played by acoustic-gravity waves in thermospheric dynamics. After a thorough discussion on the properties of acoustic-gravity waves in an ideal isothermal atmosphere, the effects produced by horizontal winds, sharp boundary discontinuities, and dissipative processes are discussed. The generation of these waves by stationary or moving sources is then treated. It is shown that the atmospheric response to a stationary impulse source can be described by the emission of three waves: acoustic, buoyancy, and gravity. These discussions are then followed by reviewing propagation effects in a realistic atmosphere for both free waves and guided waves. Recent numerical results are given. When acoustic-gravity waves propagate through the ionosphere, interaction between the wave and the ionosphere will take place. The physical processes involved in such an interaction are examined.

Acoustic‐gravity waves in the upper atmosphere

[1] The dissipation of high-frequency gravity waves (GWs) in the thermosphere is primarily due to kinematic viscosity and thermal diffusivity. Recently, an anelastic GW dispersion relation was derived which includes the damping effects of kinematic viscosity and thermal diffusivity in the thermosphere and which is valid before and during dissipation. Using a ray trace model which incorporates this new dispersion relation, we explore many GW properties that result from this dispersion relation for a wide range of thermospheric temperatures. We calculate the dissipation altitudes, horizontal distances traveled, times taken, and maximum vertical wavelengths prior to dissipation in the thermosphere for a wide range of upward-propagating GWs that originate in the lower atmosphere and at several altitudes in the thermosphere. We show that the vertical wavelengths of dissipating GWs, λz(zdiss), increases exponentially with altitude, although with a smaller slope for z > 200 km. We also show how the horizontal wavelength, λH, and wave period spectra change with altitude for dissipating GWs. We find that a new dissipation condition can predict our results for λz(zdiss) very well up to altitudes of ∼500 km. We also find that a GW spectrum excited from convection shifts to increasingly larger λz and λH with altitude in the thermosphere that are not characteristic of the initial convective scales. Additionally, a lower thermospheric shear shifts this spectrum to even larger λz, consistent with observations. Finally, we show that our results agree well with observations.

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Horizontal and vertical propagation and dissipation of gravity waves in the thermosphere from lower atmospheric and thermospheric sources

The guided acoustic-gravity modes that can propagate in a realistic dissipative atmosphere are computed for periods between 30 sec and 2 hours. The analysis differs from previous treatments by including dissipation (viscosity and thermal conductivity) and by using a realistic sound speed profile throughout the thermosphere. The presence of realistic dissipation allows a unique determination of the acoustic-gravity mode spectrum free from ambiguities related to the choice of the upper boundary condition. Since the atmospheric model below the thermosphere is similar to those used in previous studies, the characteristics of the lower atmospheric modes (e.g., the Lamb mode) are essentially unchanged from previous analyses of fully ducted modes in dissipationless atmospheres. The attenuation distances of these lower atmospheric modes, defined as the distances required for the modes to be attenuated by a factor of 1/e, are of the order of the earth's circumference or greater. The upper atmospheric modes, on the other hand, exhibit several characteristics that disagree with the results of previous analyses. The calculations reported here reveal the existence of several long-period modes with average speeds between 300 and 700 m/sec, all composed at F region heights of internal gravity waves with nearly horizontal phase fronts. Their attenuation distances are of the order of 1/8 of the way around the earth or less. All the properties of these upper atmospheric modes agree qualitatively with the observed characteristics of large-scale F region traveling disturbances (TID's). Ion drag is not found to be particularly important, but changes of thermospheric temperature due to diurnal and solar cycle variations are found to have quite appreciable effects on the upper atmospheric modes.

Acoustic-gravity modes and large-scale traveling ionospheric disturbances of a realistic, dissipative atmosphere

Analysis of the excitation of atmospheric gravity waves leads to a hypothesis concerning the nature of medium-scale traveling ionospheric disturbances (TID's) and their relation to large-scale TID's Stationary phase techniques are used to derive the asymptotic fluctuations of velocity and electron density generated by a line source (eg, the auroral electrojet) Dissipation by viscosity and thermal conductivity is included in an approximate manner The gravity wave response includes a discrete spectrum of upper-atmospheric guided modes and a continuous spectrum of freely propagating waves; these two classes of waves can be associated with large-scale and medium-scale TID's, respectively The medium-scale TID's, in turn, are primarily composed of two families of waves: those that propagate obliquely upward from the source and reach the F region directly and those that propagate initially downward and reach the F region only after reflection from the earth For an E region source the earth-reflected waves appear as nearly monochromatic wave packets, whereas the direct waves appear for the most part as single pulses; this qualitative difference corresponds to J E Titheridge's observational distinction between periodic and nonperiodic TID's It is suggested that this dichotomy may imply the dominance of upper-atmospheric (as opposed to tropospheric) sources In this connection the average fluctuations of the auroral electrojet are shown to be sufficient to generate sizable medium-scale TID's that propagate to large horizontal distances with no loss of amplitude despite the absence of ducting mechanisms Such propagation is possible because amplitude attenuation due to cylindrical spreading is counteracted by the fact that the lower-frequency waves, which reach the F region at the greater horizontal distances, are generated with larger amplitudes Several properties of the calculated response, including the linear increase of period and wavelength with distance from the source, might be useful in future observational studies designed to identify the sources of medium-scale TID's

A theory of medium-scale traveling ionospheric disturbances

Determination of striation scale sizes for plasma clouds in the ionosphere

Calculations reveal that the earth's atmosphere can support only two strongly ducted lower-atmospheric gravity modes, the S1 and S2 modes previously found by Friedman, with speeds of 312 m/sec and 253 m/sec at long periods. The principal finding of the present work is that Friedman's higher-order modes Si (i > 2), with speeds less than 250 m/sec, are weakly ducted and do not propagate for long distances. Since the bulk of observed medium-scale TID travel at speeds between 100 and 250 m/sec, below the speeds of the S1 and S2 modes, it is concluded that the hypothesis relating medium-scale TID to ducted modes is untenable. Instead, it is suggested that medium-scale TID are manifestations of freely propagating gravity waves.

Lower‐atmospheric gravity modes and their relation to medium‐scale traveling ionospheric disturbances

The refraction of internal acoustic-gravity waves by the earth's spherically symmetric gravitational field is investigated by using the approximation of geometrical acoustics in an isothermal atmosphere. Instead of following straight lines the ray paths of internal acoustic waves bend away from the earth, whereas the ray paths of internal gravity waves bend toward the earth so as to nearly match the earth's curvature. Consequently, the use of a flat-earth approximation for describing long-distance propagation around a spherical earth is well justified for internal gravity waves, although not for internal acoustic waves.

Samuel H. Francis

Papers

Acoustic-gravity modes and large-scale traveling ionospheric disturbances of a realistic, dissipative atmosphere

A theory of medium-scale traveling ionospheric disturbances

Determination of striation scale sizes for plasma clouds in the ionosphere

Lower‐atmospheric gravity modes and their relation to medium‐scale traveling ionospheric disturbances

Propagation of internal acoustic‐gravity waves around a spherical Earth