[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

Anomalous increases of the ionization density at middle latitudes (positive ionospheric storms) and anomalous increases of the neutral gas density at low latitudes (the geomagnetic activity effect) are prominent features of upper atmospheric storms. The present study investigates the idea that both phenomena are caused by traveling atmospheric disturbances (TADs). According to theory, such TADs are generated during magnetic substorm activity and propagate with high velocity from polar to equatorial latitudes. To examine the above hypothesis, magnetic, ionospheric, and neutral atmospheric data are compared for five different disturbance events. These case studies demonstrate that (1) there is a good temporal correlation between magnetic substorm activity at high latitudes, daytime positive ionospheric storms at middle latitudes, and the geomagnetic activity effect at low latitudes; (2) the initial phase of positive ionospheric storms propagates with high velocity toward lower latitudes; (3) this velocity is roughly consistent with the time lag of the geomagnetic activity effect at low latitudes; (4) the ionospheric disturbance is a conjugate phenomenon of global extent; and (5) it cannot be explained as an electric field effect. In summary, our data are fully consistent with the idea that TADs are responsible for both positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes.

Common origin of positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes

The upper atmosphere constitutes the outer region of the terrestrial gas envelope above about 100 km altitude. The energy budget of this outer gas layer is partly controlled by the dissipation of solar wind energy. Since this energy input is largely irregular, the resulting density changes are considered as perturbations. The properties and physics of such density perturbations are reviewed here. Besides being an important link in the complex chain of solar-terrestrial relations, such disturbances are also of practical interest because they affect the orbits of satellites and space stations and are responsible for ionospheric disturbance effects.

Density Perturbations in the Upper Atmosphere Caused by the Dissipation of Solar Wind Energy

[1] We investigate the role convection plays in the thermosphere using deep convection and ray trace models with a general dissipative anelastic GW dispersion relation. In the absence of dissipation, a GW's vertical wavelength λz increases by , or greater if its intrinsic frequency ωIr is close to the smaller thermospheric buoyancy frequency N = N0. Here, and 0 are the asymptotic temperatures in the thermosphere and lower atmosphere, respectively, and N0 is the buoyancy frequency in the lower atmosphere. In the presence of dissipation, λz also increases in the thermosphere by a factor of ∼2–3 when ωIr > 0.2N0 and λz > 25 km during active solar conditions. GW dissipation altitudes and maximum vertical wavelengths, which increase as increases, are displayed for small-scale and midscale GWs. GWs excited from deep convection encounter horizontal shears, which impose anisotropy on the spectrum. Along with dissipative filtering, momentum flux divergence and body forces result. The thermospheric body forces resulting from our convection model achieve maximum accelerations at z 180–200 km, extend down to z 130 km, last for the duration of deep convection, are ∼600 km × 600 km × 40–80 km in the x, y, and z directions, and are very strong with accelerations ≃0.5–0.75 m s−2 and ≃0.25–0.4 m s−2 during extreme solar minimum ( = 600 K) and active solar conditions ( = 2000 K), respectively. During extreme solar minimum, there is negligible forcing above z ≃ 230 km, whereas the forcing extends up to z ≃ 360 km during active solar conditions. These horizontal, thermospheric body forces may be a new source of large-scale, long-period secondary GWs and induced TIDs (traveling ionospheric disturbances) at high altitudes in the thermposphere.

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Influence of solar variability on gravity wave structure and dissipation in the thermosphere from tropospheric convection

GW drag and wave‐induced heating/cooling are shown to be smaller below ∼170 km at high solar activity, and larger above. The maxima of GW momentum deposition occur much higher in the upper thermosphere, but their peaks are half as strong, 120 vs 240 m s −1 day −1 in the winter hemisphere when the insolation is large. Instead of strong net cooling in the upper thermosphere, GWs produce a weak heating at high solar activity created by fast harmonics less affected by dissipation. Molecular viscosity increases with solar activity at fixed pressure levels, but seen in Cartesian altitude grids it can either increase or decrease in the lower thermosphere, depending on the height. Therefore, in pressure coordinates, in which most GCMs operate, the influence of larger temperatures can be viewed as a competition between the enhanced dissipation and vertical expansion of the atmosphere.

/pdf/internal-gravity-waves-in-the-thermosphere-during-low-and-3zbvz56iel.pdf

K.D. Cole

Papers

Energy transfer by gravity wave dissipation

Electromagnetic field dissipation and corpuscular bombardment and their implications in atmospheric modelling