Recent analyses of observational data reveal the presence of perturbations in the E and F regions of the equatorial ionosphere with periods ranging from 2 to 45 days. The characteristic periods of many of these perturbations suggest an association with free Rossby (resonant mode) oscillations, perhaps excited either in the lower atmosphere or in situ. In the present work we analyze hourly magnetic observations from Huancayo Observatory, Peru (12.00°S, 75.30°W geographic; 0.72°S, 4.78°W geomagnetic), for the presence and persistence of these oscillations during the whole year of 1979. The measured variations can be interpreted in terms of oscillations of the wind field in the E region (approximately 100‐160 km), which in turn cause perturbations in the electric fields generated by the wind‐driven atmospheric dynamo and in the magnetic field intensity measured at the ground. The observations suggest that the effects of planetary wave oscillations with periods close to 2.5, 3, 6, 7, 9, 10.5, and 16 days may regularly propagate into the thermosphere and ionosphere, causing oscillations which are significant in magnitude. On the basis of an averaged periodogram analysis, we estimate that planetary wave effects may account for up to 75% of the total energy in ΔH values in the 2 to 35 day period range, suggesting that planetary waves may provide an important contribution to the dynamics and electrodynamics of the lower ionosphere and thermosphere. EUV fluxes during 1979 are noted to have a predominant 13.5‐day periodicity during the first half of the year and the more typical 27‐day oscillation during the latter half of 1979. These features can in principle affect the ΔH variations through their influence on the E region conductivity. We examine such influences here, especially those that affect the interpretation of the quasi 16‐day oscillation.

Measurements of atmospheric density near 200 km from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment are used to delineate the temporal, seasonal‐latitudinal, and day/night dependences of the response to magnetic storm‐related perturbations in high‐latitude energetic inputs. Five periods of geomagnetic activity are analyzed and yield consistent results which can be interpreted within the framework of recent thermosphere‐ionosphere simulations by Fuller‐Rowell et al. [1995]: In response to a change in magnetic activity level from quiet (Kp ≈ 1–2) to active (Kp ≈ 4–7) conditions, an increase in daytime (1030 LT) density of order 50–70% occurs between 60 and 80° geographic latitude in the summer hemisphere, with about half the maximum response in the winter hemisphere. This difference is mainly due to the difference in ionization/conductivity levels (and hence joule heating rates) between the hemispheres. On the dayside, penetration of the disturbance at about the 50% intensity level is realized at the equator, whereas in the winter hemisphere equatorward penetration is much weaker. These effects are connected with the prevailing solar‐driven circulation; the net summer‐to‐winter meridional flow facilitates equatorward advection of the disturbance bulge in the summer hemisphere but hinders advection in the winter hemisphere. In both hemispheres the daytime component of the solar‐driven diurnal circulation tends to oppose equatorward penetration to the same degree. However, on the nightside (2230 LT) penetration at nearly the 100% level of both summer and winter disturbance bulges are realized to within 20° of the geographic equator. This behavior is associated with the equatorward advection in both hemispheres consistent with the nighttime component of the solar‐driven circulation. Comparisons with the MSISE90 model [Hedin, 1991] show it to capture the salient features of the daytime behavior but exhibits little day/night asymmetry, in contrast to the experimental results.

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1] To help understand the large disparity in the results of circulation modeling for the atmospheres of Titan and Venus, where the whole atmosphere rotates faster than the surface (superrotation), the atmospheric angular momentum budget is detailed for two General Circulation Models (GCMs). The LMD GCM is tested for both Venus (with simplified and with more realistic physical forcings) and Titan (realistic physical forcings).The Community Atmosphere Model is tested for both Earth and Venus with simplified physical forcings. These analyses demonstrate that errors related to atmospheric angular momentum conservation are significant, especially for Venus when the physical forcings are simplified. Unphysical residuals that have to be balanced by surface friction and mountain torques therefore affect the overall circulation. The presence of topography increases exchanges of angular momentum between surface and atmosphere, reducing the impact of these numerical errors. The behavior of GCM dynamical cores with regard to angular momentum conservation under Venus conditions provides an explanation of why recent GCMs predict dissimilar results despite identical thermal forcing. The present study illustrates the need for careful and detailed analysis of the angular momentum budget for any GCM used to simulate superrotating atmospheres.

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