Abstract. Recent investigations of atmospheric gravity waves (AGW) and travelling ionospheric disturbances (TID) in the Earth\\'s thermosphere and ionosphere are reviewed. In the past decade, the generation of gravity waves at high latitudes and their subsequent propagation to low latitudes have been studied by several global model simulations and coordinated observation campaigns such as the Worldwide Atmospheric Gravity-wave Study (WAGS), the results are presented in the first part of the review. The second part describes the progress towards understanding the AGW/TID characteristics. It points to the AGW/TID relationship which has been recently revealed with the aid of model-data comparisons and by the application of new inversion techniques. We describe the morphology and climatology of gravity waves and their ionospheric manifestations, TIDs, from numerous new observations.

The quasi‐periodic DP 2 magnetic fluctuations (period of 30–40 min) appearing coherently at the auroral and equatorial latitudes during the day are analyzed based on the high time resolution magnetometer data recorded at the International Monitor for Auroral Geomagnetic Effects (IMAGE) stations in Scandinavia and at the Brazilian and African equatorial stations. It is shown that the correlation between the DP 2 magnetic fluctuations at both latitudes is excellent (correlation coefficient of 0.9). No discernible time shift has been found within the resolution of 25 s. The European incoherent scatter (EISCAT) radar observations in Scandinavia show that the DP 2 fluctuations at auroral latitudes are caused by an ionospheric Hall current which is controled by the convection electric field. The DP 2 fluctuations exhibit a strong decrease in magnitude with decreasing latitude, however, it is enhanced considerably at the dip equator with an amplitude comparable to that at the subauroral latitude. The considerable equatorial enhancement of the magnitude of the DP 2 fluctuations with an enhancement ratio of 4 is due to the concentration of the electric current along the highly conductive dayside equatorial ionosphere. These observational facts can be explained in terms of an ionospheric current which is generated by the magnetospheric electric field at the high latitude and extends to the equatorial ionosphere almost instantaneously. From the viewpoint of the electric field penetration, we conclude that the magnetospheric electric field penetrates to the equatorial ionosphere through the polar ionosphere almost instantaneously within the time resolution of 25 s. The nearly instantaneous propagation of the electric field to the equator can be explained primarily by a parallel plane transmission line model composed of the conductive Earth and ionosphere. In addition to our finding of the fast propagation of the DP 2 electric field, it is found that an impulsive magnetic change with a timescale of 100 s appears at the dayside dip equator with a time delay of about 10 s, which requires to include the effect of the high conductivity of the dayside equatorial ionosphere in future studies of the propagation model.

Abstract. We have studied the negative magnetic bay associated with the substorm that occurred on April 20, 1993, and have found that it is markedly enhanced at the daytime dip equator, coherent with that at afternoon subauroral latitudes. The amplitude of the negative bay decreases monotonously with the latitude, but it is amplified at the dip equator by a factor of 2.5 compared to the low-latitude negative bay. This latitudinal profile implies that in addition to the three-dimensional current system in the magnetosphere, DP ionospheric currents originating in the polar ionosphere contribute greatly to negative bays. Penetration of the convection electric field and the effect of a shielding electric field due to Region 2 (R2) field-aligned currents (FACs) are examined on the basis of European Incoherent Scatter (EISCAT) and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations made in the afternoon sector. The northward electric field at EISCAT (66 ø corrected geomagnetic latitude (CGMLAT)) is well correlated with the magnetic field X component at Nurmijfirvi (56 ø CGMLAT) during the presubstorm period, but the coherency breaks down during the substorm cycle. By assuming that the R2 FACs intensify the northward electric field at EISCAT but reduce it at Nurmijfirvi, we demonstrate that the R2 FACs grow concurrently, although delay by some 17 min, with the convection electric field. Our analytical results indicate that the convection electric field decreases abruptly during the substorm and that the shielding electric field overcomes the convection electric field at around the peak of the negative bay, owing to its delayed reaction. The equatorial negative bay is thus due to an overshielding effect caused by the electric field associated with the R2 FACs. IntroductionPenetration of the magnetospheric convection electric field to the midlatitude and low-latitude ionosphere has been studied using data obtained from In this paper we first report a new observational finding: a negative magnetic bay associated with a substorm is markedly enhanced at the daytime dip equator, coherent with the negative bay at afternoon high-latitude stations. In addition, we provide direct evidence based on EISCAT data for a rapid decrease in the convection electric field that penetrates to the equator, causing a large-amplitude negative deflection at dip 23,251