Global propagation of atmospheric gravity waves: A review

Abstract

The theoretical and observational evidence concerning the global propagation of atmospheric gravity waves is reviewed, with special emphasis on waves generated in the auroral zones. Gravity-wave theory predicts that the response to an auroral source mechanism consists of a discrete spectrum of upper-atmospheric guided modes and a continuous spectrum of freely propagating internal waves. Ionospheric observations confirm the existence of this dichotomy by revealing two distinct classes of traveling ionospheric disturbances (TIDs): large-scale TIDs associated with the guided modes and medium-scale TIDs associated with the freely propagating waves. TIDs of both classes have been observed to propagate for thousands of km without apparent attenuation. Large-scale TIDs observed at midlatitudes invariably travel toward the Equator, and their occurrence is strongly correlated with severe magnetic storms. Taken together, these facts imply that the only natural sources of large-scale TIDs are in the auroral zones. Theory supports this conjecture with the prediction that the guided modes associated with large-scale TIDs can be excited only by upper-atmospheric sources (in the E-region or above), since these modes consist of surface gravity waves ducted by the steep temperature gradients at the base of the thermosphere. Unlike these guided modes, the continuous spectrum of internal waves can be excited by sources at any altitude, so that their concomitant medium-scale TIDs are much more common than large-scale TIDs, and it is therefore difficult to identify their origins. Some observational evidence does exist, however, relating midlatitude medium-scale TIDs with auroral sources. In this connection, theoretical calculations predict that the average fluctuations of the auroral electrojet are sufficient to generate observable internal waves which propagate freely to large horizontal distances with no loss of amplitude, despite geometrical spreading, the Earth's curvature, and the absence of conventional ducting mechanisms. The effects of geometrical spreading are complicated by the anisotropic nature of gravity-wave propagation, with the result that even spherically spreading gravity waves observed at a fixed height can appear to grow as they travel away from their source. The Earth's curvature presents no barrier to long-distance propagation of such waves because they are refracted around the Earth by the gravitational field. The observed atmospheric response to nuclear explosions clearly illustrates these effects.