TOTAL ELECTRON-CONTENT VARIATIONS DUE TO NONCLASSICAL TRAVELING IONOSPHERIC DISTURBANCES - THEORY AND GLOBAL POSITIONING SYSTEM OBSERVATIONS

Measurements of total electron content (TEC) variations are easy to pe rform. As several authors have pointed out, however, TEC variations mu st be interpreted carefully since TEC is an integrated quantity. Prior studies of TEC: variations have considered these variations to stem f rom ''classical'' gravity-wave interaction with the ionosphere at midl atitudes; that is, the component of gravity-wave neutral wind perturba tions along the local magnetic field moves the charged particles up an d down the field lines. Recent evidence suggests that at night, in the absence of photoproduction and when E layer conductivity becomes smal l, electrodynamic effects come into play and gravity-wave perturbation s can yield vertical movement of the F layer, rather than movement alo ng the field lilies. Here we consider the TEC variations resulting fro m horizontally propagating disturbances in ionospheric height. First, we develop an analytic model, assuming sinusoidal height variations an d a geostationary satellite, to estimate the magnitude of TEC variatio ns produced. In this case, the dependence of TEC variations on the ori entation of the line of sight, relative to the direction of disturbanc e propagation, takes on a different form from the classical case. Tota l electron content variations may also be observed using nonstationary satellites, such as Global Positioning System (GPS) satellites. We pr esent GPS measurements from Puerto Rico for the night of August 22-23, 1995, which show TEC variations of similar to +/-5 x 10(15) m(-2) in the presence of a southwesterly propagating ionospheric height disturb ance (as independently corroborated hy 630-nm airglow images at Arecib o and Ramey digisonde measurements). Although GPS satellite motion doe s not permit direct application of the geostationary theory, geostatio nary theory still allows us to make a crude estimate of the magnitude of TEC variations that result.