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Faraday rotation data from a pair of polarimeters located in Tucson, Arizona during the first half of 1990 have been examined for evidence of traveling ionospheric disturbances. With the use of digital filtering techniques, oscillations in total electron content (TEC) with periods ranging from 20 to 90 minutes and amplitudes of 1–5 × 1016 m−2 have been detected in the data. When present, the oscillations persist for varying time intervals (usually several hours) and show considerable variation from day to day. Although the viewing geometry (nearly along the local magnetic field line) is not ideal for observation of TIDs, these oscillations do appear to be real ionospheric oscillations. We have examined the possibility that the oscillations are not caused by actual changes in electron content but rather are due to changes in the Faraday rotation resulting from the motion of the ionospheric plasma along a magnetic field line and the resulting change in effective magnetic field strength. Our calculations indicate waves with displacement amplitudes of a few tens of kilometers would be sufficient to explain the observations. We also examined the possibility that the oscillations could be due to actual changes in TEC by using a one-dimensional F region model and imposing a plausible wind oscillation (simulating a gravity wave). We found that a wave with a velocity amplitude of 15 m s−1 was also sufficient to explain the observations. We expect that the observations are best explained by a combination of the two effects, that is, an actual oscillation in electron content and an oscillation in Faraday rotation, both caused by gravity waves in the thermosphere. We estimate that for the 90-min oscillations, 25% of the amplitude is due to the Faraday rotation effect and 75% is due to an actual change in TEC.

[1] Lookup tables have been constructed that address group delay and RF carrier phase advance for transionospheric signal propagation referenced to a ground-based transmitter. The tables are based on parameterizations of ray-tracing results using the Reilly ray trace/ICED, Bent, Gallagher (RIBG) model with the magnetic field turned off. The quantities of interest are incremental group and phase path lengths referenced to straight-line distance between transmitter and receiver and are designated by Δlg and Δlp (as defined, both quantities are positive). Equations are presented that relate these quantities to group delay and ionospheric Doppler shifts. Tables currently in use are three-dimensional, expressed in terms of frequency (f), elevation angle (θ), and straight-line total electron content (TECSL). At high frequencies, Δlg and Δlp can be accurately expressed by a straight-line propagation formula that is proportional to TECSL. With decreasing frequency, these quantities become greater than their straight-line counterparts due to additional refraction effects, including ray bending. Extensive ray-tracing runs have been performed under a variety of ionospheric conditions to produce scatterplots of Δlg and Δlp versus TECSL for given pairs of [f, θ] values. The current ranges in these variables are from 20 to 100 MHz and 5° to 90°, respectively. The tables contain coefficients from fourth-degree polynomial fits to the distributions of points within the scatterplots. Where scatter is problematic, the introduction of a fourth parameter, slab thickness, is shown to significantly reduce this scatter. Magnetic field effects are discussed and are shown to have a small effect on table coefficients. Discussion is also included on the use of the tables with a model ionosphere.