Estimating Horizontal Phase Speeds of a Traveling Ionospheric Disturbance From Digisonde Single Site Vertical Ionograms

Horizontal phase speeds for a medium-scale traveling ionospheric disturbance (TID) are calculated from three different atmospheric gravity wave (AGW) dispersion relations using vertical phase speeds derived from vertical ionograms measured by a single ionosonde. Observed heights from a network of four ionosondes in southern New Mexico provide the measured phase velocities. Horizontal phase speeds calculated from the dispersion relations are compared to measured TID speeds as a function of altitude and show general agreement. However, the linear relationship between the vertical and calculated horizontal AGW speeds for this TID frequency and wavenumber range predicts larger variations than the observations. The inclusion of viscosity and thermal diffusion terms in the dispersion relations increases the agreement with measurements. This technique provides a new method of predicting horizontal TID phase speeds from measurements at a single ionosonde site.