Thomas A. Croft

The influence of ionospheric irregularities on sweep-frequency backscatter

Abstract Using computer simulation it is possible to predict the effect of traveling ionospheric irregularities upon the data obtained by HF sweep-frequency ground-backscatter sounders. The simulation process makes use of an ionospheric model in which radio rays are traced to determine the time distribution of energy along the distant ground after skywave travel. Subsequent computation yields a synthetic record of the echo amplitude as a function of radio frequency and time delay, much like the experimental data. The degree of realism in these synthesis is limited primarily by the need for economy; the process used here is a simplified version of one previously described. ( Croft , 1967 ) It is shown that a localized ionospheric irregularity causes a discontinuity in backscatter echo amplitude with a limited extent in frequency and with a delay which is comparatively independent of frequency. This discontinuity has the appearance of a short streak on the synthetic records and such a feature is common in experimental data. When an ionospheric irregularity exists from the ground to an unlimited altitude and is formed with its long axis tilted about 45° away from the observer, a backscatter streak appears which is spread over the entire frequency range of the background echo. Furthermore, this streak is tilted on the record, i.e., its time delay increases with an increase in frequency. When a similar anomaly is programmed so that it tilts inward toward the observer, then the resulting streak has a delay which decreases with an increase in frequency. It is thus demonstrated that there is a relation between the tilt of the anomaly and the tilt of the backscatter streak. The relation is in accord with a limited observation that tilted streaks in experimental backscatter move only in the predicted direction of anomaly tilt. Concentric anomalies (having constant altitude) are shown to produce new, short-range leading edges on the backscatter. Experimental data having this form can thus be interpreted as an indication of the presence of small ‘layerlets’ within the ionosphere, but it is noted that there are at least three other possible causes for this form of backscatter echo.