Morphology and characteristics of disturbed HF skywave channels

The normal HF skywave channel is one in which propagation via ionospheric reflection predominates. A disturbed HF skywave channel is defined to be one in which ionospheric scatter plays a significant, often dominant, role in point-to-point radiowave propagation. From a communications perspective, a disturbed channel is one which exhibits reduced signal level in combination with the extensive delay and Doppler spread. Channels which are regularly disturbed include the trans-equatorial, trans-polar and trans-auroral channels. This report covers a number of topics including: (1) measurements that have contributed to our knowledge about the structure and behavior of disturbed channels, (2) solar-terrestrial control factors (e.g., diurnal, seasonal, solar cycle, and geomagnetic), (3) irregularity source regions and current theories of irregularity generation processes, and (4) sample data, available from channel probes, for each of the disturbed channels.