Ionospheric irregularity diagnostics from the phase structure functions ofMF/HF radio echoes

We present a new approach to investigating ionospheric irregularities, usin g the temporal structure function of totally reflected radio echo phase var iations. Modern digital ionosondes (e.g., the dynasonde) measure the echo p hase with very high resolution and precision, at closely spaced antennas, f requencies, and times. A "stringing" procedure gives continuous and unambig uous phase variation data for time intervals of any desired length. Quasi-p eriods of tens of seconds up through several minutes are caused by large-sc ale movements of the ionospheric plasma, while shorter-period phase variati ons result from the interaction of the sounding signal with small-scale irr egularities. The relevant irregularity spatial domain extends from decamete r radio wavelengths to the first Fresnel scale, a few kilometers. We obtain a theoretical relation between structure functions of the temporal phase v ariations and spatial irregularities with a simple model of frozen horizont al drift. The relation permits solutions of both the direct and inverse pro blems. Although long-period phase measurements are practicable and essentia l to exploring larger irregularity scales, they require observing modes ded icated to multiple fixed-frequency time series, and this undesirably limits the number of altitudes that can be monitored simultaneously. An alternati ve "rudimentary structure function" is obtainable from standard dynasonde " B-mode" ionograms; it offers good altitude and time resolution for irregula rity studies while permitting other established diagnostics (electron densi ty profiles, vector velocities, critical frequencies, etc.) with the same d ata. We show some example analyses by these methods as applied to auroral a nd magnetic-equatorial dynasonde observations. We find irregularity amplitu des in the range 0.001 < DeltaN/N < 0.1 (for a nominal scale of 1 km) and s pectral indices in the range 2 < nu < 4, with evidence of diurnal variation in both quantities at both locations.