Effects on transionospheric HF propagation observed by ISIS at middle and auroral latitudes Original Research Article

In 1978, an experiment on transionospheric HF propagation was carried out using a transmitter at Ottawa and the sounder receivers of the ISIS-I and ISIS-II spacecraft. Over 100 ISIS-II passes were successfully recorded using a fixed frequency of 9.303 MHz. A survey of the data has allowed some reproducible characteristics of transionospheric propagation to be identified. A number of ISIS-II ionograms are published here to illustrate those characteristics. A systematic feature of the pulses is their partial splitting into ordinary (O) and extraordinary (X) parts, producing a tripartite compound pulse at the satellite. Equatorward pulses are comparatively sharp and occasionally exhibit periodic fades with beat frequencies between about 1 and 4 Hz. Features of the fades indicate that focussing of rays is a better explanation for the fades than diffraction. Rays near the limits of the reception zone can result in dispersed pulses, thought to indicate forward scattering. Swept-frequency ionograms interleaved with fixed-frequency measurements allowed two-dimensional density distributions to be modeled in altitude and latitude. Three-dimensional ray tracing plus a Newton’s-iteration algorithm were used to find rays that connected the transmitter with the position of the satellite at any time along its path. The latitudinal extent of the zone irradiated at ISIS-II altitude thus computed is approximately as observed, albeit sensitively dependent upon north–south density gradients. Within this “iris” of accessibility, the maximum intensity of waves recorded at the spacecraft is within 10 dB of what is computed with a link calculation based on ray optics, but there are many dropouts of 20–30 dB below this maximum envelope. Toward the equator, propagation directions come to within about 10° of the magnetic-field axis. This research supports planning for coordinated ground-space radio experiments in the upcoming e-POP satellite mission.