Wideband probing of the transauroral HF channel: Solar minimum

Abstract

The Naval Research Laboratory HF Channel Prober is a high‐resolution, computer‐controlled, coded‐pulse sounder designed to study the time variable behavior of the wideband HF radio channel. Multispectral time histories of the channel pulse response and the channel scattering function are tools used in the study. Pulse response delay resolution down to 1 μs and unambiguous Doppler up to ±30 Hz are possible, depending on the choice of experiment format and operating parameters. Measurements were made quarterly between October 1985 and July 1986 on a 2300‐km transauroral path between a transmitter site at Frobisher Bay, Canadian Northwest Territories, and a receiver site near Rome, New York. Data are presented in the form of ionograms, pulse response time histories (power and coherent video), and channel scattering functions. Data for mid‐latitude and transauroral paths are compared, and their similarities and differences are discussed. The mid‐latitude channel is multimodal but is usually characterized by a single specular return per mode. The character of transauroral channel is quite variable, depending strongly on the degree of magnetic disturbance, the location of the midpath reflection point relative to the position of the auroral oval, and the mode. During quiet magnetic conditions, and with the path midpoint well below the auroral oval, channel behavior is shown to resemble a mid‐latitude channel at certain times and a specular multipath channel at other times (i.e., one where each mode comprises several distinct multipath components). Doppler spread in these cases tends to be small. At night, when the midpath point lies within the auroral oval, the return signal is diffuse, and the Doppler spread appears to be larger by an order of magnitude.