DELAY, DOPPLER, AND AMPLITUDE CHARACTERISTICS OF HF SIGNALS RECEIVED OVER A 1300-KM TRANSAURORAL SKY WAVE CHANNEL

Channel probe observations of propagation conditions along a 1294-km t ransauroral path between Sondrestrom, Greenland, and Keflavik, Iceland , were made during the period from March 13 to April 2, 1992. The midp oint of this path was located at a corrected geomagnetic latitude of 7 2 degrees. The objective of these measurements was to supplement the e xisting data base describing propagation conditions on the HF transaur oral channel with data pertaining to a period around the time of solar maximum. Received signals for this path fell into three distinct grou ps depending on their amplitude and delay and Doppler spread character istics. These are (1) strong, specularly reflected ionospheric returns characteristic of a quiescent daytime ionospheric channel during magn etically quiet conditions; (2) strong specular multipath signals refle cted from horizontal gradients of electron density and regularly encou ntered at night; and (3) weak scatter returns that are also a persiste nt nighttime phenomenon. The scatter returns are usually observed at d elays exceeding those anticipated for the one-hop return and, very oft en, at frequencies that are well above the MUF for the great circle pr opagation path. The multipath and scatter returns exhibit large delay and Doppler spreads indicative of spatially extensive distributions of drifting and randomly moving irregularities. Two measurement events a re discussed which illustrate these conclusions: a noontime measuremen t with Kp = 3, and a midnight measurement with Kp = 2. The noontime me asurement exhibited a scatter return from an isolated irregularity reg ion in addition to the usual ionospheric reflected signals. A simple i rregularity drift model produced delay and Doppler shift curves that w ere consistent with those observed for the scatter component of the re ceived signal and supported a hypothesis of an irregularity region dri ft speed of 1200 m s(-1) parallel to the great circle propagation path .