Long delayed radio echoes - The illusive secret of the ionosphere

The first radio echoes with long delays were reported in 1927 by Jørgen Hals in Oslo, Norway on 9.54 MHz signals from the Netherlands. His first report exists at the National Library in Oslo and was later reported in Nature [Størmer, 1928]. Controlled experiments where delays ranged from 3 to 30 seconds confirmed the phenomenon and convinced most skeptics that the phenomenon was real [van der Pol, 1928]. About 15 different hypotheses have been presented over time for their cause [Shlionskiy 1985]. The five most likely ones will be presented here. They involve phenomena with long-distance travel such as around the world many times or reflection from plasma clouds. Another scenario is where the speed of propagation is substantially slower than normal as if there is mode conversion to mechanical waves in the ionosphere with or without nonlinearity [Vidmar, Crawford 1985]. The only one of the five which is understood well is when a signal is ducted in the magnetosphere and reflected from the opposite hemisphere. It may occur for frequencies up to 4 MHz and will give delays of up to 0.5 seconds. The further north, the longer the delay. Therefore, such medium delay echoes may sometimes be confused with round-the-world echoes of 138 ms, which however seldom occur at such low frequencies. There are examples of such echoes from Tasmania and St. Petersburg in the scientific literature. Amateur operator reports from New York, Georgia, California, Newfoundland, Denmark, and the UK [Martinez, 2007] from the last decades will be presented and analyzed and audio from some of them will be demonstrated. The delay is predictable from geomagnetic latitude using a simplified dipole model for the geomagnetic field line [Holm 2009]. The effect occurs during the dark hours and is most likely during winter months of years of low solar activity. A good report of such a phenomenon should involve a digital recording of audio using CW and not SSB signals. Delay measurement with amateur radio equipment is a challenge and needs to compensate for the usual delay between the onsets of the transmitter and sidetone signals.