Norwegian civil engineer and amateur radio operator Jørgen Hals noticed something odd in 1927 while listening to shortwave broadcasts from a station in Eindhoven, The Netherlands. Signals from the station were coming back to him as strange echoes roughly three seconds after the original transmission, and sometimes they arrived alongside it, according to Space. Hals reported the finding to Norwegian physicist Carl Størmer, who investigated with physicist Balthasar van der Pol. The pair managed to reproduce the effect but could not work out what produced it.
Nearly a hundred years later, the phenomenon — now called long delayed echoes, or LDEs — still has no confirmed explanation. Space reports that the echoes are generally defined as radio signals that return more than 2.7 seconds after they were sent, though the term has sometimes been used more loosely. The unpredictable nature of the echoes, and wide variation in how long the delays last, have made it hard for researchers to pin down a single cause.
Why it matters
The mystery sits at the intersection of atmospheric physics and radio engineering. If the cause could be identified, it would improve understanding of how the upper atmosphere and the space environment around Earth affect radio waves. Space notes that most proposed explanations involve unexplained natural processes in the atmosphere, while a few more speculative ideas — including the possibility of extraterrestrial technology — have also been raised over the years.
Trapped signals and magnetic field lines
One line of research dates to the mid-1980s, when researchers R. J. Vidmar and F. W. Crawford ran experiments in Alaska aimed at detecting long delayed echoes, building on earlier work they published involving possible detections in 1970. According to Space, the two hypothesised that radio signals could be caught between ionised layers of the atmosphere and guided across long distances.
Sverre Holm, a researcher at the University of Oslo, has advanced a related idea he calls the magnetospherically ducted echo, or MDE. In this model, radio waves can become trapped in electrically charged regions of the upper atmosphere and travel along magnetic field lines that stretch far into the magnetosphere. Under the right conditions, a signal could reach the opposite hemisphere before being sent back toward Earth, producing a delayed echo. Holm told Space that MDE is the best understood of five promising explanations he outlines, and that the model allows such echoes to be predicted to what he described as about 5 milliseconds of accuracy using a simple model of the magnetic field lines.
A theory with limits
Holm is also clear about where the MDE idea falls short. Space reports that the mechanism produces relatively short delays, which may not explain the much longer, multi-second echoes in some historical records. Because radio waves travel at close to the speed of light, a signal could in theory circle the planet several times within a single second — which does not prove any signal took such a path, but suggests some reported echoes might correspond to dozens of laps around Earth.
Asked about the historical cases with far longer delays, Holm told Space in an email that magnetospheric duct echoes "cannot [account] for those historical LDEs at all." He pointed to two main differences: the historical echoes were detected at about 9.55 MHz, well above the 1–4 MHz range usually linked to MDEs, and the historical delays ran from 3 to 30 seconds, while MDEs typically last only a few hundred milliseconds.
Other explanations may help fill the gap. Space reports that one proposal holds that electromagnetic radio energy could couple into slower plasma-wave modes while moving through the upper atmosphere, potentially creating far longer delays than ordinary radio propagation would allow. After that coupling, the signals could convert back into ordinary electromagnetic waves and return to lower levels of the atmosphere.
What happens next depends on further observations of a phenomenon that has resisted explanation for generations. Watch for new measurements of echo timing and frequency that could test the competing theories.
