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What causes mysterious long delayed radio echoes in Earth's atmosphere?
The exact cause of this unusual phenomenon remains unknown.
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In 1927, Norwegian civil engineer and amateur radio operator Jørgen Hals made an unusual discovery: while monitoring transmissions from a shortwave radio station in Eindhoven, The Netherlands, he began hearing odd, "delayed" radio echoes, arriving approximately three seconds after the original transmissions, that sometimes accompanied the station's signals.
"From where this echo comes I cannot say for the present," Hals wrote of his peculiar discovery, adding that he could "only confirm that I really heard it."
Almost a century after Hals' discovery, the exact cause of this unusual phenomenon — known today as long delayed echoes, or LDEs — remains unknown. Their baffling nature has prompted a range of potential explanations, most of which focus on a variety of unexplained natural phenomena occurring in Earth's atmosphere. However, a few more far-out possibilities, including potential evidence of extraterrestrial technology, have also been considered.
The enigma of long-delayed echoes has perplexed scientists since their initial detection by Hals, who at the time reported his odd findings to Norwegian physicist Carl Størmer. This prompted an investigation by Størmer with physicist Balthasar van der Pol, during which the duo was able to reproduce the phenomenon but were unsuccessful in determining its cause.
Since those early observations, ongoing reports of long-delayed echoes have continued to be recorded. Under normal conditions, radio echoes occur whenever a radio signal reflects or "bounces" off an object, although other conditions such as signals traveling through a delayed path can also cause echoes to occur.
Long delayed echoes, by contrast, are generally characterized by radio signals that return after more than 2.7 seconds following their original transmission, although the terminology has sometimes been applied more broadly.
Complicating investigations into this unusual brand of radio phenomena over the years has been their unpredictable nature, along with significant reported variances in the timing and nature of the delays, which has made it difficult for researchers to narrow down a single cause.
Despite the challenges presented by this unique class of space mysteries, there are several theories that have been put forward over the years about what could cause long-delayed echoes.