Long delayed echo

"LDE" redirects here. For the LiteStep distribution, see LDE(X).

Long delayed echoes (LDEs) are radio echoes which return to the sender several seconds after a radio transmission has occurred. Delays of longer than 2.7 seconds are considered LDEs.[1][2] LDEs are scientific anomalies of uncertain origin.

Contents

History

These echoes were first observed in 1927 by civil engineer Jørgen Hals from his home near Oslo, Norway. Hals had repeatedly observed an unexpected second radio echo with a significant time delay after the primary radio echo ended. Unable to account for this strange phenomenon, he wrote a letter to Norwegian physicist Carl Størmer, explaining the event:

At the end of the summer of 1927 I repeatedly heard signals from the Dutch short-wave transmitting station PCJJ at Eindhoven. At the same time as I heard these I also heard echoes. I heard the usual echo which goes round the Earth with an interval of about 1/7 of a second as well as a weaker echo about three seconds after the principal echo had gone. When the principal signal was especially strong, I suppose the amplitude for the last echo three seconds later, lay between 1/10 and 1/20 of the principal signal in strength. From where this echo comes I cannot say for the present, I can only confirm that I really heard it.[3]

The pair, joined by physicist Balthasar van der Pol[4] researched the echoes for some years, but failed to come up with an explanation. The reason for this is that the effect only occurs sporadically and that the time-delay of the echoes varies dramatically. One would expect that, if these were echoes off some region of the atmosphere or something in space (e.g., the moon) the echoes would exhibit a predictable time delay (the time taken to travel to, and back from, the deflecting entity).[5]

Long delayed echoes have been heard sporadically from the first observations in 1927 and up to our time.

Five hypotheses

Shlionskiy lists 15 possible natural explanations in two groups: reflections in outer space, and reflections within the Earth's magnetosphere.[6][7] Vidmar and Crawford suggest five of them are the most likely.[8] Sverre Holm, professor of signal processing at the Centre for Imaging, University of Oslo, details those five[9]; in summary,

Signals may pass the ionosphere and then be ducted in the magnetosphere out to a distance of several earth radii over to the opposite hemisphere where they will be reflected on top of the ionosphere. The round-trip time varies with the geomagnetic latitude of the transmitter and is typically in the 140 - 300 ms range. The further North the station, the larger the delay. Due to the short delay, this cannot be considered to be a real long-delayed echo. For completeness it is still included here.
"Goodacre [11][12] reports that he pointed his antenna towards the horizon and received his own 28 MHz signal delayed by up to about 9 seconds.... His measurement implies travel up to 65 rounds around the earth." Probably the upper frequency limit for such effects.
The most popular current theory is that the radio signals are trapped between two ionized layers in the atmosphere and then are guided around the world many times over until they fall out of a gap in the bottom layer. (Ducting propagation between air layers in the lower atmosphere is a well-understood phenonemon. See Radio propagation.)
Investigated experimentally by Crawford et. al.; they recorded echoes with delays up to 40 seconds at 5-12 MHz.[8][13].
Freyman [14] did experiments at 9.9 MHz and detected several thousand echoes of delay up to 16 seconds at times when solar plasma probably entered the magnetosphere.
Could explain amateur VHF/UHF echoes. Hans Rasmussen found echoes delayed by 4.6 seconds at 1296 MHz,[15] Yurek recorded a 5.75 second delay at 432 MHz.[16]

None of these hypotheses can explain everything. Only the first mechanism is well established, and none of the other four are well-established enough to deserve the term "theory". The phenomena are often fleeting and non-repeatable. Our understanding of how the magnetosphere interacts with the solar wind is still evolving.

Alternative hypotheses

Some believe that the aurora activity that follows a solar storm is the source of LDEs.

Still others believe that LDEs are double EME (EMEME) reflections, i.e. the signal is reflected by the moon and that reflected signal is reflected by the Earth back to the moon and reflected again by the moon back to the earth.

Duncan Lunan proposed the radio echoes observed by Størmer and Van der Pol in 1928 might have been transmissions from a Bracewell probe, an artifact of aliens trying to communicate with us by bouncing back our own signals.[1] This concept is also addressed by Holm.[9]

Deception

It must be kept in mind that it is not easy to identify whether an LDE is 'natural'. Grassman warns: "Attempts at deception can in no case be ruled out, and it is to be feared that less serious radio amateurs contribute to deliberate falsification.... Short transmissions using different frequencies are a relatively simple procedure for excluding potential troublemakers."[5]

See also

References

  1. ^ a b [1] ARRL: Stan Horzepa, Radio Ghosts
  2. ^ ARRL: Stan Horzepa,Long-Delayed Echoes Again
  3. ^ Carl Stormer, "Short Wave Echoes and the Aurora Borealis," Nature, 122, 681, (1928)
  4. ^ Balthus van der Pol, "Short Wave Echoes and the Aurora Borealis," Nature, 122, 878-879 (1928)
  5. ^ a b [2] V. Grassmann, Long-delayed radio echoes, Observations and interpretations, VHF Communications vol 2, pp. 109-116, 1993.
  6. ^ Holm summary, Shlionskiy's 15 possible explanations for Long Delayed Echoes
  7. ^ A. G. Shlionskiy, "Radio echos with multisecond delays," Telecomm. and Radio Eng., Vol 44, No. 12, pp. 48-51, Dec. 1989.
  8. ^ a b R. J. Vidmar and F. W. Crawford, "Long-delayed radio echoes: Mechanisms and observations," Journ. Geophys. Res., vol. 90, no. A2, pp. 1523-1530, Feb. 1985.
  9. ^ a b Sverre Holm, The Five Most Likely Explanations for Long Delayed Echoes
  10. ^ Sverre Holm, Unusual HF Propagation Phenomena - MDE
  11. ^ A. K. Goodacre (VE3HX), "Observations of long-delayed echoes on 28 MHz," QST, March 1980, pp. 14-16.
  12. ^ A. K. Goodacre (VE3HX), "Some observations of long-delay wireless echoes on the 28-MHz amateur band," Journ. Geophys. Res., Vol. 85, No. A5, pp. 2329-2334, May 1980
  13. ^ F. W. Crawford, D. M. Sears, R. L. Bruce, "Possible observations and mechanism of very long delayed radio echoes," Journ. Geophys. Res., Space Physics, vol. 75, no. 34, pp. 7326-7332, Dec. 1970.
  14. ^ R. W. Freyman, "Measurements of long delayed radio echoes in the auroral zone," Geophys. Res. Letters, Vol. 8, No. 4, pp. 385-388, April 1981.
  15. ^ H. L. Rasmussen (OZ9CR), "Ghost echoes on the Earth-Moon path," Nature, Vol. 257, p 36, Sept. 4, 1975.
  16. ^ J. Yurek (K3PGP), “Echoes: An amateur observation and a professional reply,” QST, 62, pp. 35-36, May 1978.

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