The Five Most Likely Explanations for Long Delayed Echoes

Radio waves travel so fast that most people consider them to reach their destination instantaneously. Perhaps the only exception is a TV newscast with a live interview to another continent. The connection via a geostationary satellite gives a delay of about half a second from the end of a question to the start of the response. Still, much longer delays have been experienced from the beginning of the radio era. Such echoes occur very seldom and are called long delayed echoes (LDE).

A Radionette R-3 from 1927, the first European receiver to run from the mains supply. This regenerative receiver was a modern radio when Hals heard the first long-delayed echoes in Oslo. (With permission from Norsk Radiohistorisk Forening (1201))

My interest in this was started when I read the intriguing short essay on the ARRL web-site by Stan Horzepa in October 2003 entitled "Surfin': Radio Ghosts" where he lists some of the current theories about these echoes. In July 2007 Horzepa revisited the topic in his column and to my amazement he now referred to my page. The next week's follow-up column referred to my accompanying page with unusual HF propagation phenomena.

An interesting aspect of the LDE phenomenon for me is that the first observations took place in Oslo, actually at what is today called Bygdøy (previously Bygdø), a Western suburb on a peninsula which I pass on my daily commute to work. The first report was published more than 80 years ago by professor Carl Størmer, University of Oslo, known for being the first to measure the height of the Northern Lights, and it starts like this [Størmer, 1928]:

On Feb. 29 of this year I received a letter from Engineer Jørgen Hals, Bygdø, in which he says: "I herewith have the honour to advise you that at the end of the summer 1927 I repeatedly heard signals from the Dutch short-wave transmitter station PCJJ (Eindhoven). At the same time as I heard the telegraph-signals I also heard echoes. I heard the usual echo, which goes round the earth with an interval of about 1/7 second, as well as a weaker echo about 3 seconds after the principal signal had gone. When the principal signal was especially strong, I suppose that the amplitude for the last echo 3 seconds after 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 will only herewith confirm that I really heard this echo."

Delay between direct signal and echo. Several of them were observed simultaneously in Oslo, Norway and at two locations in Eindhoven, Holland on 11. Oct 1928, 16-17 UTC. B. van der Pol, Short Wave Echoes and the Aurora Borealis, Nature, Dec. 1928

After having spent several days at the National Library in Oslo going through the files of Professor Størmer, I found that J. Hals (1890-1942), a civil engineer with a keen interest in radio, had already been listening for echoes for a while [Hals, 1934]. His results led to more investigations and collaboration between van der Pol at Philips in the Netherlands and the group in Oslo. The experiments involved regular test transmissions from the Netherlands and at times from Indonesia. They lasted for two years from early 1928 and is probably the largest effort ever undertaken to study LDEs. The most spectacular result was simultaneous observations in Norway and the Netherlands of echoes of the PCJJ signals at a wavelength of 31.4 meters (9.55 MHz) on 24. October 1928 [v. d. Pol, 1928], see figure to the right. These results convinced most sceptics at the time that the effect is real, and it is still probably the most compelling documentation there is of the existence of long delayed echoes.

Echo signals were also heard in the UK by a group under the leadership of E. V. Appleton (Nobel prize in 1947 for his studies of the ionosphere). In January 1930 the test transmissions from PCJJ were terminated due to the lack of conclusions regarding the mechanism behind the echoes and the cost of the experiment. There were also other experiments that were undertaken at the time, e.g. in France. In 1934 the World Radio Research League also conducted a series of experiments with transmitters in the UK and in Switzerland. There were many observations of echoes, but no new understanding of the phenomenon was gained.

At that time, radio was a novel and unexplored field; it was for instance only a few years since 1924 when the US government had tried to get all transmitters in the country to observe radio silence for 5 minutes every hour for two days to listen for signals from Mars.

I decided to study the topic of long-delayed echoes (LDE) in detail and I went through the papers listed at the end, with the aim of finding out the status of possible natural explanations. This resulted in an essay published on the main site for popular research in Norway. Parts of the fascination and mystique about long-delayed radio echoes are that they are still not properly explained. This has also led to some rather exotic explanations involving extra-terrestrials, listed at the end here.

Natural mechanisms

Shlionskiy [1979] divides possible explanations in two groups: Reflections outside the earth system and effects in the earth's ionosphere or magnetosphere. He lists four hypotheses in the first group and eleven in the second. A mind-boggling fact is that the several of the explanations involve media where radio waves no longer travel in straight lines, or at much lower speeds than 300,000 km/sec. An excellent historical review is given by Muldrew [1979].

I follow Vidmar and Crawford [1985] and discuss here the five most likely explanations, listed roughly according to the frequencies they apply for.

  1. Ducting in the magnetosphere and ionospheric reflection.
    MDE-paths
    Ducts for transmitters in GA (USA), Northern England,
    Hobart (Tasmania), and the longest one for St. Petersburg (Russia)

    Magnetic field lines in the magnetosphere (Illustration by NASA)

    This is the effect which is best understood of all the effects listed here. The radio signal has to pass through the lower ionosphere near the transmitter site. Then it has to be ducted along the earth's magnetic field lines to the other hemisphere of the earth. This duct makes the radiowave travel in a curved path out to a distance of 1 to more than 3 earth radii from the earth's surface. On the other side of the earth, the radio signal is reflected from the upper ionosphere and then it follows the same path back [Muldrew 1979]. The electromagnetic wave will follow the magnetic field line closest to earth on its night-side as shown for different locations in the figure to the right.

    Characteristics: Frequencies in the 1-4 MHz range and duration up to 0.5 sec. The delay varies with the geomagnetic latitude and can be predicted quite accurately. It is longer, the closer to the magnetic North/South pole the transmitter is located. The short delay time places this effect in a different league than the next four effects, and therefore it is more appropriate to call them MDE "medium delay echoes" or "magnetospherically ducted echoes" rather than LDE.

    Observations:

    The effect only occurs during the dark hours, and it is most likely to occur between 1900 and midnight local time during winter months of years of low solar activity, and at about 2 MHz [Villard et al, 1980 and Ellis and Goldstone, 1990].

  2. Travel many times around the world.
    Signals that travel once around the earth are a common phenomenon and occur with a delay of about 1/7 of a second. Signals that travel a few times around the world are also not uncommon (listen to an example here). It is conceivable that a signal can travel many times around the earth and that there may exist mechanisms that cause focusing or amplification of the wave so that attenuation will be much less than expected [Shlionskiy, 1989]. One such mechanism can be explained by considering an omnidirectional antenna. The energy spreads in all directions, but will combine at the opposite point of the earth (antipodal point), and then again at the transmitter site and so on [Appleton, 1928].

    Goodacre [Goodacre, 1980] reports that he pointed his antenna towards the horizon and received his own 28 MHz signal delayed by up to about 9 seconds. A statistical analysis of the delays revealed a periodicity of 0.138 seconds, i.e. the travel time for a signal around the earth. His measurement implies travel up to 65 rounds around the earth. This happened in Nov. and Dec. 1978 and Jan. 1979, i.e. about one year before the peak of solar cycle 21. This frequency is probably about the upper limit that this effect could apply for.

  3. The ionosphere (Illustration by NASA)
  4. Mode conversion involving coupling to mechanical waves in the ionosphere.
    This phenomenon may take place on the top of the ionosphere where the incident electromagnetic wave is coupled to a longitudinal plasma wave of low group velocity (~ 1 km/sec), like an ion acoustic wave. The energy will travel along the magnetic field lines, and an amplification of the plasma wave will take place by beam-plasma interaction. The extent of the region where this could take place is in the order of 10 km. Then a new mode-conversion will take place back to electromagnetic energy. The effect may occur for frequencies near the maximum frequency reflected from the F2 ionospheric layer (f0F2), typically 5-10 MHz, but occasionally up to some 20 MHz.

    This effect was investigated experimentally [Crawford et al 1970] and [Vidmar and Crawford, 1985]. Although several echoes were recorded with delays up to about 40 seconds, the authors do not draw any firm conclusions. The tests took place at 5-12 MHz in 1967-1970, and in Alaska at 5.8 MHz in 1978.

    Lawton and Newton [1974] are also advocates of this theory. They hypothesize that it may be more likely to occur when the trailing Moon-Earth Lagrangian point is above the horizon as described in the next paragraph. They also performed several unsuccessful experiments.

  5. The five Lagrange points, L1-L5, of either
    the Sun-Earth or the Earth-Moon system (Illustration from Wikipedia)
  6. Reflection from distant plasma clouds.
    This hypothesis assumes a cloud of ionized gases and particles, coming originally from the sun. The main problem with this hypothesis is that Doppler shifts may easily be too great and received signals too weak.

    Possible exceptions are if the plasma cloud is located in one of the Earth-Moon Lagrangian points, where mass can be trapped. Candidate points are L4 or L5 (delay about 2.5 sec), which follow the Earth's movement. One point in favor of this hypothesis is the finding of Sassoon [1973] who went through all available observations and found that there was a statistically significant higher occurrence of LDEs when the Earth-Moon trailing Lagrange point (L5) was above the horizon. Another possibility is the Sun-Earth Langrangian point L1 (delay of about 10 sec). The Lagrange points could give echoes for signals that pass through the ionosphere, i.e. mostly high HF and VHF/UHF signals.

    Budden and Yates [1952] conducted experiments from 1947-1949 at 13.5 and 20.7 MHz with vertical transmission to test if they could receive such echoes. In 27000 transmissions they did not detect a single echo, but this could be because they set out to test Størmer's original hypothesis (see no. 1.4 here).

    On the other hand Freyman [1981] did experiments from Alaska in the auroral zone at 9.9 MHz. He wanted to test if radio waves would be guided along magnetic field lines and be reflected off solar plasma. He detected several thousand echoes of delay up to 16 seconds at times when there was a change in the magnetic field and solar plasma probably entered the magnetosphere. His paper shows examples of echoes that were received at the same delay even when transmissions were minutes apart.

  7. Non-linearity in addition to mode conversion.
    Another possible explanation for VHF and UHF echoes was proposed by Muldrew in 1979. It assumes the presence of an unknown second transmitter, and that non-linearity generates a difference frequency that falls in the range of f0F2. The difference frequency is then subject to the former coupling effect to mechanical waves. The difference frequency then propagates as a plasma wave, and then it couples back via the second transmitter's frequency to the original frequency and propagates normally back to the observer.

    This effect could account for larger variations in delay time than the former hypothesis. It could explain radio amateur observations of echoes in the 50, 144, 432 and 1296 MHz bands, especially during attempts at Earth-Moon-Earth communication.

    One UHF example is Hans Rasmussen's report on echoes delayed by 4.6 seconds at 1296 MHz [Rasmussen, 1975]. Another example, documented by a strip-chart recording, is an observation at 432 MHz of a delay of about 5.75 seconds [Yurek, 1978].

Extra-terrestrial explanations

At the dawn of the space age, Bracewell wrote on how an advanced galactic community might communicate to us by using an unmanned space probe. He said:

To ensure use of a wave-length that could both penetrate our ionosphere and be in a band certain to be in use, the probe could first listen for our signals and then repeat them back. To us, its signals would have the appearance of echoes having delays of seconds or minutes, such as were reported thirty years ago by Størmer and van der Pol and never explained. Further: Should we be surprised if the beginning of its message were a television image of a constellation? [Bracewell, 1960].

This suggestion combined with the lack of consensus on the explanation of long delayed echoes, has led to some rather imaginative explanations. They are based on interpretation of the delay times recorded in the first reports of long-delayed echoes.

It is easy to come up with objections to these interpretations:
  1. Measurement round-off. The delays are all integer values, while papers after the early 1930's all report delays with fractional seconds. Some of the early results were obtained using 'oscillographs' for recording signals, which means reading out a distance on a piece of paper or film (e.g. 60 cm/sec). But most of the time they seem to have used rather improvised measurement setups. In addition to Størmer's comment above on accuracy, note what van der Pol said [v. d. Pol, 1928] on how measurements were done during the simultanteous reception of echoes in Oslo and Eindhoven on 24 October 1928, 16-17 UTC (figure above): The timing of the (first set of) observations was done with a stop watch, while for the (second set of) observations the second hand of an ordinary watch was used.
  2. One second unit. The second is not a universal unit, but may be related to human physiology, as a typical heart beat lasts for one second. If the extra-terrestrial civilization knew us so well that they knew our units for time, then they for sure would know a lot of other things about us as well. Why didn't they choose to communicate to us in a much more obvious way, with all that background knowledge about us already?
  3. Probability of sequence. Any interpretation of a sequence of numbers should be based on an underlying calculation of probabilities. How likely is this sequence if the numbers are drawn from a random generator? The delay sequences are relatively short, so I would not be surprised if there is a considerable likelihood that they can be generated from random numbers.
These objections withstanding, I cannot but admire the imagination of the people who have come up with these interpretations. I am not the only one who is fascinated by this, judging from the large number of internet pages that deal with SETI (Search for Extraterrestrial Intelligence) and LDE (Long Delayed Echoes) interpretations, many more than the natural explanations occupy. Just try a search using these words: Størmer (or Störmer, Stormer, Stermer, Schtermer), Hals, LDE, SETI...

Acknowledgement

Thanks to Anne Melgård and Nina Korbu at the National Library of Norway, and Knut Hegna at the University of Oslo Library for help in finding material.

References

  1. C. Størmer, "Short wave echoes and the aurora borealis," Nature, No. 3079, Vol. 122, p. 681, Nov. 3, 1928.
  2. B. v. d. Pol, "Short wave echoes and the aurora borealis," Nature, No. 3084, Vol. 122, pp. 878-879, Dec. 8, 1928.
  3. E. V. Appleton, "Short wave echoes and the aurora borealis," Nature, 122, p. 879, 1928.
  4. J. Hals, "The discovery of echoes of long delay," World Radio (BBC), p 731, 770, 806, 844, 848, in 4 issues in Nov-Dec 1934.
  5. K. G. Budden and G. G. Yates, "A search for radio echoes of long delay," J. Atmos Terr. Phys., 2, pp. 272-281, 1952.
  6. C. Størmer, The Polar Aurora, Oxford University Press, London, 1955.
  7. R. N. Bracewell, "Communications from Superior Galactic Communities," Nature 186, pp 670-671, May 1960.
  8. F. W. Crawford, D. M. Sears, R. L. Bruce, "Possible observations and mechanism of very long delayed radio echoes," Journ. Geophys. Res., vol. 75, no. 34, pp. 7326-7332, Dec. 1970.
  9. D. A. Lunan, "Spaceprobe from Epsilon Bootes," Spaceflight (British Interplanetary Society), 1973.
  10. G. Sassoon, "Correlation of long-delayed radio echoes and the moon's orbit," Spaceflight (British Interplanetary Society), 1974.
  11. A. T. Lawton and S. J. Newton, "Long delayed echoes - the Trojan ionosphere," Journ. British Interplanetary Society, 1974.
  12. H. L. Rasmussen (OZ9CR), "Ghost echoes on the Earth-Moon path," Nature, Vol. 257, p 36, Sept. 4, 1975.
  13. J. Yurek (K3PGP), “Echoes: An amateur observation and a professional reply,” QST, 62, pp. 35-36, May 1978.
  14. D. B. Muldrew, "Generation of long-delay echoes," Journ. Geophys. Res., vol. 84, no. A9, pp. 5199-5215, Sep. 1979.
  15. A. K. Goodacre (VE3HX), "Observations of long-delayed echoes on 28 MHz," QST, March 1980, pp. 14-16.
  16. 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.
  17. O. G. Villard (W6QYT), D. B. Muldrew, and F. W. Waxham (K7DS), "The magnetospheric echo box - A type of long-delayed echo explained," QST, Oct. 1980, pp. 11-14.
  18. 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.
  19. 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.
  20. G. T. Goldstone and G. R. A. Ellis, "Observations of 1.91 MHz echoes from the magnetic conjugate point after propagation through a magneto-ionic duct," Proceedings of the Astronomical Society of Australia, vol. 6, no. 3, 1986, p. 333-335
  21. A. G. Shlionskiy, "Radio echos with multisecond delays," Telecomm. and Radio Eng., Vol 44, No. 12, pp. 48-51, Dec. 1989.
  22. G. R. A. Ellis and G. T. Goldstone, "The probability of observing ducted magnetospheric echoes from the ground," Journ. Geophys. Res., vol. 95, no. A5, pp. 6587-6590, May. 1990.
  23. V. Grassmann (DF5AI), "Long-delayed radio echoes, Observations and interpretations," VHF Communications vol 2, pp. 109-116, 1993.
  24. D. V. Blagoveshchensky, K. A. Dobroselsky, O. A. Maltseva, "Main ionospheric trough as a channel for MF propagation in the magnetosphere," Radio Science, Vol. 32, No. 4, pp 1477-1490, Jul-Aug 1997.
  25. P. Martinez (G3PLX), "Long Delayed Echoes, A Study of Magnetospheric Duct Echoes 1997-2007," Radcom, Oct 2007, pp. 60-63.
  26. S. Holm (LA3ZA), "Magnetospheric ducting as an explanation for delayed 3.5 MHz signals," QST, p. 54, March 2009.
  27. P.-E. Karlshøj (OZ4UN), "Observation of long delayed echoes on 80 meters," QST, pp. 72-73, Nov 2009.
Papers by Muldrew (Refs. 14 and 17) may be downloaded from Gabriel Sampol's (EA6VQ) LDE page (Scroll down to the bottom of the page). This page also contains several interesting accounts of LDEs.

First created 16 March 2004, last updated 5 April 2014.

© Sverre Holm, University of Oslo
(Norwegian radio amateur LA3ZA)

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