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 satellite connection
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).
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, a Western suburb which I pass on my
daily commute to work. The first report was published more than 80 years ago by
Størmer, University of Oslo, known for being the first to
measure the height of the Northern Lights, and it starts like this
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."
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. (With
permission from Norsk
Radiohistorisk Forening (1201))
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]. These results convinced most sceptics at the time that the effect is real.
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 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
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.
Shlionskiy  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 .
I follow Vidmar and Crawford  and discuss here the five most
likely explanations, listed roughly according to the frequencies they
- Ducting in the magnetosphere and ionospheric reflection.
|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.
- A recent report that goes to great depths in explaining them is based on hearing a total of 59 echoes with delay 210-220 ms in the North of England over 10 years on frequencies between 2 and 3.9 MHz [Martinez, 2007]. He also heard double echoes from time to time. (listen to an example from Martinez here).
- Another example of observed delays is: 260-270 ms at 1.91 MHz in Hobart, Tasmania [Goldstone and Ellis, 1986] with simultaneous reception of the echoes from time to time at a second receiver 20 km away from the transmitter and main receiver.
- Other observations of echoes of delay 284-305 ms at 1.8 MHz have been made in St Petersburg, Russia [Blagoveshchensky et al, 1997]. These observations sometimes also had a second echo at twice the travel time (590 ms).
- Here is an observation report on 3.915 MHz from the fall of 1974, which most likely is this effect.
- It may even happen in both the 1.8 and 3.5 MHz bands at the same time (scroll to W1TR's report from the mid 90's).
Here is a report of 214-219 ms echoes heard in New York at 3.530 MHz in February 2008 over a duration of about 50 minutes.
- The effect can probably be heard in the AM broadcast band also, like here at 1200 kHz.
A likely echo of 143 ms at 3.5 MHz from February 2007 in GA, USA is described in [Holm, 2009].
Echoes with delay 237 ms were heard in January 2009 at 1745 local time North of Copenhagen in Denmark at 3.5 MHz [Karslhøj, 2009], listen to audio file where also OZ7BQ very close by heard the echoes.
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].
- 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.
(Illustration by NASA)
- 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  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.
The five Lagrange points, L1-L5,
of either the Sun-Earth or the Earth-Moon system
(Illustration from Wikipedia)
- 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  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  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  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.
- 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].
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:
- Message from extra-terrestrials located in the trailing moon-earth Lagrangian
point (L5) with a message about
their origin. This theory builds on Bracewell's suggestion. In the plot are the delays recorded by van der Pol on 11 October 1928, 20-21 local time in the Netherlands [v. d. Pol, 1928]. The points plotted to the right of the axis at a delay of 8 seconds are
supposedly how the constellation Bootes
looked 12,600 years ago [Lunan, 1973]. To achieve this image, the point for delay no
6 at 3 seconds, has to be moved to 13 seconds (the red point). See original figure halfway down this page. This kind of data massaging is usually not considered to be good science!
To be fair it seems that Duncan Lunan later withdrew this theory, see Wikipedia article.
- Here is another
interpretation of the same data set by Rashid
Faizullin, leading to an arrow pointing in space to the origin of the civilization. A translation of the text in the figure is: "The arrow which is shown here in the equatorial spherical coordinate system points to the stars with numbers that correspond to the fifth Størmer series: 8-Procyon, 3-Alpha Centauri, 11-Altair, 12-Betelgeuse, 13-Alpha of the Southern Cross, 15-Pollux (Hipparcos). The arrow is preserved in the ecliptic spherical coordinate system." The delay times are used to look up stars according to their brightness in one version of the ordered list of the brightest stars.
(Russian language report)
- Filipenko uses the integer delays as a means of looking up
elements in the periodic table, and finds that extra-terrestrials may
have communicated to us already in 1928 how to make modern
semiconductors. The five delay time series that Filipenko uses were recorded by Størmer on 11. October 1928, 15.45-16.00 UTC [Størmer, 1955]. Note Størmer's words on accuracy: I noted in seconds the following intervals of time between signals and echo: ... The times noted by me can lay no claim to great accuracy, because I was not adequately prepared, but in any case they give a qualitative idea of the phenomenon.
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,
- 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): 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.
- 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?
- 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.
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.
Papers by Muldrew (Refs. 14 and 17) may be downloaded from Gabriel Sampol's (EA6VQ)
(Scroll down to the bottom of the page). This page also contains
several interesting accounts of LDEs.
- C. Størmer, "Short wave echoes and the aurora borealis,"
Nature, No. 3079, Vol. 122, p. 681, Nov. 3, 1928.
- B. v. d. Pol, "Short wave echoes and the aurora borealis,"
Nature, No. 3084, Vol. 122, pp. 878-879, Dec. 8, 1928.
- E. V. Appleton, "Short wave echoes and the aurora borealis," Nature, 122, p. 879, 1928.
- 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.
- K. G. Budden and G. G. Yates, "A search for radio echoes of long
delay," J. Atmos Terr. Phys., 2, pp. 272-281, 1952.
- C. Størmer, The Polar Aurora, Oxford University Press, London, 1955.
- R. N. Bracewell, "Communications from Superior Galactic Communities," Nature 186, pp 670-671, May 1960.
- 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.
- D. A. Lunan, "Spaceprobe from Epsilon Bootes," Spaceflight (British Interplanetary Society), 1973.
- G. Sassoon, "Correlation of long-delayed radio echoes and the moon's orbit," Spaceflight (British Interplanetary Society), 1974.
- A. T. Lawton and S. J. Newton, "Long delayed echoes - the Trojan ionosphere," Journ. British Interplanetary Society, 1974.
- H. L. Rasmussen (OZ9CR), "Ghost echoes on the Earth-Moon path," Nature,
Vol. 257, p 36, Sept. 4, 1975.
- J. Yurek (K3PGP), “Echoes: An amateur observation and a professional reply,” QST, 62, pp. 35-36, May 1978.
- D. B. Muldrew, "Generation of long-delay echoes," Journ. Geophys.
Res., vol. 84, no. A9, pp. 5199-5215, Sep. 1979.
- A. K. Goodacre (VE3HX), "Observations of long-delayed echoes on 28 MHz,"
QST, March 1980, pp. 14-16.
- 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.
- 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.
- R. W. Freyman, "Measurements of long delayed radio echoes in the
auroral zone," Geophys. Res. Letters, Vol. 8, No. 4, pp. 385-388, April
- 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.
- 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
- A. G. Shlionskiy, "Radio echos with multisecond delays,"
Telecomm. and Radio Eng., Vol 44, No. 12, pp. 48-51, Dec. 1989.
- 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.
- V. Grassmann (DF5AI), "Long-delayed
radio echoes, Observations and interpretations," VHF Communications
vol 2, pp. 109-116, 1993.
- 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
- P. Martinez (G3PLX), "Long Delayed Echoes, A Study of Magnetospheric Duct Echoes 1997-2007,"
Radcom, Oct 2007, pp. 60-63.
- S. Holm (LA3ZA), "Magnetospheric ducting as an explanation for delayed 3.5 MHz signals," QST, p. 54, March 2009.
- P.-E. Karlshøj (OZ4UN), "Observation of long delayed echoes on 80 meters," QST, pp. 72-73, Nov 2009.
First created 16 March 2004, last updated 9 June 2012.
© Sverre Holm, University of Oslo
(Norwegian radio amateur LA3ZA)