Long delayed echoes (LDEs) are

radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...

echoes which return to the sender several seconds after a radio transmission has occurred. Delays of longer than 2.7

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

s are considered LDEs.
ARRL: Stan Horzepa, ''Radio Ghosts'' (dead link. use https://web.archive.org/web/20031105155129/http://www.arrl.org/news/features/2003/10/31/1/) LDEs are considered Anomaly (natural sciences), anomalous and have a number of proposed scientific origins.

History

These echoes were first observed in 1927 by civil engineer and

amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...

operator Jørgen Hals from his home near

Oslo Oslo ( or ; ) is the capital and most populous city of Norway. It constitutes both a county and a municipality. The municipality of Oslo had a population of in 2022, while the city's greater urban area had a population of 1,064,235 in 2022 ...

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...

. 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 Fredrik Carl Mülertz Størmer () (3 September 1874 – 13 August 1957) was a Norwegian mathematician and astrophysicist. In mathematics, he is known for his work in number theory, including the calculation of and Størmer's theorem on con ...

, 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.

Physicist Balthasar van der Pol helped Hals and Stormer investigate the echoes, but due to the sporadic nature of the echo events and variations in time-delay, did not find a suitable explanation.
V. Grassmann, ''Long-delayed radio echoes, Observations and interpretations,'' VHF Communications, vol. 2, pp. 109-116, 1993. Long delayed echoes have been heard sporadically from the first observations in 1927 and up to the present day.

Five hypotheses

Shlionskiy lists 15 possible ''natural'' explanations in two groups: reflections in outer space, and reflections within the Earth's magnetosphere. Vidmar and Crawford suggest five of them are the most likely.R. J. Vidmar and F. W. Crawford, "Long-delayed radio echoes: Mechanisms and observations," Journal of Geophysical Research, vol. 90, no. A2, pp. 1523–1530, February 1985. Sverre Holm, professor of signal processing at the

University of Oslo The University of Oslo (; ) is a public university, public research university located in Oslo, Norway. It is the List of oldest universities in continuous operation#Europe, oldest university in Norway. Originally named the Royal Frederick Univ ...

details those five;Sverre Holm, ''The Five Most Likely Explanations for Long Delayed Echoes''
/ref> in summary, * Ducting in the

Earth's magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...

and

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

at low HF frequencies (1–4 MHz). Some similarities with

whistlers Whistler may refer to: * Someone who whistles Places Canada * Whistler, British Columbia, a resort town ** Whistler railway station ** Whistler Secondary School * Whistler Blackcomb, a ski resort in British Columbia * Whistler Mountain, Bri ...

. :Signals may pass the ionosphere and then be conducted 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 Geomagnetic latitude, or magnetic latitude (MLAT), is a parameter analogous to geographic latitude, except that, instead of being defined relative to the geographic poles, it is defined by the axis of the geomagnetic dipole, which can be accurat ...

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. Radio waves of frequency less than about 7 MHz can become trapped in magnetic field-aligned ionization ducts with L values (distance from the center of the Earth to the

field line A field line is a graphical Scientific visualization, visual aid for visualizing vector fields. It consists of an imaginary integral curve which is tangent to the field Euclidean vector, vector at each point along its length. A diagram showing ...

at the magnetic equator) less than about 4. These waves after being trapped can propagate to the opposite hemisphere where they become reflected in the topside ionosphere. They can return along the duct, leave it, and propagate to the receiver. * Travel many times around the world. Signals can travel around the Earth seven times in one second. Such signals are also not uncommon. :"Goodacre 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 Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...

layers in the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

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 phenomenon. See

Radio propagation Radio propagation is the behavior of radio waves as they travel, or are wave propagation, propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio w ...

.) * Mode conversion: Signals couple to plasma waves in the upper ionosphere. :Investigated experimentally by Crawford et al., they recorded echoes with delays up to 40 seconds at 5–12 MHz. The signals from two separated transmitters T1 and T2, T2 transmitting CW or quasi-CW signals, interact nonlinearly in the ionosphere or magnetosphere. If the

wave vector In physics, a wave vector (or wavevector) is a vector used in describing a wave, with a typical unit being cycle per metre. It has a magnitude and direction. Its magnitude is the wavenumber of the wave (inversely proportional to the wavelength) ...

and frequency of the forced oscillation at the difference frequency of the two signals satisfies the dispersion relation for electrostatic waves, such waves would exist and begin to propagate. This wave could grow in amplitude due to wave-particle interaction. At a later time it could interact with the CW signal and propagate to T1. * Reflection from distant plasma clouds coming originally from the Sun. :Freyman 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. * Non-linearity in addition to mode conversion. Two transmitted signals combine to generate a difference frequency, which travels with a plasma wave, and then it is converted back. : It could explain amateur VHF/

UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...

echoes. Hans Rasmussen found echoes delayed by 4.6 seconds at 1296 MHz, and Yurek recorded a 5.75 second delay at 432 MHz.

Alternative hypotheses

Some believe that the

aurora An aurora ( aurorae or auroras), also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in Earth's sky, predominantly observed in high-latitude regions (around the Arc ...

activity that follows a

solar storm A solar storm is a disturbance on the Sun, which can emanate outward across the heliosphere, affecting the entire Solar System, including Earth and its magnetosphere, and is the cause of space weather in the short-term with long-term patterns compr ...

is the source of LDEs. Still others believe that LDEs are double EME (EMEME) reflections, i.e. the signal is reflected by the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

and that reflected signal is reflected by the Earth back to the Moon and reflected again by the Moon back to the Earth. When discussing the use of automated probes as a potential means of contact with extraterrestrial civilizations, American physicist

Ronald Bracewell Ronald Newbold Bracewell AO (22 July 1921 – 12 August 2007) was the Lewis M. Terman Professor of Electrical Engineering of the Space, Telecommunications, and Radioscience Laboratory at Stanford University. Education Bracewell was bor ...

proposed that such probes might try to attract attention by sending back to us our own signals, citing the long delayed echoes as a possible case. This concept was expanded upon by

Duncan Lunan Duncan Alasdair Lunan, born October 1945, is a Scottish author with emphasis on astronomy, spaceflight and science fiction, undertaking a wide range of writing and speaking on those and other topics as a researcher, tutor, critic, editor, lecture ...

, and also addressed by Holm.

Deception

Volker Grassmann writing in ''VHF Communications'' noted the possibility of individuals hoaxing LDEs, saying, "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." To reduce the possibilities of errors or hoaxes a worldwide logging system has been developed.Long Delayed Echo detection automated
/ref>

Notes

References

* J. H. Dellinger, "Observations on Long-Delay Radio Echoes," ST, 8, 42, 88 (1934). * K. C. Budden, C. C. Yates, "A Search for Radio Echoes of Long Delay," Journal of Atmospheric and Terrestrial Physics, 2, 272-281 1952). * O. G. Villard, Jr., A. F. Fraser-Smith, R. T. Cassan, "LDE's, Hoaxes, and the Cosmic Repeater Hypothesis,,' ST, 55, 54-58 (1971) * A. T. Lawton, S. J. Newton, "Long Delayed Echoes: The Search for a Solution," Spaceflight, 16, 181-187, 195 (1974). * George Sassoon, "A Correlation of Long-Delay Radio Echoes and the Moon's Orbit," Spaceflight, 16, 258-264 (1974). * 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. * 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 * Ellis, G. R. A. and G. T. Goldstone, "Observations of long delayed echoes", Journal of Atmospheric and Terrestrial Physics, v49 #10 (1987) pp. 999–1005. * P. Martinez (G3PLX), "Long Delayed Echoes, A Study of Magnetospheric Duct Echoes 1997-2007," Radcom, Oct 2007, pp. 60–63. * Duncan A. Lunan, "Space Probe from Epsilon Boötis" ''Spaceflight'', 16:122-31 (April 1973) *Muldrew, D. B., Generation of long delay echoes, Journal of Geophysical Research, vol. 84, 5199–5215, 1979.

External links

Propagation Studies, Dr. Volker Grassmann, DF5AI.NET (3 papers)

{{DEFAULTSORT:Long Delayed Echo Radio frequency propagation Unsolved problems in physics Unexplained phenomena