Long delayed echo
Encyclopedia
Long delayed echoes are radio
echoe
s which return to the sender several seconds after a radio transmission has occurred. Delays of longer than 2.7 second
s are considered LDEs. LDEs are scientific anomalies
of uncertain origin.
, 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:
The pair, joined by physicist Balthasar van der Pol
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).
Long delayed echoes have been heard sporadically from the first observations in 1927 and up to our time.
, details those five; in summary,
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.
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. This concept is also addressed by Holm.
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
echoe
Echo (phenomenon)
In audio signal processing and acoustics, an echo is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single...
s which return to the sender several seconds after a radio transmission has occurred. Delays of longer than 2.7 second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s are considered LDEs. LDEs are scientific anomalies
Anomaly
Anomaly may refer to:-Astronomy and celestial mechanics :* In astronomy, an anomaly is a quantity measured with respect to an apsis, usually the periapsis...
of uncertain origin.
History
These echoes were first observed in 1927 by civil engineer Jørgen Hals from his home near OsloOslo
Oslo is a municipality, as well as the capital and most populous city in Norway. As a municipality , it was established on 1 January 1838. Founded around 1048 by King Harald III of Norway, the city was largely destroyed by fire in 1624. The city was moved under the reign of Denmark–Norway's King...
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
. 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
Carl Størmer
Fredrik Carl Mülertz Størmer was a Norwegian mathematician and physicist, known both for his work in number theory and for studying the movement of charged particles in the magnetosphere and the formation of aurorae....
, 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.
The pair, joined by physicist Balthasar van der Pol
Balthasar van der Pol
Balthasar van der Pol was a Dutch physicist.Van der Pol studied physics in Utrecht, and in 1920 he was awarded his doctorate . He studied experimental physics with John Ambrose Fleming and Sir J. J. Thomson in England...
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).
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. Vidmar and Crawford suggest five of them are the most likely. Sverre Holm, professor of signal processing at the Centre for Imaging, University of OsloUniversity of Oslo
The University of Oslo , formerly The Royal Frederick University , is the oldest and largest university in Norway, situated in the Norwegian capital of Oslo. The university was founded in 1811 and was modelled after the recently established University of Berlin...
, details those five; in summary,
- Ducting in the Earth's magnetosphere and ionosphereIonosphereThe ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
at low HF frequenciesHigh frequencyHigh frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
(1-4 MHz).. Some similarities with Whistlers.
- 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.
- 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 layers in the atmosphereAtmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
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 atmosphereAtmospheric ductIn telecommunication, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals are guided or ducted, tend to follow the curvature of the Earth, and experience less attenuation in the ducts than they would if...
is a well-understood phenonemon. See Radio propagationRadio propagationRadio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
.)- Mode conversion: Signals couple to plasmaPlasma (physics)In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
waves in the upper ionosphere.
- Mode conversion: Signals couple to plasma
- Investigated experimentally by Crawford et. al.; they recorded echoes with delays up to 40 seconds at 5-12 MHz..
- 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, then is converted back.
- Could explain amateur VHF/UHF echoes. Hans Rasmussen found echoes delayed by 4.6 seconds at 1296 MHz, Yurek recorded a 5.75 second delay at 432 MHz.
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
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
is still evolving.
Alternative hypotheses
Some believe that the auroraAurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
activity that follows a solar storm
Solar storm
Solar storm can refer to:*Solar flare, a large explosion in the Sun's atmosphere*Coronal mass ejection , a massive burst of solar wind associated with solar flares*Geomagnetic storm, the interaction of the Sun's outburst with Earth's magnetic field...
is the source of LDEs.
Still others believe that LDEs are double EME
EME (communications)
Earth-Moon-Earth, also known as moon bounce, is a radio communications technique which relies on the propagation of radio waves from an Earth-based transmitter directed via reflection from the surface of the Moon back to an Earth-based receiver....
(EMEME) reflections, i.e. the signal is reflected by the moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
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
Duncan Lunan
Duncan Alasdair Lunan, who hails from Troon, born on the 24 October 1945 is a Scottish author, with emphasis on astronomy, spaceflight and science fiction, as well as astronomer, science reporter, and teacher...
proposed the radio echoes observed by Størmer and Van der Pol in 1928 might have been transmissions from a Bracewell probe
Bracewell probe
A Bracewell probe is a hypothetical concept for an autonomous interstellar space probe dispatched for the express purpose of communication with one or more alien civilizations. It was proposed by Ronald N...
, an artifact of aliens
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...
trying to communicate with us by bouncing back our own signals. This concept is also addressed by Holm.