Interplanetary scintillation
Encyclopedia
In astronomy
, interplanetary scintillation refers to random fluctuations in the intensity of radio wave
s of celestial
origin, on the timescale of a few seconds. It is analogous to the twinkling
one sees looking at star
s in the sky
at night, but in the radio part of the electromagnetic spectrum
rather than the visible one
. Interplanetary scintillation is the result of radio waves traveling through fluctuations in the density of the electron
and proton
s that make up the solar wind
.
was observed as early as 1951 by Antony Hewish
, and he then reported irregularities in radiation received during an observation of a bright radio source in Taurus
in 1954. Hewish considered various possibilities, and suggested that irregularities in the solar corona would cause scattering
by refraction
and could produce the irregularities he observed. A decade later, while making astrometric observations of several bright sources of celestial radio waves using a radio interferometer, Hewish and two collaborators reported "unusual fluctuations of intensity" in a few of the sources. The data strongly supported the notion that the fluctuations resulted from irregularities in the density of the plasma
associated with the solar wind
, which the authors called interplanetary scintillation, and is recognized as the "discovery of the interplanetary scintillation phenomenon."
In order to study interplanetary scintillation, Hewish built the Interplanetary Scintillation Array
at the Mullard Radio Astronomy Observatory
. The array consisted of 2,048 dipole
s over almost five acre
s of land, and was built to constantly survey the sky at a time resolution of about 0.1 second
s. This high time resolution set it apart from many other radio telescope
s of the time, as astronomers did not expect emission from an object to feature such rapid variation. Soon after observations were under way, Hewish's student Jocelyn Bell turned this assumption on its head, when she noticed a signal which was soon recognized as emanating from a new class of object, the pulsar
. Thus "it was an investigation of interplanetary scintillation that led to the discovery of pulsars, even though the discovery was a by-product rather than the purpose of the investigation."
of the medium through which waves are traveling. The solar wind
is a plasma
, composed primarily of electron
s and lone proton
s, and the variations in the index of refraction are caused by variations in the density
of the plasma. Different indices of refraction result in phase
changes between waves traveling through different locations, which results in interference. As the waves interfere, both the frequency
of the wave and its angular size are broadened, and the intensity varies.
, measurements of interplanetary scintillation "be utilized as valuable and inexpensive probes of the solar wind." As already noted, the observed information, the intensity fluctuations, is related to the desired information, the structure of the solar wind, through the phase change experienced by waves traveling through the solar wind. The root mean square
(RMS) intensity fluctuations are often expressed relative to the mean intensity from the source, in a term called the scintillation index, which is written as
This can be related to the phase deviation caused by turbulence in the solar wind by considering the incident electromagnetic
plane wave, and yields
The next step, relating the phase change to the density structure of the solar wind, can be made more simple by assuming that the density of the plasma is highest towards the sun, which allows the "thin screen approximation." Doing so eventually gives an RMS deviation for the phase of
where
is the wavelength of the incoming wave, 
is the classical electron radius
,
is the thickness of the "screen," or the length scale over which the majority of the scattering takes place, 
is the typical size scale of density irregularities, and 
is the root mean squared variation of the electron density about the mean density. Thus interplanetary scintillation can be used as a probe of the density of the solar wind. Interplanetary scintilation measurements may also be used to infer the velocity of the solar wind.
Stable features of the solar wind can be particularly well studied. At a given time, observers on Earth
have a fixed line of sight through the solar wind, but as the Sun rotates over an approximately month long period, the perspective on Earth changes. It is then possible to do "tomographic reconstruction
of the distribution of the solar wind" for the features of the solar wind which remain static.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, interplanetary scintillation refers to random fluctuations in the intensity of radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s of celestial
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...
origin, on the timescale of a few seconds. It is analogous to the twinkling
Scintillation (astronomy)
Scintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through a medium, most commonly the atmosphere ....
one sees looking at star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s in the sky
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...
at night, but in the radio part of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
rather than the visible one
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. Interplanetary scintillation is the result of radio waves traveling through fluctuations in the density of the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
and proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s that make up 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...
.
Early study
Scintillation in radio waves due to the ionosphereIonosphere
The 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...
was observed as early as 1951 by Antony Hewish
Antony Hewish
Antony Hewish FRS is a British radio astronomer who won the Nobel Prize for Physics in 1974 for his work on the development of radio aperture synthesis and its role in the discovery of pulsars...
, and he then reported irregularities in radiation received during an observation of a bright radio source in Taurus
Taurus (constellation)
Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:...
in 1954. Hewish considered various possibilities, and suggested that irregularities in the solar corona would cause scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
by refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
and could produce the irregularities he observed. A decade later, while making astrometric observations of several bright sources of celestial radio waves using a radio interferometer, Hewish and two collaborators reported "unusual fluctuations of intensity" in a few of the sources. The data strongly supported the notion that the fluctuations resulted from irregularities in the density of the plasma
Plasma (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...
associated 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...
, which the authors called interplanetary scintillation, and is recognized as the "discovery of the interplanetary scintillation phenomenon."
In order to study interplanetary scintillation, Hewish built the Interplanetary Scintillation Array
Interplanetary Scintillation Array
The Interplanetary Scintillation Array was built at the Mullard Radio Astronomy Observatory in 1967 and originally covered four acres . It was extended in 1978 to nine, and re-furbished in 1989. It operates at 81.5 MHz , and is made up of 4096 dipoles in a phased array. 14 beams can map the...
at the Mullard Radio Astronomy Observatory
Mullard Radio Astronomy Observatory
Mullard Radio Astronomy Observatory is home to a number of large aperture synthesis radio telescopes, including the One-Mile Telescope, 5-km Ryle Telescope, and the Arcminute Microkelvin Imager...
. The array consisted of 2,048 dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
s over almost five acre
Acre
The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land.The acre is related...
s of land, and was built to constantly survey the sky at a time resolution of about 0.1 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. This high time resolution set it apart from many other radio telescope
Radio telescope
A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
s of the time, as astronomers did not expect emission from an object to feature such rapid variation. Soon after observations were under way, Hewish's student Jocelyn Bell turned this assumption on its head, when she noticed a signal which was soon recognized as emanating from a new class of object, the pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
. Thus "it was an investigation of interplanetary scintillation that led to the discovery of pulsars, even though the discovery was a by-product rather than the purpose of the investigation."
Cause
Scintillation occurs as a result of variations in the refractive indexRefractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of the medium through which waves are traveling. 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 a plasma
Plasma (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...
, composed primarily of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and lone proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s, and the variations in the index of refraction are caused by variations in the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the plasma. Different indices of refraction result in phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
changes between waves traveling through different locations, which results in interference. As the waves interfere, both the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the wave and its angular size are broadened, and the intensity varies.
Solar wind
As interplanetary scintillation is caused by the solar windSolar 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...
, measurements of interplanetary scintillation "be utilized as valuable and inexpensive probes of the solar wind." As already noted, the observed information, the intensity fluctuations, is related to the desired information, the structure of the solar wind, through the phase change experienced by waves traveling through the solar wind. The root mean square
Root mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
(RMS) intensity fluctuations are often expressed relative to the mean intensity from the source, in a term called the scintillation index, which is written as
This can be related to the phase deviation caused by turbulence in the solar wind by considering the incident electromagnetic
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
plane wave, and yields
The next step, relating the phase change to the density structure of the solar wind, can be made more simple by assuming that the density of the plasma is highest towards the sun, which allows the "thin screen approximation." Doing so eventually gives an RMS deviation for the phase of
where
is the wavelength of the incoming wave, is the classical electron radiusClassical electron radius
The classical electron radius, also known as the Lorentz radius or the Thomson scattering length, is based on a classical relativistic model of the electron...
,
is the thickness of the "screen," or the length scale over which the majority of the scattering takes place, is the typical size scale of density irregularities, and is the root mean squared variation of the electron density about the mean density. Thus interplanetary scintillation can be used as a probe of the density of the solar wind. Interplanetary scintilation measurements may also be used to infer the velocity of the solar wind.Stable features of the solar wind can be particularly well studied. At a given time, observers on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
have a fixed line of sight through the solar wind, but as the Sun rotates over an approximately month long period, the perspective on Earth changes. It is then possible to do "tomographic reconstruction
Tomographic reconstruction
The mathematical basis for tomographic imaging was laid down by Johann Radon. It is applied in computed tomography to obtain cross-sectional images of patients...
of the distribution of the solar wind" for the features of the solar wind which remain static.