Interplanetary medium
Encyclopedia
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s, asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s and comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
s move.
Composition and physical characteristics
The interplanetary medium includes interplanetary dustInterplanetary dust cloud
The interplanetary dust cloud is cosmic dust which pervades the space between planets in the Solar System and in other planetary systems...
, cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s and hot 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...
from 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...
. The temperature of the interplanetary medium is approximately 100,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, and its density is very low at about 5 particles per cubic centimeter in the vicinity of the 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...
; it decreases with increasing distance from the sun, in inverse proportion to the square of the distance.
The density is variable, and may be affected by magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s and events such as coronal mass ejection
Coronal mass ejection
A coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
s. It may rise to as high as 100 particles/cm³.
Since the interplanetary medium 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...
, it has the characteristics of a plasma, rather than a simple gas; for example, it carries with it the Sun's magnetic field, is highly electrically conductive (resulting in the Heliospheric current sheet
Heliospheric current sheet
The heliospheric current sheet is the surface within the Solar System where the polarity of the Sun's magnetic field changes from north to south. This field extends throughout the Sun's equatorial plane in the heliosphere. The shape of the current sheet results from the influence of the Sun's...
), forms plasma double layer
Double layer (plasma)
A double layer is a structure in a plasma and consists of two parallel layers with opposite electrical charge. The sheets of charge cause a strong electric field and a correspondingly sharp change in voltage across the double layer. Ions and electrons which enter the double layer are accelerated,...
s where it comes into contact with a planetary magnetosphere or at the heliopause, and exhibits filamentation (such as in aurora
Aurora (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...
).
The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's 10−4 tesla magnetic dipole field would reduce with the cube of the distance to about 10−11 tesla. But satellite observations show that it is about 100 times greater at around 10−9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like a MHD dynamo.
Extent of the interplanetary medium
The outer edge of the solar system is the boundary between the flow of the solar wind and the interstellar mediumInterstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
. This boundary is known as the heliopause and is believed to be a fairly sharp transition of the order of 110 to 160 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s from the sun. The interplanetary medium thus fills the roughly spherical volume contained within the heliopause.
Interaction with planets
How the interplanetary medium interacts with planets depends on whether they have magnetic fields or not. Bodies such as the MoonMoon
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...
have no magnetic field and the solar wind can impact directly on their surface. Over billions of years, the lunar regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
has acted as a collector for solar wind particles, and so studies of rocks from the moon's surface can be valuable in studies of the solar wind.
High energy particles from the solar wind impacting on the Moon's surface also cause it to emit faintly at X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
wavelengths.
Planets with their own magnetic field, such as the Earth and Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, are surrounded by a magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
within which their magnetic field is dominant over the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's. This disrupts the flow of the solar wind, which is channelled around the magnetosphere. Material from the solar wind can 'leak' into the magnetosphere, causing aurora
Aurora (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...
e and also populating the Van Allen Belts with ionised material.
Observable phenomena of the interplanetary medium
The interplanetary medium is responsible for several effects which can be seen from earth. The Zodiacal lightZodiacal light
Zodiacal light is a faint, roughly triangular, whitish glow seen in the night sky which appears to extend up from the vicinity of the sun along the ecliptic or zodiac. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders...
is a broad band of faint light sometimes seen after sunset and before sunrise, stretched along the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
and brightest near the horizon. It is caused by sunlight scattering off dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
particles in the interplanetary medium between the Earth and the Sun.
A similar effect is the Gegenschein
Gegenschein
The gegenschein is a faint brightening of the night sky in the region of the antisolar point.- Explanation :Like the zodiacal light, the gegenschein is sunlight reflected by interplanetary dust...
, which is seen directly opposite to the sun's position in the sky. It is much fainter than the Zodiacal light, and is caused by sunlight reflecting off dust particles outside the Earth's orbit.
History
The term "interplanetary" appears to have been first used in print in 1691 by the scientist Robert BoyleRobert Boyle
Robert Boyle FRS was a 17th century natural philosopher, chemist, physicist, and inventor, also noted for his writings in theology. He has been variously described as English, Irish, or Anglo-Irish, his father having come to Ireland from England during the time of the English plantations of...
: "The air is different from the æther (or vacuum) in the... interplanetary spaces" Boyle Hist. Air.
The notion that space is considered to be a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
filled with an "aether
Aether (classical element)
According to ancient and medieval science aether , also spelled æther or ether, is the material that fills the region of the universe above the terrestrial sphere.-Mythological origins:...
", or just a cold, dark vacuum continued up until the 1950s (see below).
In 1898, American astronomer Charles Augustus Young
Charles Augustus Young
Charles Augustus Young was an American astronomer.He graduated from Dartmouth and later became a professor there in 1865, remaining until 1877 when he went to Princeton....
wrote: "Inter-planetary space is a vacuum, far more perfect than anything we can produce by artificial means..." (The Elements of Astronomy, Charles Augustus Young, 1898).
And Akasofu recounted that: "The view that interplanetary space is a vacuum into which the Sun intermittently emitted corpuscular streams was changed radically by Ludwig Biermann
Ludwig Biermann
Ludwig Franz Benedict Biermann was a German astronomer.He made important contributions to astrophysics and plasma physics...
(1951, 1953) who proposed on the basis of comet tails, that the Sun continuously blows its atmosphere out in all directions at supersonic speed" (Syun-Ichi Akasofu
Syun-Ichi Akasofu
is the Founding Director of the International Arctic Research Center of the University of Alaska Fairbanks , serving in that position from the center's establishment in 1998 until January 2007. Previously he had been director of the university's Geophysical Institute from 1986.-Background:Akasofu...
, Exploring the Secrets of the Aurora, 2002)
Tufts University Professor of astronomy, Kenneth R. Lang, writing in 2000 noted, "Half a century ago, most people visualized our planet as a solitary sphere traveling in a cold, dark vacuum of space around the Sun".
See also
- Interplanetary space
- Interplanetary dust
- Interplanetary dust cloudInterplanetary dust cloudThe interplanetary dust cloud is cosmic dust which pervades the space between planets in the Solar System and in other planetary systems...
- Interplanetary magnetic fieldInterplanetary Magnetic FieldThe interplanetary magnetic field is the term for the solar magnetic field carried by the solar wind among the planets of the Solar System....
- Interstellar spaceInterstellar SpaceInterstellar Space was one of the final studio albums recorded by the saxophonist John Coltrane before his death in 1967, originally-released posthumously by Impulse! Records on LP in 1974.-Composition:...
- Interstellar mediumInterstellar mediumIn astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
- interstellar dust
- Intergalactic space
- Intergalactic medium
- Intergalactic dustIntergalactic dustIntergalactic dust is cosmic dust in between galaxies in intergalactic space. Evidence for intergalactic dust has been suggested as early as 1949, and study of it grew throughout the late 20th century. There are large variations in the distribution of intergalactic dust...