Interplanetary medium

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The heliospheric current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium. Heliospheric-current-sheet.gif
The heliospheric current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium.

The interplanetary medium (IPM) consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move. The IPM stops at the heliopause, outside of which the interstellar medium begins. Before 1950, interplanetary space was widely considered to either be an empty vacuum, or consisting of "aether".


Composition and physical characteristics

The interplanetary medium includes interplanetary dust, cosmic rays, and hot plasma from the solar wind. [2] [ failed verification ] The temperature of the interplanetary medium varies. For dust particles within the asteroid belt, typical temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU. [3] The density of the interplanetary medium is very low, decreasing in inverse proportion to the square of the distance from the Sun. It is variable, and may be affected by magnetic fields and events such as coronal mass ejections. At about 5 particles per cubic centimeter in the vicinity of the Earth,[ citation needed ] it may rise to as high as 100 particles/cm3.

Since the interplanetary medium is a plasma, or gas of ions, the interplanetary medium has the characteristics of a plasma, rather than a simple gas. For example, it carries the Sun's magnetic field with it, is highly electrically conductive (resulting in the heliospheric current sheet), forms plasma double layers where it comes into contact with a planetary magnetosphere or at the heliopause, and exhibits filamentation (such as in aurorae).

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 generate magnetic fields, and in this respect it behaves like an MHD dynamo.

Extent of the interplanetary medium

The outer edge of the heliosphere is the boundary between the flow of the solar wind and the interstellar medium. This boundary is known as the heliopause and is believed to be a fairly sharp transition of the order of 110 to 160 astronomical units 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 Moon have no magnetic field and the solar wind can impact directly on their surface. Over billions of years, the lunar regolith has acted as a collector for solar wind particles, and so studies of rocks from the lunar surface can be valuable in studies of the solar wind.

High-energy particles from the solar wind impacting on the lunar surface also cause it to emit faintly at X-ray wavelengths.

Planets with their own magnetic field, such as the Earth and Jupiter, are surrounded by a magnetosphere within which their magnetic field is dominant over the Sun'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 aurorae and also populating the Van Allen radiation belts with ionised material.

Observable phenomena of the interplanetary medium

The interplanetary medium is responsible for several optical phenomena visible from Earth. Zodiacal light is a broad band of faint light sometimes seen after sunset and before sunrise, stretched along the ecliptic and appearing brightest near the horizon. This glow is caused by sunlight scattered by dust particles in the interplanetary medium between Earth and the Sun.

A similar phenomenon centered at the antisolar point, gegenschein is visible in a naturally dark, moonless night sky. Much fainter than zodiacal light, this effect is caused by sunlight backscattered by dust particles beyond Earth's orbit.


The term "interplanetary" appears to have been first used in print in 1691 by the scientist Robert Boyle: "The air is different from the æther (or vacuum) in the... interplanetary spaces" Boyle Hist. Air. In 1898, American astronomer Charles Augustus Young 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).

The notion that space is considered to be a vacuum filled with an "aether", or just a cold, dark vacuum continued up until the 1950s. 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". [4] In 2002, Akasofu stated "The view that interplanetary space is a vacuum into which the Sun intermittently emitted corpuscular streams was changed radically by Ludwig Biermann (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, Exploring the Secrets of the Aurora, 2002)

See also

Related Research Articles


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Magnetosphere Region around an astronomical object in which its magnetic field affects charged particles

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Solar wind Stream of charged particles released from stars

The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of materials found in the solar plasma: trace amounts of heavy ions and atomic nuclei C, N, O, Ne, Mg, Si, S, and Fe. There are also rarer traces of some other nuclei and isotopes such as P, Ti, Cr, Ni, Fe 54 and 56, and Ni 58,60,62. Embedded within the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field.

A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft. A magnetic sail could also thrust directly against planetary and solar magnetospheres.

Hannes Alfvén

Hannes Olof Gösta Alfvén was a Swedish electrical engineer, plasma physicist and winner of the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics (MHD). He described the class of MHD waves now known as Alfvén waves. He was originally trained as an electrical power engineer and later moved to research and teaching in the fields of plasma physics and electrical engineering. Alfvén made many contributions to plasma physics, including theories describing the behavior of aurorae, the Van Allen radiation belts, the effect of magnetic storms on the Earth's magnetic field, the terrestrial magnetosphere, and the dynamics of plasmas in the Milky Way galaxy.

Outer space Void between celestial bodies

Outer space is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins. The plasma between galaxies accounts for about half of the baryonic (ordinary) matter in the universe; it has a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins. Local concentrations of matter have condensed into stars and galaxies. Studies indicate that 90% of the mass in most galaxies is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces. Observations suggest that the majority of the mass-energy in the observable universe is dark energy, a type of vacuum energy that is poorly understood. Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space.

Bow shock Boundary between a magnetosphere and an ambient magnetized medium

In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of the stellar wind abruptly drops as a result of its approach to the magnetopause. For stars, this boundary is typically the edge of the astrosphere, where the stellar wind meets the interstellar medium.

Heliosphere Region of space dominated by the Sun

The heliosphere is the vast, bubble-like region of space that surrounds and is created by the Sun. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstellar medium. The "bubble" of the heliosphere is continuously "inflated" by plasma originating from the Sun, known as the solar wind. Outside the heliosphere, this solar plasma gives way to the interstellar plasma permeating the Milky Way galaxy. Radiation levels inside and outside the heliosphere differ; in particular, galactic cosmic rays are less abundant inside the heliosphere, so that the planets inside are partly shielded from their impact. The word "heliosphere" was likely coined by Alexander J. Dessler, who is credited with first use of the word in scientific literature in 1967. The scientific study of the heliosphere is heliophysics, which includes space weather and space climate.

Astrophysical plasma

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Magnetosphere of Saturn

The magnetosphere of Saturn is the cavity created in the flow of the solar wind by the planet's internally generated magnetic field. Discovered in 1979 by the Pioneer 11 spacecraft, Saturn's magnetosphere is the second largest of any planet in the Solar System after Jupiter. The magnetopause, the boundary between Saturn's magnetosphere and the solar wind, is located at a distance of about 20 Saturn radii from the planet's center, while its magnetotail stretches hundreds of Saturn radii behind it.

Heliospheric current sheet

The heliospheric current sheet is the surface in 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 rotating magnetic field on the plasma in the interplanetary medium. A small electrical current flows within the sheet, about 10−10 A/m2. The thickness of the current sheet is about 10,000 km near the orbit of the Earth.

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Heliophysics science of the Sun

Heliophysics is the science of the physical connections between the Sun and the solar system. NASA defines heliophysics as "(1) the comprehensive new term for the science of the Sun - Solar System Connection, (2) the exploration, discovery, and understanding of Earth's space environment, and (3) the system science that unites all of the linked phenomena in the region of the cosmos influenced by a star like our Sun. Heliophysics concentrates on the Sun, and its effects on Earth, the other planets of the solar system, and the changing conditions in space. Heliophysics is concerned with the magnetosphere, ionosphere, thermosphere, mesosphere, and upper atmosphere of the Earth and other planets. Heliophysics combines the science of the Sun, corona, heliosphere and geospace. Heliophysics encompasses cosmic rays and particle acceleration, space weather and radiation, dust and magnetic reconnection, nuclear energy generation and internal solar dynamics, solar activity and stellar magnetic fields, aeronomy and space plasmas, magnetic fields and global change, and the interactions of the solar system with our galaxy."

Interplanetary magnetic field

The interplanetary magnetic field (IMF), now more commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar System.

Magnetosphere of Jupiter Cavity created in the solar wind

The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere. Wider and flatter than the Earth's magnetosphere, Jupiter's is stronger by an order of magnitude, while its magnetic moment is roughly 18,000 times larger. The existence of Jupiter's magnetic field was first inferred from observations of radio emissions at the end of the 1950s and was directly observed by the Pioneer 10 spacecraft in 1973.

Comet tail

A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. Separate tails are formed of dust and gases, becoming visible through different phenomena; the dust reflects sunlight directly and the gases glow from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.

Energetic neutral atom

Energetic neutral atom (ENA) imaging, often described as "seeing with atoms", is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere, even to its outer boundary. This constitutes the far-flung edge of the solar system.

The Arctowski Medal is awarded by the U.S. National Academy of Sciences "for studies in solar physics and solar-terrestrial relationships." Named in honor of Henryk Arctowski, it was first awarded in 1969.

Heliophysics Science Division

The Heliophysics Science Division of the Goddard Space Flight Center (NASA) conducts research on the Sun, its extended solar system environment, and interactions of Earth, other planets, small bodies, and interstellar gas with the heliosphere. Division research also encompasses geospace—Earth's uppermost atmosphere, the ionosphere, and the magnetosphere—and the changing environmental conditions throughout the coupled heliosphere.

Pickup ion

In solar science, heliospheric pickup ions are created when neutral particles inside the heliosphere are ionized by either solar ultraviolet radiation, charge exchange with solar wind protons or electron impact ionization. Pickup ions are generally characterized by their single charge state, a typical velocity that ranges between 0 km/s and twice the solar wind velocity (~800 km/s), a composition that reflects their neutral seed population and their spatial distribution in the heliosphere. The neutral seed population of these ions can either be of interstellar origin or of lunar-, cometary, or inner-source origin. Just after the ionization, the singly charged ions are picked up by the magnetized solar wind plasma and develop strong anisotropic and toroidal velocity distribution functions, which gradually transform into a more isotropic state. After their creation, pickup ions move with the solar wind radially outwards from the Sun.


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  2. NASA (12 March 2019). "What scientists found after sifting through dust in the solar system". EurekAlert! . Retrieved 12 March 2019.
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  4. Kenneth R. Lang (2000). The Sun from Space. Springer Science & Business Media. p. 17. ISBN   978-3-540-66944-9.