The interplanetary medium is the material which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planets, asteroids, and comets, move.
Composition and physical characteristics
The interplanetary medium includes interplanetary dust, cosmic rays, and hot plasma from the solar wind. The temperature of the interplanetary medium varies. For dust particles within the asteroid belt, typical temperatures range from 200 K at 2.2 AU down to 165 K at 3.2 AU. The density of the interplanetary medium is very low, about 5 particles per cubic centimeter in the vicinity of the Earth; it decreases with increasing distance from the Sun, in inverse proportion to the square of the distance. It is variable, and may be affected by magnetic fields and events such as coronal mass ejections. It may rise to as high as 100 particles/cm. Since the interplanetary medium is a plasma, 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, forms plasma double layers where it comes into contact with a planetary magnetosphere or at the heliopause, and exhibits filamentation. 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 tesla magnetic dipole field would reduce with the cube of the distance to about tesla. But satellite observations show that it is about 100 times greater at around tesla. Magnetohydrodynamic theory predicts that the motion of a conducting fluid 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.
History
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 in the... interplanetary spaces" Boyle Hist. Air. 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. 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...". 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 who proposed on the basis of comet tails, that the Sun continuously blows its atmosphere out in all directions at supersonic speed" 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".
