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Theinterplanetary medium (IPM) orinterplanetary space consists of the mass and energy which fills theSolar System, and through which all the larger Solar System bodies, such asplanets,dwarf planets,asteroids, andcomets, move. The IPM stops at theheliopause, outside of which theinterstellar medium begins. Before 1950, interplanetary space was widely considered to either be an empty vacuum, or consisting of "aether".
The interplanetary medium includesinterplanetary dust and gas,cosmic rays, and hotplasma from thesolar wind.[2] The density of the medium is very low, with the solar wind component decreasing ininverse proportion to the square of the distance from the Sun.[3] The solar wind is variable, and may be affected bymagnetic fields andsolar activity events such ascoronal mass ejections. Typical particle densities in the interplanetary medium are about 5-40 particles/cm3, but exhibit substantial variation.[4]: Figure 1 In the vicinity of theEarth, it contains about 5 particles/cm3,[5]: 326 but values as high as 100 particles/cm3 have been observed.[4]: Figure 2
The temperature of the interplanetary medium varies through the solar system.Joseph Fourier estimated that interplanetary medium must have temperatures comparable to those observed atEarth's poles, buton faulty grounds: lacking modern estimates ofatmospheric heat transport, he saw no other means to explain the relative consistency ofEarth's climate.[6] A very hot interplanetary medium remained a minor position among geophysicists as late as 1959, when Chapman proposed a temperature on the order of 10000 K,[7] but observation inLow Earth orbit of theexosphere soon contradicted his position.[citation needed] In fact, both Fourier and Chapman's final predictions were correct: because the interplanetary medium is sorarefied, it does not exhibitthermodynamic equilibrium. Instead, different components have different temperatures.[4]: 4 [5][8] The solar wind exhibits temperatures consistent with Chapman's estimate incislunar space,[5]: 326, 329 [8][9] and dust particles near Earth's orbit exhibit temperatures 257–298 K (3–77 °F),[10]: 157 averaging about 283 K (50 °F).[11] In general, the solar wind temperature decreasesproportional to the inverse-square of the distance to the Sun;[7] the temperature of the dust decreases proportional to the inversecube root of the distance.[10]: 157 For dust particles within theasteroid belt, typical temperatures range from 200 K (−100 °F) at 2.2 AU down to 165 K (−163 °F) at 3.2 AU.[12]
The solar wind component of the interplanetary medium is aplasma, or gas ofions, and has the physical characteristics of a plasma, rather than a simple gas. For example, it carries the Sun's magnetic field with it, is highly electrically conductive[13] (resulting in theheliospheric current sheet), forms plasmadouble layers where it comes into contact with a planetary magnetosphere or at theheliopause,[citation needed] and exhibits filamentation (such as inaurorae).
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 an ideal 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 around7 nT.[13]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 anMHD dynamo.[citation needed]
The outer edge of theheliosphere is the boundary between the flow of the solar wind and theinterstellar medium. This boundary is known as theheliopause and is believed to be a fairly sharp transition[14] that was measured at a distance of around120 AU from the Sun by theVoyager missions.[15] The interplanetary medium thus fills the volume contained within the heliopause. The actual shape of this volume remains uncertain, extending only100 AU toward thesolar apex but up to800 AU in the polar direction.[16]
How the interplanetary medium interacts with planets depends on whether they havemagnetic fields or not. Bodies such as theMoon have no magnetic field and thesolar wind can impact directly on their surface. Over billions of years, thelunar regolith has acted as a collector for solar wind particles, and so studies of rocks from thelunar 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 atX-ray wavelengths.
Planets with their own magnetic field, such as the Earth andJupiter, are surrounded by amagnetosphere within which their magnetic field is dominant over theSun'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, causingaurorae and also populating theVan Allen radiation belts with ionised material.
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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 theecliptic and appearing brightest near the horizon. This glow is caused by sunlightscattered bydust particles in the interplanetary medium between Earth and the Sun.
A similar phenomenon centered at theantisolar point,gegenschein is visible in a naturally dark, moonlessnight sky. Much fainter than zodiacal light, this effect is caused by sunlightbackscattered bydust particles beyond Earth's orbit.
The term "interplanetary" appears to have been first used in print in 1691 by the scientistRobert Boyle: "The air is different from the æther (or vacuum) in the... interplanetary spaces" BoyleHist. Air. In 1898, American astronomerCharles 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 avacuum 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".[18] In 2002, Akasofu stated "The view that interplanetary space is a vacuum into which the Sun intermittently emitted corpuscular streams was changed radically byLudwig 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)
