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The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move. 行星际物质泛指充满太阳系空间的物质，而其他较大的太阳系天体如行星，小行星，及彗星在太阳系中运动时皆穿越这些物质。
组成及物理性质
[编辑]The interplanetary medium includes interplanetary dust, cosmic rays and hot plasma from the solar wind. The temperature of the interplanetary medium is approximately 10,000 K, and its density is very low at about 5 particles per cubic centimeter in the vicinity of the Earth; it decreases with increasing distance from the sun, in proportion with the inverse square of the distance.
行星际物质由行星际尘埃，宇宙射线，及等离子所构成。尘埃的大小约...？宇宙射线则泛指各种高能粒子（微中子，质子，电子及其他离子），可能由太阳产生，也可能来自太阳系外的其他事件（如超新星爆发）。等离子由太阳风吹出，温度可达绝对温度一万度，但是密度却是非常的低：在地球附近只有每立方厘米五个粒子，而且随着与太阳距离成平方反比递减。但是这些等离子的密度也会随着磁场大小的变化与太阳风的强度骤变（如日冕物质喷发）而改变，当太阳风强度增强时其密度甚至可达每立方厘米一百个粒子。
The density 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/cm³.
（以下所说似乎假设行星际物质大部分是等离子？或是只描述等离子部分？-> 移至可观测之现象）
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 (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 aurora).
The plasma in the interplanetary medium is also responsible for the strength of the Sun's magenetic 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.
行星际物质的等离子性质也导致了一些有趣的现象。
最远可及之处
[编辑]The outer edge of the solar system 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.
行星际物质充满整个太阳系，一直到太阳系的边界——太阳风最远可及，被称为“太阳系顶”的位置。太阳系顶约位于距太阳110至160天文单位之处，定义为行星际物质与系外星际物质的交界。
与行星的交互影响
[编辑]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 many billions of years, the lunar regolith 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 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 Belts with ionised material.
观测上之现象
[编辑]The interplanetary medium is responsible for several effects which can be seen from earth. The 黄道光 is a broad band of faint light sometimes seen after sunset and before sunrise, stretched along the ecliptic and brightest near the horizon. It is caused by sunlight scattering off dust particles in the interplanetary medium between the Earth and the Sun.
A similar effect is the 对日照, 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.
历史沿革
[编辑]Originally, astronomers thought that space was an empty vacuum. In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720)
参考书目
[编辑]Bill Arnett's Nine Planets page about the interplanetary medium