The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour,[27] creating a very tenuous atmosphere (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.[28]

Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.[29]

The interplanetary medium is home to at least two disclike regions of cosmic dust. The first, which lies in the inner solar system, is known as the zodiacal dust cloud and is responsible for the phenomenon of zodiacal light*. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.[30] The second, which extends from about 10 AU to about 40 AU, was probaby created by similar collisions within the Kuiper belt.[31] [32]


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*Zodiacal light

The zodiacal light in the eastern sky before the beginning of morning twilight. A reddish auroral glow can also be seen near the horizon, especially at the left.
The zodiacal light in the eastern sky before the beginning of morning twilight. A reddish auroral glow can also be seen near the horizon, especially at the left.

The 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. In mid-northern latitudes, the zodiacal light is best observed in the western sky in the spring after the evening twilight has completely disappeared, or in the eastern sky in the autumn just before the morning twilight appears. It is so faint that it is completely masked by either moonlight or light pollution. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed in a band completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible for 60% of the total skylight on a moonless night. There is also a very faint, but still slightly increased, oval glow directly opposite the Sun which is known as the gegenschein.

The zodiacal light is produced by sunlight reflecting off dust particles which are present in the solar system and known as cosmic dust. Consequently, its spectrum is the same as the solar spectrum. The material producing the zodiacal light is located in a lens-shaped volume of space centered on the sun and extending well out beyond the orbit of Earth. This material is known as the interplanetary dust cloud. Since most of the material is located near the plane of the solar system, the zodiacal light is seen along the ecliptic. The amount of material needed to produce the observed zodiacal light is amazingly small. If it were in the form of 1 mm particles, each with the same albedo (reflecting power) as Earth's moon, each particle would be 8 km from its neighbors. The gegenschein may be due to the fact that particles directly opposite the sun as seen from Earth would be in full phase.

The Poynting-Robertson effect causes the particles to spiral slowly into the Sun, thus requiring a continuous source of new particles to maintain the zodiacal cloud. Cometary dust and dust generated by collisions among the asteroids are believed to be mostly responsible for the maintenance of the dust cloud producing the zodiacal light and the gegenschein. In recent years, observations by a variety of spacecraft have shown significant structure in the zodiacal light including dust bands associated with debris from particular asteroid families and several cometary trails.

This phenomenon was first investigated by the astronomer Giovanni Domenico Cassini in 1683 and first explained by Nicolas Fatio de Duillier in 1684.

Interplanetary medium


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.

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 is approximately 100,000 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.

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.

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 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 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 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.



Observable phenomena of the interplanetary medium

The interplanetary medium is responsible for several effects which can be seen from earth. The Zodiacal light 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 Gegenschein, 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.