interplanetary medium

The matter contained in the Solar System in the space between the planets. The term usually refers to the plasma and neutral gas (which comes from the local interstellar medium), but there is also interplanetary dust. The Solar System is filled with a tenuous plasma – the solar wind – that streams outward from the Sun at supersonic speeds and in which is embedded the Sun's magnetic field wound into a spiral by the Sun's rotation (see plasma). This is called the interplanetary magnetic field. At the Earth the interplanetary medium has a density of about eight particles per cm³ moving at a velocity of 300–400 km s^–1, and a magnetic flux density that averages about 5 × 10^–9 tesla. The interplanetary magnetic field is crucial in coupling the solar wind plasma to the magnetosphere of the Earth and other planets. The interplanetary medium moves at supersonic speed relative to the Earth, creating a shock front called the bow shock, rather like that set up by supersonic aircraft. Irregularities in the density of the solar wind cause interplanetary scintillations (see scattering) on small radio sources viewed through the medium.

Interplanetary Medium

the material filling the space between the planets of the solar system. The interplanetary medium does not include the outer atmospheres of the planets (extended hydrogen envelopes), comets and their remnants, the part of the solar corona nearest the sun, or cosmic rays, including those of solar origin. The interplanetary medium is closely associated with such observable phenomena as the zodiacal light and the F- and K- components of the solar corona. The medium may be divided into gaseous (neutral and ionized) and solid (dust) components.

Before the 1950’s it was assumed that the solar system was filled with a stationary gas with an equilibrium level of ionization. A dynamic theory was subsequently developed, according to which the gaseous component of the interplanetary medium consists of the expanding matter from the solar corona, which carries a “frozen in” magnetic field; that is, the field is carried along by the matter. This conclusion was arrived at through the analysis of comet tails, which always point away from the sun. A mathematical theory of the expanding solar corona was developed in 1958. The widely used term “solar wind” was introduced to designate the continuous, although strongly varying, stream of solar coronal plasma, which accelerates near the sun and sweeps out the stationary gas in the solar system.

At the end of the 1950’s, systematic experimental studies of the interplanetary medium began using instruments placed on artificial earth satellites and lunar and planetary probes. These studies have been conducted mainly with plasma sondes, magnetic and electrostatic analyzers, and highly sensitive magnetometers. This has made it possible to study the energy, mass, and charge spectra of solar-wind particles and the microstructure and macrostructure of the topography of the magnetic field between the orbits of Venus and Mars, as well as to trace changes in these quantities with time and solar activity.

The distributions of the velocities and density of the solar wind may be profitably investigated by radiolocation studies of the solar corona using large terrestrial radar units. During a quiet period, the solar wind’s proton stream near the earth is usually about 3 X 10⁷ to 3 X 10⁸ particles/cm².sec within ±5° of the sun’s direction and with a mean velocity of 350-450 km/sec and an energy of 1 keV. During periods of higher solar activity, the stream of particles increases to 10⁹-10¹⁰ particles/cm²-sec, and the velocity to 1,000 km/sec or more. The electron component of the solar wind has an almost isotropic angular distribution with a mean electron energy of 15 eV. The solar wind carries a “frozen in” magnetic field, whose intensity is 3-5 gammas (1 gamma = 10^-5oersted). It has been established that the magnetic field in the solar system has a zonal structure, which is associated with a change in the field’s polarity over large distances; the number of zones varies from three to six. The stream-lining of the earth’s magnetic field by the solar wind results in the phenomenon of the Van Allen radiation belts and a variety of complex effects in the magnetosphere, including auroras and magnetic storms. In this case, a stationary shock wave front is formed at a distance of 10-15 earth radii from the illuminated half of the earth.

In addition to the ionized component, the interplanetary medium also includes atoms of neutral hydrogen, which can be observed from equipment in space by means of resonance scattering of solar radiation at the 1215.7 Å Lα line. These observations have revealed that the entire solar system is moving with a velocity of 20 km/sec relative to the interstellar neutral hydrogen. Interaction of the latter with -the solar wind leads to the formation of a collisionless shock wave at the distance of the orbit of Jupiter in a direction 40° from the direction of the motion of the sun relative to the nearest stars. At this wave front, the directed velocity of the protons in the solar wind is transformed into random thermal motion corresponding to a temperature of 3 X (10⁶-10⁷) °K. The neutral hydrogen atoms in turn form two components—one hot and one cold. The hot component originates at the shock wave front as a result of charge transfers between the protons of the solar wind and the neutral atoms of the interstellar medium. At velocities of 100-200 km/sec, such atoms pass through the solar system in about 0.1 year and thus have little time to be ionized by the solar ultraviolet radiation and remain neutral. The density of this component depends somewhat on the distance from the sun. The cold component is formed by the action of the sun’s gravity on the atoms of the interstellar medium. The density of these atoms falls off sharply on approaching the sun. At the earth’s distance from the sun, the density of neutral atoms is 10^-2-10^-3 atoms/cm³.

Fundamental problems that remain to be studied concerning the structure of the interplanetary medium include the mechanism causing the acceleration of the solar-wind plasma near the sun, the distribution of density and temperature outside the plane of the ecliptic, and the behavior of the solar wind near the shock wave front and at the periphery of the solar system.

The dust component of the interplanetary medium is being studied by astronomical methods (optical observations of the F-component of the solar corona, optical and radar observations of meteors) and by means of piezoelectric and other types of transducers carried on space vehicles. This component appears as the result of the disintegration of asteroids and comets, but there exists the possibility that it has been preserved from the time of the solar system’s formation from a gas-dust cloud.

Studies in the 1960’s and 1970’s have shown that previous estimates of the meteor hazard in interplanetary and orbital flights were two or three orders too high; in particular, they have not supported the hypothesis of the existence of a dust cloud around the earth. Problems in the study of the interplanetary medium’s dust component are reduced to obtaining the distributions of the sizes and masses and finding the velocities of the particles as a function of the distance from the sun and the plane of the ecliptic, and eventually finding the velocities outside the plane.

REFERENCES

Parker, E. N. Dinamicheskie protsessy v mezhplanetnoi srede. Moscow, 1965. (Translated from English.)
Solnechnyi veter. Moscow, 1968. (Collection of articles; Translated from English.)

V. G. KURT