Interplanetary Medium is thinly scattered matter which exists between the Planets and which fills the Solar System. It is composed of primarily - Gas and Dust. It fills the Space with extremely small amounts of neutral (non-ionized) hydrogen throughout much of Interplanetary space. At the distance of Earth’s orbit from the Sun, for example, the concentration of neutral hydrogen is about one atom per 100 cubic cm (6 cubic inches). The energy, in the form of electromagnetic radiation, that occupies the same volume is the interstellar radiation field. The material components of the Interplanetary Medium consist of neutral hydrogen, plasma gas comprising electrically charged particles from the Sun, Cosmic Rays, and Dust particles. The source of the ionized particles comprising this gas is mostly outflows and outbursts of the Sun. Some of this gas is due to outflow of particles from Planets, comets, and asteroids. And some of this gas originates from the in-fall of particles of Gas and Dust from the surrounding interstellar space. Approximately 60% of the light on a moonless night comes from sunlight reflecting off of the particles that make up the Interplanetary Dust Cloud found in the Interplanetary Medium.
Interplanetary Medium Boundaries
Inner Boundary - The inner boundary of the Interplanetary Medium (IPM) is made up of specific models of gases in the outer Atmosphere of the Sun called the Corona. The boundary between the Corona and the IPM can be defined as that point where the solar Corona becomes less dense than other constituents of the IPM. However a possible alternative boundary point between the Corona and the IPM could exist where the subsonic dynamics of the plasma of the Corona transit into the supersonic flow, known as the solar wind. This boundary is best understood by examining the hydrostatic balances of gases that comprise the solar Corona. Because the Sun is in pressure equilibrium, the outward thermal and magnetic pressures balance the gravitational attraction of the mass of the Sun. The solar wind derives from those particles that escape this boundary.
Outer Boundary - The Outer Boundary is also known as the Heliosphere. The heliosphere is the region that extends from the exterior boundaries of the Sun to the outermost reaches of the influence of the Sun. The heliosphere is a magnetic bubble formed by the effects of the Sun's magnetic fields as it interacts with local interstellar winds. As the solar wind flows outward, it interacts with the flow of local interstellar wind and with in-falling neutral particles and dust grains. Because the solar wind is a supersonic flow, the transition from the heliosphere into the local interstellar medium, it is believed, occurs as a shock called the termination shock. Farther out from the termination shock is the Heliopause boundary layer. The termination shock is the backup of the pressure wave that develops from the Heliopause boundary itself. It occurs due to the initial ''collision'' of the plasma that composes the interstellar wind with the magnetic forces due to the Sun. The location of the termination shock and the Heliopause varies significantly based on the activity of the Sun. During the solar maximum, the solar wind is weaker so that the external pressure on the Heliopause forces the heliosphere to shrink.
Composition, Characteristics and Physical Features of Interplanetary Medium
- Interplanetary Dust - The Interplanetary Dust is composed of cloud which is made up of Cosmic Dust (small particles floating in space) which pervade the space between Planets in the Solar System and in other Planetary systems. The total mass of the Interplanetary Dust Cloud is about the mass of an Asteroid of radius 15 km (with density of about 2.5 g/cm3). Interplanetary Dust Particles (or IDPs) are extremely small (0.001 cm in diameter) grains or particles found on Earth. The dust particles are among the most primitive materials in the Solar System. Large enrichment's of deuterium relative to hydrogen have been found in Interplanetary Dust Particles. Their chemical compositions are strongly suggestive of extraterrestrial origin, and they have high abundances of helium, a major component of the solar wind. They also have high densities of fossil nuclear tracks, produced as heavy atoms in solar flares penetrated the mineral grains. The main physical processes affecting Interplanetary Dust Particles are: expulsion by radiation pressure, inward Poynting-Robertson (PR) radiation drag, solar wind pressure (with significant electromagnetic effects), sublimation, mutual collisions, and the dynamical effects of Planets.
- Cosmic Rays - The highest energy particles in the Interplanetary Medium are called cosmic rays. Some are of solar origin; the most energetic, however, originate in some other unknown and very energetic processes outside our Solar System. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper Atmosphere causes the Auroras. Other Planets with significant magnetic fields, especially Jupiter, have similar effects.
- Interplanetary Magnetic Field - The other major component of the IPM is the Interplanetary magnetic field (IMF) that fills the entire region of the heliosphere. The IMF is mostly due to the transport of solar magnetic structures into the IPM. As the outward moving plasma of the solar wind goes from subsonic in the Corona to supersonic just outside the Corona, the magnetic field becomes locked within the solar wind plasma. The solar wind then carries the Coronal magnetic fields into the IPM.
- High Temperature - The temperature of the Interplanetary Medium is approximately 100,000 K.
- Variation in Density - The density of Interplanetary Medium 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.
- Presence of Hot Plasma and Solar Winds - Interplanetary Medium has the characteristics of a plasma, rather than a simple gas. Hot Plasma is composed of electrons, protons and other ions. It carries with it the Sun's magnetic field. It 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) from the solar wind. 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. The primary constituents of the solar wind are approximately 95% protons, 4% alpha particles, and 1% minor ions including multiple ionization states of C, O, Si, and Fe. The solar wind also contains electrons in number approximately equal to the ions, and hence the solar wind is considered an electrically neutral plasma. The solar wind contains approximately 1-10 particles per cubic centimeter. The solar wind is a fast stream of particles that leaves the Corona at approximately 400km/s in the ecliptic plane. The velocity of this stream varies significantly and ranges from 300-1000 km/s. At high Heliolatitudes (above 45°) during the solar minimum, the solar wind leaves the Corona at approximately 800 km/s, again with a very large range of velocities.
Importance of Interplanetary Medium
- The IPM modulates the flux of Cosmic Rays in the Inner Heliosphere and the details of cosmic ray diffusion and drift through the IPM need to be understood in detail in order to infer the spectrum and composition of the Cosmic Rays incident on The Heliosphere. The heliosphere is the area in which the total pressure of the Solar Wind Plasma is stronger than that of the Interstellar Medium around it. It is therefore the result of an interaction between the solar wind and the Interstellar Medium into which the Solar Wind eventually merges.
- The IPM is itself modulated by solar activity, leading to fast and slow solar wind streams, corotating shocks, and in-situ particle acceleration.
- The details of the structure of the IPM - the role of turbulence, waves, and the magnetic field, and their effect on the spatial spectrum of the IPM - are of great interest for their own sake.
- 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.
