MEETING OF SOLAR WINDS AND INTERSTELLAR SPACE

The heliopause is the outermost boundary of the solar wind, where the interstellar medium restricts the outward flow of the solar wind and confines it within a magnetic bubble called the heliosphere.

The solar wind is made up of electrically charged atomic particles, composed primarily of ionized hydrogen, that stream outward from the Sun.

This is the perceived "edge of the solar system" that the Voyager Instellar Mission is hoped to return new and exciting information about. As the Voyager cruises gracefully in the solar wind, its fields, particles and waves instruments are studying the space around them. In May 1993, scientists concluded that the plasma wave experiment was picking up radio emissions that originate at the heliopause -- the outer edge of our solar system.

Exactly where the heliopause is has been one of the great unanswered questions in space physics. By studying the radio emissions, scientists now theorize the heliopause exists some 90 to 120 astronomical units (AU) from the Sun. (One AU is equal to 150 million kilometers (93 million miles), or the distance from the Earth to the Sun.

Except near some of the planets, interplanetary space is filled with the Sun's magnetic field. Its interactions with the solar wind are very complicated. Within a few solar radii of the Sun the magnetic field determines the flow of the solar wind; much of the flow is trapped in magnetic loops. But some regions of the Sun's magnetic field are open allowing the solar wind to escape. Farther out the plasma dominates and the magnetic field is entrained in the particle flow.

As the solar wind moves out into space, it creates a magnetized bubble of hot plasma around the Sun, called the heliosphere. Eventually, the expanding solar wind encounters the charged particles and magnetic field in the interstellar gas. The boundary created between the solar wind and interstellar gas is the heliopause. The precise shape and location of the heliopause is not known but it is probably similar in shape to the Earth's magnetosphere and the bow shock is probably about 110 - 160 AU from the Sun.

The interstallar medium surrounding us currently is of low density, and contains mainly neutral hydrogen. It has been estimated that as our Sun rotates around the galactic centre (T=200 million years) it will also hit regions with higher density and more molecular hydrogen. This could lead to modifications to the heliospheric structure, and changes in the cosmic ray fluxes.

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 aurora. Other planets with significant magnetic fields, especially Jupiter, have similar effects.

Scientists speculate that Voyager 1 will reach the "edge of our solar system" within the decade.