Heliosphere

I

Introduction

Heliosphere name given to the magnetosphere that surrounds the Sun. The heliosphere is the region of space dominated by the Sun’s magnetic field and the charged particles streaming from the Sun (the solar wind). Because the Sun orbits inside the Milky Way Galaxy, it carries the heliosphere through the magnetic field and the gas and dust of the galaxy.

The heliosphere resembles a huge bubble in space filled with electrified particles (negatively charged electrons and positively charged protons and ions). These particles stream outward in all directions from the corona of the Sun in the solar wind at a velocity of about 400 km/sec (250 mi/sec). The bubble of the heliosphere extends far beyond the planets and may be regarded as defining the region of space influenced by the Sun (Solar System).

As the Sun moves in its orbit through the galaxy it is also passing through a hot, thin cloud of interstellar gas and dust. This material is enough to shape the trailing side of the heliosphere into an elongated heliotail resembling a huge windsock. Studies of the motion of nearby stars show that the Sun is traversing the cloud at a velocity of 25.5 km/sec (15.8 mi/sec).

The density of the solar wind decreases as the inverse square of its distance from the Sun. For example, the density of the solar wind at 2 AU (about 300 million km/186 million mi) is only one quarter its density at 1 AU (about 150 million km/93 million mi). The Sun’s magnetic field also weakens with distance, but at a slower rate. The boundaries of the heliosphere occur where charged particles (or plasma) from the interstellar medium dominate the charged particles from the Sun, effectively blocking the outward motion of the solar wind. The boundary between the interstellar medium and the heliosphere is called the heliopause.

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The flow of interstellar plasma is sometimes called the interstellar wind. Although electrically neutral atoms, cosmic rays, and dust particles from interstellar space can penetrate the heliosphere, virtually all of the material in the heliosphere originates in the Sun itself or from material that formed the solar system.

II

Structure of the Heliosphere

The interaction of the heliosphere with the interstellar medium takes place in several stages. For a spacecraft traveling out of the solar system away from the Sun’s direction of movement, the first boundary to be reached is the termination shock, which is at least about twice as far from the Sun as the orbit of Pluto, at about 78 AU (11.7 billion km/7.2 billion mi). This is a standing shock wave where the density and magnetic field of the supersonic solar wind begin to decrease, slowing it dramatically from more than 100 km/sec (63 mi/sec) at this point to about half that speed.

Beyond the termination shock is a region known as the heliosheath, where solar wind and interstellar gas mix. Further out, at between 106 and 160 AU (between 16 billion km/10 billion mi and 24 billion km/15 billion mi), is the heliopause, which separates solar plasma from interstellar plasma. At this boundary the pressure of the solar wind balances the pressure of particles in interstellar space. The precise shape of the heliosphere and the distance of the heliopause are thus determined by the motion of the Sun relative to the interstellar medium, the density of the interstellar plasma, and the pressure exerted on its surroundings by the outflowing solar wind. Beyond the heliopause, the flow of interstellar ions is diverted around the heliosphere, creating the long heliotail.

Still further out, at about 227 AU (34 billion km/21 billion mi) from the Sun, there is probably a bow shock, another shock surface where the flow of the interstellar medium is suddenly slowed as it approaches the heliosphere.

All of these boundaries are thought to be moving back and forth at speeds of up to 100 km/sec (63 mi/sec) as the heliosphere is squeezed and released due to gusts in the solar wind and variations in the pressure exerted by the interstellar medium.

III

Observations from Space Probes

Much of what scientists know about the outer heliosphere comes from the two Voyager space probes, launched by NASA in 1977. The two probes are heading in different directions towards interstellar space. By the end of 2006, Voyager 1 was more than 100 AU (about 15 billion km/9.38 billion mi) from the Sun, while Voyager 2 was over 80 AU (12 billion km/7.4 billion mi) away.

For some years before this, the Voyagers had detected extremely energetic charged particles that could have been ionized and accelerated near the termination shock, in addition to bursts of radio noise not associated with any known source. Scientists concluded that they were approaching the first of the heliosphere’s outer boundaries.

Careful study of Voyager 1 data showed that it finally crossed the termination shock and entered the heliosheath on December 16, 2004, at a distance from the Sun of 94 AU (about 14 billion km/8.8 billion mi). Analysis of the data suggests that Voyager 1 may have experienced several close encounters with the termination shock between 2002 and 2005, when the boundary region moved inward before expanding away from the Sun just ahead of the chasing spacecraft. Traveling at a speed of about 1.6 million km per day (1 million mi per day), Voyager 1 could even cross into interstellar space by 2017.

NASA intends to launch the Interstellar Boundary Explorer (IBEX) probe in 2008. Placed in Earth orbit, IBEX is designed to take energetic neutral atom (ENA) images of the inner heliosheath that should answer many questions about what happens to particles at the termination shock and beyond. Looking further ahead, The European Space Agency is studying proposals to launch an interstellar heliopause probe that could send back data at a distance of 200 AU (about 29.9 billion km/18.6 billion mi) from the Sun. This spacecraft would be dedicated to studying the outer heliosphere and local interstellar medium. To reach interstellar space in the shortest possible time, the probe will have to be launched towards the heliosphere’s leading edge and not the heliotail.

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