FEATURE ARTICLE
The Galactic Environment of the Sun
The heliosphere appears to protect the inner solar system from the vagaries of the interstellar medium
Priscilla Frisch
The Heliosphere
As the Local Interstellar Wind blows through our solar system, it must pass through another wind—that produced by our own sun. The solar wind is a hot plasma—consisting of charged particles (mostly protons, helium nuclei and electrons)—that streams outward from the sun at high speed. Its source is the solar corona, the tenuous, million-degree plasma surrounding the sun that is evident during a total solar eclipse as a halo of brilliant "hair" surrounding the darkened disk. The solar wind also contains an embedded magnetic field that is wrapped into a tight spiral pattern by solar rotation. From the coronal region the supersonic solar wind blows out far beyond the orbit of Pluto before it is stopped by the charged component of the interstellar gas.
As the solar wind blows out toward the outer solar system, its density decreases. At a distance of 1 astronomical unit (or AU, the distance from the sun to the earth), the solar wind typically has a density of about 5 particles per cubic centimeter and a speed of about 400 kilometers per second. At about 80 to 100 AU, a termination shock is formed as the solar wind slows down from supersonic to subsonic speeds. The solar wind finally stops at the heliopause—the "stagnation" surface between the solar wind and the ions of the interstellar medium about 130 to 150 AU from the sun—which forms the boundary of the heliosphere. Both the average density and velocity of the solar wind vary with the activity cycle of the sun (and with the sun's latitude).
Models of the heliosphere suggest that it is shaped something like a water droplet. This shape is largely determined by the flow of the charged component of the interstellar gas around the solar-wind plasma. Although the interstellar medium is only about 30 percent ionized (again mostly protons and electrons), these charged particles do not want to traverse the magnetic fields embedded in the solar wind (because the Lorentz force binds them to the magnetic field). Thus the interstellar plasma is compressed and diverted around the heliosphere. Since neutral interstellar hydrogen atoms trade electrons back and forth with interstellar protons, a small portion of the neutral interstellar hydrogen is also compressed and diverted at the boundary of the heliosphere, forming an observable phenomenon called the hydrogen wall.
Another phenomenon that may be found outside the heliosphere is a bow shock, which would be produced if the heliosphere were moving faster than the speed of sound through the Local Interstellar Cloud. Since the speed of sound in the cloud is about 9.6 kilometers per second, and the relative sun/cloud velocity is 26 kilometers per second, one would assume that the heliosphere would have a Mach 2.5 bow shock. However, there is a good possibility that the Local Interstellar Cloud contains a weak magnetic field, which, if it were stronger than about 3 or 4 microgauss would inhibit the formation of a bow shock . Bow shocks form when the substrate material is unable to carry off disturbances faster than the object causing the disturbance is itself moving. If a magnetic field is present, then the tension in the magnetic field lines can transport the disturbance away (similar to the way in which the whole string vibrates when a violin string is plucked). The stronger the interstellar magnetic field, the faster it carries away pressure disturbances. Current estimates of the strength of the interstellar magnetic field lie between 1 and 5 microgauss.
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