#18B.   The Heliosphere

The Boundary of the Solar Wind

    The solar wind may be viewed as a rarefied gas of charged particles, expanding outwards. It is a somewhat unusual gas, a "plasma" of charged particles: its particles are bound together by the magnetic field surrounding them, not by mutual collisions, the way molecules in the air we breathe interact. But like an ordinary gas, it too can exert a pressure

    As the solar wind spreads out radially, its density keeps decreasing, like light spreading from a lamp--as its distance R from the center of the Sun increases, its density drops in proportion to 1/R², getting smaller and smaller. The magnetic field also weakens, approximately like 1/R; the field of a bar magnet in empty space would weaken at a much faster rate, but here the magnetic intensity is kept up by the charged particles it contains, belonging to the solar wind.

    The pressure exerted by the solar wind depends on both density and magnetic field; because both these get increasingly weaker as the gas spreads out from the Sun, so does the pressure. But another gas, extremely rarefied, fills space outside the solar system, the "interstellar medium." One may well expect the solar wind's expansion to end wherever its pressure balances that of the interstellar gas.

    Some cautions need to be observed, though. First, the balancing pressure can only come from interstellar plasmas, from charged particles in interstellar space and from the magnetic fields which accompany them. Neutral particles--such as molecules of ordinary air--will see no obstacle: the magnetic field exerts no force on them, and solar wind particles are so far apart (centimeters or inches) that collisions practically never occur.

Standing Shocks

    A supersonic flow which encounters an obstacle undergoes a double transition. First it forms a "standing shock," a transition where the flow abruptly slows down below the speed of sound, while its density increases. This transition is also associated with the conversion into heat of much of the kinetic energy of the ordered supersonic flow. That makes a great difference for space vehicles re-entering the atmosphere from orbit: the space shuttle, for instance, must lose an enormous amount of kinetic energy before landing, and it benefits from the fact that most of that energy is converted to heat not when the supersonic air flow hits the heat-shield tiles, but some distance before that, in the standing shock ahead of the vehicle.

    Similarly, when the solar wind hits an obstacle, it forms a "bow shock" ahead of it, slows down below the Alfvén speed and only then, some distance further downstream, interacts with the obstacle. Planetary magnetic fields, such as that of the Earth, are a good example. The Earth's bow shock forms about 13 Earth radii sunward of Earth--that is the closest point in a curved surface, similar to what you get when you rotate a hyperbola around its axis of symmetry. The solar wind then continues, slower and denser, until it is finally deflected by the Earth's magnetic field at the "magnetopause," at a distance of about 10.5 Earth radii.

The termination Shock

    Theories suggested that the shock would occur at about 50-100 AU (astronomical units; one AU is the mean Sun-Earth distance), well beyond the orbits of Neptune (30 AU) and Pluto (40 AU). Front runner among NASA's long-distance space probes is Voyager 1, launched in 1977 towards Jupiter and Saturn. As its distance grew, its signals weakened and needed special handling by NASA's engineers, with a very slow signal rate. Its electricity generators, using heat from radioactive plutonium, also lost some power, and its solar wind detector has died.

    Year after year the investigators waited--interpreting noisy periods as the approach of the termination shock, only to see the noise quiet down again. No doubt betting pools wagered good money on when and where the crossing would happen.

And then it happened, just when no one was watching.

The Crossing

Magnetic Field change at the Heliospheric Termination Shock

    What next? The flow of the solar wind is not constant, and at some time in the future its volume and/or speed could increase to where the termination shock is pushed outwards and passes over Voyager 1 again. Spacecraft close to the Earth's bow shock often observe such swings. However, another instrument aboard Voyage 1 suggests this is not imminent, a detector for low energy cosmic ray ions of 70 MeV and up. Since the crossing of the termination shock the flow of these particles has steadily increased (see figure below).

Flux of low-energy cosmic rays at Voyager 1, 2004-5     It has long been known that the flow intensity of the slowest cosmic rays, the bottom of the energy distribution of such particles, is reduced inside the heliosphere, at least near the ecliptic. This may be caused by irregularities in the magnetic field, and is evidenced by the observed variation of such particles with the 11-year sunspot cycle. The steady growth of their flow since December 2004 suggests that the termination shock plays an important role in their exclusion.     So, what comes next? Somewhere beyond the termination shock lies the heliopause, the true interstellar boundary, and interesting things might be observed when it is reached. Because its solar wind detector no longer works, Voyager won't see if the solar wind is bent away there, but its magnetic field lines would also bend, and that could still be observed. Outside the heliopause yet another bow shock may exist, because the solar system itself is moving fairly rapidly relative to the surrounding part of the galaxy.     No one knows when might this happen, and how well Voyager will be operating when the heliopause is reached. As insurance there also exists Voyager 2, lagging by some tens of AU, but still returning solar wind data. Then again, the entire Voyager mission is operating under a shadow: a move was made in early in 2005 (but later reversed) to cut expenses by stopping its tracking. We can only keep our fingers crossed that tracking continues as long as data keep arriving, and meanwhile wish both Voyagers a long and fruitful old age!

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   education("at" symbol)phy6.org

Co-author: Dr. Mauricio Peredo