The Voyagers and the Heliosphere
Published on Nov 13, 2006 at 5:42 pm.
2 Comments.
Filed under Sun, interstellar medium.
Far from the Sun, two tiny spacecraft sail onward heading out of the Solar System into interstellar space. Voyager 1, is now a distance of just about 101 Astronomical Units from the Sun, and Voyager 2 is over 81 Astronomical Units out. Both spacecraft were launched in 1977 to study the outer planets of our Solar System. Voyager 1 passed Jupiter and Saturn, and Voyager 2 went by Jupiter, Saturn, Uranus, and Neptune. Both kept going, and are now leaving our Solar System. They had two cousins that went ahead of them, Pioneer 10 and Pioneer 11, launched in 1972 and 1973, but these two forerunners have since quit operating. The Voyagers, though, are stil clinging to life.
Though far past the most distant planet, and not expected to pass near any known Kuiper Belt objects, one would be tempted to think of the Voyager missions being over. However, they are still returning useful data, and these spacecraft are now able to study the very edge of the Sun’s magnetosphere.
The Sun has a magnetic field, just as do the planets. And the Sun is constantly spewing out particles (the solar wind). The solar wind, together with magnetic energy from the Sun, travel outward, dissipating as they go.  But, this can’t go on forever. The space between the stars isn’t empty.  Yes, it is pretty close to empty, but it isn’t totally empty. There are actually atoms, molecules, and dust grains between the stars. There is a galactic magnetic field. All of this is called the interstellar medium. The solar wind pushes back on the interstellar medium, creating a bubble of sorts around the Sun. But, the solar wind particles slow down and get thinner as they move away from the Sun. The magnetic fields embedded in the solar wind interact with the galactic magnetic field. They can only push the interstellar medium back so far. The region of space where the solar wind dominates over the interstellar medium is called the heliosphere.
The interstellar medium and the Sun are moving at slightly different speeds and directions, so the pressure of the interstellar medium on the heliosphere isn’t symmetric. This means that the heliopshere itself is not symmetric. Rather, it is elongated, much like the magnetosphere of the Earth. In fact, another way to look at this is that the heliosphere is dominated by the Sun’s magnetic field, and beyond is dominated by the galaxy’s magnetic field. This gives much the same definition to the heliosphere as to the magnetosphere of a planet.
Where the solar wind particles slow to the point that they have difficulty pushing aside the interstellar medium, they bunch up. This causes compression, heating, changes in energy, radiation, and a change in magnetic field. We call this the termination shock. The Sun continues to influence its environment somewhat beyond the termination shock, but at a severely reduced level. Eventually, the solar wind is so weak that it has no effect on the interstellar medium at all. This marks the edge so the heliosphere, and we call this the heliopause. As the Sun moves through the interstellar medium, it pushes it out of the way, creating a bow shock, much like the bow shock at the sunward side of Earth’s magnetosphere. The charged particles streaming from the Sun suddenly slowing at the termination shock, produce radio waves. Charged particles in the interstellar medium being pushed out of the way also creat radio signals. Between the termination shock and the heliopause is a region that we call the heliosheath, which marks what some would consider the edge of the Solar System. Beyond the heliosheath is interstellar space.
Prior to the Voyager missions, many astronomers believed that the termination shock should be somewhere near the orbits of the outer planets, perhaps as close in as Uranus, and the heliopause slightly farther than Pluto. However, the Voyagers and Pioneers went sailing past these distances without even a hint of a change in the space environment. Finally, though, Voyager 1 passed trough the termination shock and into the heliopause in about December of 2004. Voyager 2 seems to have reached the termination shock earlier this year. While we don’t know exactly where the heliopause is located, we expect both spacecraft to be past it within ten years. At that point, they will finally have reached interstellar space, and we will have our very first opportunity to directly measure the interstellar environment. (That assumes that the Voyager mission continues to be extended. There is talk of budget cuts putting an end to the program before the craft reach interstellar space. If that happens, it would be a shame, since it took them 30 years to get this far, and they are still operational, except for one instrument aboard Voyager 1.)
One thing that has been a major surprise, though, from the Voyagers is that they did not find the source of medium energy cosmic rays. Low energy cosmic rays are believed to originate from the Sun. Very high energy cosmic rays likely originate with supernova explosions. However, it had been speculated that medium energy cosmic rays were caused by particles accelerated between the heliopause and the termination shock. The source of these medium energy cosmic rays, though, has yet to be found, suggesting that their origin lies farther out, perhaps even beyond the heliopause. At present, it is unclear what mechanism may be responsible for them. Hopefully, we can find out if the Voyagers are still functioning when they reach the source of these cosmic rays.Â
-Astroprof
(Images courtesy of NASA)
A Ler…-- Rastos de Luz on November 15, 2006 at 6:00 am: 1
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Lindsey on July 24, 2010 at 4:07 pm: 2
Fascinating! I really enjoy your blog.