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Where the Solar Wind Hits the Wall
생명과학 ScienceNow (2000-03-20)

Every so often, the stream of particles coming from the sun knocks out a satellite or disrupts power grids on Earth. But that's nothing compared to the collisions in interstellar space, where the solar wind bangs into a huge particle cloud. Now an analysis of the faint glow of photons caroming inside this turmoil has given astronomers their first peek at the huge wake the solar wind creates when the solar system ploughs through this cloud.

The solar wind, consisting mainly of protons and electrons, follows the sun's magnetic field lines out into space. At about 180 times the distance between the sun and Earth [180 astronomical units (AUs)], just past the Oort cloud, it collides with a huge interstellar cloud more than 6 light-years across and containing neutral hydrogen atoms, helium atoms, and protons. As a result, the solar wind slows down drastically, forming a bow shock. The hydrogen atoms from interstellar space slow down as well, creating a 150-AU-wide region of higher density called the "hydrogen wall." Until recently, astronomers had no way to locate this zone.

Now, a French-Polish team has analyzed data from ultraviolet (UV) spectrometers aboard the Hubble Space Telescope and Voyager 1, a space probe that flew by Jupiter in 1979 and Saturn in 1980 and is now at about 70 times the sun-Earth distance. They looked at faint UV radiation from photons that ricochet inside the transition region several times, picking up energy from collisions with hydrogen atoms, before escaping. During this so-called Fermi acceleration, the photons' wavelengths become shorter; the resulting "Fermi glow" enabled the team to reconstruct the three-dimensional shape of the bow shock, says team leader Lotfi Ben-Jaffel of the Institute for Astrophysics in Paris.

The team found that the bow shock is tilted about 12 degrees with respect to the sun's path through the galaxy. That's because the magnetic field in the nearby interstellar space, although extremely weak, deflects the flow of charged particles making up the bow shock, explains Ben-Jaffel, who says the results will be published in the May issue of the Astrophysical Journal.

Using the Fermi glow is a "very clever method" to detect the bow shock, says Gary Zank, an astronomer at the University of Delaware, and Ben-Jaffel's team is the first to detect the interstellar magnetic field, he says. The findings also confirm earlier observations of a mysterious dimming of UV light coming from certain stars, such as Alpha Centauri, says Zank; since those stars are right behind the bow shock, their dimming is probably caused by the hydrogen wall.


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