image: The yellow shape in this figure is the heliopause, the boundary between the heliosphere and the local interstellar medium. The sun sits at the center of this large bubble, but is too small to be seen in this image. The gray lines are the solar magnetic field lines and the red lines are the interstellar magnetic field. view more
Credit: Merav Opher
As the sun skims through the galaxy, it flings out charged particles in a stream of plasma called the solar wind. The solar wind creates a bubble, known as the heliosphere, which extends far outside the solar system. For decades, scientists have visualized the heliosphere as comet-shaped, with a very long tail extending thousands of times farther than the distance from the Earth to the sun.
New research suggests that the sun's magnetic field controls the large-scale shape of the heliosphere much more than expected. A new model, described in a paper published Feb. 19, 2015 in the journal Astrophysical Journal Letters, shows that the magnetic field squeezes the solar wind along the sun's north-south axis, producing two jets. These jets are then dragged downstream by the flow of the interstellar medium--the gases and dust that lie between star systems.
The model indicates that the heliospheric tail doesn't extend to large distances but is split into two jets, with a form similar to astrophysical jets observed in many other stars and around black holes. This new understanding of the heliosphere could have implications for future attempts at interstellar travel.
"Jets are really important in astrophysics," said James Drake, professor of physics and director of the Joint Space-Science Institute at the University of Maryland. "From what we can tell, the mechanism that's driving these heliospheric jets is basically the same as it is in, for example, the Crab Nebula. Yet this is really close by. If we're right about all of this, it gives us a local test bed for exploring some very important physics."
Drake suggests picturing a tube of toothpaste with rubber bands wrapped around it. In this case, the toothpaste is the jets' plasma, and the rubber bands are the rings of the solar magnetic field.
"Magnetic fields have tension just like rubber bands, and these rings squeeze in," Drake said. "So imagine you wrap your toothpaste tube very tightly with a lot of rubber bands; they will squeeze the toothpaste out the end of your tube."
The new view of the heliosphere was discovered by accident as the team studied surprising data from the Voyager 1 spacecraft and tried to understand how the galaxy's magnetic field interacts with the heliosphere. One of two identical twin spacecraft launched in 1977, Voyager 1 became the first man-made object to exit the heliosphere and plunge into interstellar space in 2012.
"Most researchers don't believe in the importance of the solar magnetic field, because the magnetic pressure on the solar wind's particles is far lower than the thermal pressure of the particles," said Merav Opher, an associate professor of astronomy and director of the Center for Space Physics at Boston University, who is the lead author of the study. However, the model shows that tension of the magnetic field controls what happens to the solar wind in the tail.
"It's also exciting that these jets are very turbulent, and will be very good particle accelerators," Opher said. The jets might, for example, play a role in the acceleration of so-called anomalous cosmic rays. "We don't know where these particles are accelerated; it's a bit of a puzzle."
Solving such puzzles will be important for future space travel. The heliosphere protects the solar system by filtering galactic cosmic rays, and upon leaving this protective shell, spacecraft will be bombarded by much higher levels of damaging radiation.
"Understanding the physical phenomena that govern the shape of the heliosphere will help us understand the filter," Opher said.
More data on the heliosphere's boundaries will become available sometime in the next few years when Voyager 2, like its twin, crosses into interstellar space.
In addition to Drake and Opher, study authors included Bertalan Zieger of Boston University and Tamas Gombosi of the University of Michigan.
This research was supported by the National Aeronautics and Space Administration (Award Nos. SMD-14-4986, NNX14AIB0G, NNX13AE04G and NNX14AF42G). The content of this article does not necessarily reflect the views of this organization.
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The research paper, "Magnetized jets driven by the sun: the structure of the heliosphere revisited," Merav Opher, James Drake, Bertalan Zieger and Tamas Gombosi, was published Feb. 19, 2015, in the journal Astrophysical Journal Letters.
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Journal
The Astrophysical Journal Letters