Interstellar magnetic field deforms the heliopause, a bubble inflated by solar wind, which reduces the incidence of cosmic rays on the planets
A Brazilian astrophysicist, Merav Opher, from George Mason University in the United States, has discovered what our Solar System looks like when seen from afar. The image of “our home in the galaxy”, as she describes it, is a type of huge bubble that contains the Sun and planets and functions like a shield that prevents the invasion of galactic cosmic rays, one of the most energy-rich types of particles and deadly to astronauts on interplanetary voyages. This bubble, called the heliopause, is inflated by the particle wind released by the Sun in the environment of extremely rarified gas existing in the stars. In the voyage of the Solar System around the galaxy’s nucleus, the heliopause collides with another gigantic moving interstellar cloud of gas and dust that crosses its path. As a result, this shock makes the heliopause take on a shape similar to that of the comets that travel against the solar wind, with a nose in front, followed by a long tail. Together with Edward Stone, from the California Institute of Technology (Caltech), Merav published, in May, a map of the heliopause’s nose in the journal Science, analyzing how the interstellar environment distorts it.
The task demanded more than just the analysis of data . From 2001 to 2004, Merav worked at the Jet Propulsion Agency of the North American Space Agency (Nasa), in Caltech, and crossed the country several times, from California, in the far west, to the University of Michigan, in the Great Lakes Region. Her objective was to learn from Tamas Gombosi how to use a computer program he developed that can simulate the interaction between magnetic fields and electrically charged particles in 3-D.
After she had learned how to use the program, Merav adapted it to reproduce the physical conditions on the outskirts of the Solar System and persuaded Stone, a veteran experimental physicist, who was the scientific director for the Voyager mission but was reluctant to become involved in any new collaboration, to work on her theoretical model. “Stone saw that I needed to examine the theory more closely if I wanted to understand the data from the Voyagers better,” says Merav. Similarly, she needed his experience with data from Voyager 1 and 2, currently more than 100 times further from the Sun than the Earth to Sun distance, which astronomers call the astronomic unit, in order to compare them with the results of her computer simulations. “It took a long time to discover how to use the program creatively so as to extract information from the data,” relates the astrophysicist.
At 100 astronomic units from the Sun, the Voyagers have another 50 astronomic units ahead of them before they reach the heliopause’s nose. They are currently close to another interesting, almost spherical region, where the solar wind collides with the gas in the interstellar environment. There, the speed of the wind drops abruptly from 400 to 40 km per second. “It’s like the Iguassu Falls, where the speed of the water is dramatically reduced after the drop,” compares Merav.
In December 2004, Voyager 1 entered the northern hemisphere of this turbulent region where the solar wind fall meets the calm interstellar flow. The whirlpool and magnetic field of the concentrated solar wind accelerate electrically charged particles, which Voyager 1 should detect in equal quantities coming from all directions as it nears this region. Contrary to expectations, however, the probe received more particles on its left side. Merav’s model showed that the data from Voyager 1 could only be explained if the shape of the sphere in the collision zone was somewhat crushed. The most surprising thing is that the deformation is caused by the magnetic force field of the interstellar environment that surrounds the heliopause. No one expected something so distant being able to influence the collision zone.
Simulations
To understand the deformation in the collision zone, Merav and Stone had to determine the precise direction of the magnetic field in our interstellar neighborhood. They concentrated, therefore, on a band in the sky in which the Voyagers had detected radio signals coming from the heliopause, where the solar and the galactic magnetic fields touch. In her simulations, Merav varied the inclination of the interstellar magnetic field until the band of radio emissions coincided with what had been observed by the Voyagers. “I tried nearly 90 different models,” she recounts. Previous studies had suggested very different values for the inclination of the magnetic field in the interstellar environment relative to the plane of the Solar System – one indicated 60o and another 90o. Merav and Stone’s work solved the contradiction, showing that the previous experiments had suggested possible extreme values for the field’s inclination. According to Merav, given the precision and data available, one can only state that the interstellar magnetic field in that region inclines between 60o and 90o.
This inclination in the field pushes both the heliopause as well as the collision zone toward the inside of the southern hemisphere of the Solar System. Data sent by Voyager 2, which is traveling to the south of the Solar System, confirm Merav’s model. In 2006 the probe started to give off signals that it was approaching the collision zone at a distance that was less than one would expect, unless this region were deformed. Calculations indicate that Voyager 2 should enter this region this year, 9 astronomic units before Voyager 1, in the northern hemisphere.
Studying the Sun’s region of influence, the heliosphere, and its frontier teaches one a lot about other stars. “It’s the only way of finding out how the stars interact with their environment,” explains Merav. The unexpected sensitivity of the heliosphere to the galactic magnetic field discovered by Merav indicates that in stars with stronger magnetic fields the effects should be even more marked.
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