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Images from the Ion and Neutral Camera (INCA), part of the Magnetospheric Imaging Instrument on NASA's Cassini spacecraft, suggest that the heliosphere may not have the comet-like shape predicted by existing models. The instrument imaged a population of hot particles that resides just beyond the boundary of where the solar wind collides with the interstellar medium, forming a termination shock.

In a paper published Oct. 15 in Science, APL researchers describe how this population of particles may be more influenced by the interstellar medium's magnetic fields than previously thought, and that the heliosphere is therefore more spherical.

In this graphic, the multicolor bubble represents new measurements of particles called energetic neutral atoms streaming in from the thick boundary known as the heliosheath, between the heliosphere (the region of our sun’s influence) and the interstellar medium (the matter between stars in our corner of the galaxy). The yellow circle is our sun. The two Voyager spacecraft, illustrated with lines showing their path, are currently traveling through the heliosheath, which is the outer layer of the heliosphere where solar wind slows down and heats up as it interacts with the interstellar medium. Cassini is not pictured, but it is still inside our solar system, orbiting Saturn. The dark inner circle represents the volume bounded by the termination shock, formed where supersonic solar wind streaming out from our sun suddenly slows down; the outer boundary of the heliosheath, where the interstellar medium and solar wind pressures balance, is called the heliopause. To the left of this bubble is the curve of the bow shock, where the interstellar medium traveling in the opposite direction slows down as it collides with the heliosphere, like the wave formed in a stream as it flows around a rock.

Courtesy: The Johns Hopkins University Applied Physics Laboratory

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