FIGURE 7.12 A side view showing the paths of Voyager 1 and 2 out of the heliosphere. In this model, the supersonic solar wind (green) streams radially outward from the Sun (gray arrows). The wind abruptly slows at the termination shock, forming the hot heliosheath (red) as it turns to flow tailward. The cooler interstellar wind from the left (blue with gray streamlines) carries the interstellar magnetic field (black arrows) with it. The wind is deflected around the heliosphere, pressing the magnetic field strongly inward in the southern hemisphere and pushing the shock closer to the Sun at Voyager 2. SOURCE: Courtesy of Merav Opher, George Mason University Department of Physics and Astronomy.

FIGURE 7.12 A side view showing the paths of Voyager 1 and 2 out of the heliosphere. In this model, the supersonic solar wind (green) streams radially outward from the Sun (gray arrows). The wind abruptly slows at the termination shock, forming the hot heliosheath (red) as it turns to flow tailward. The cooler interstellar wind from the left (blue with gray streamlines) carries the interstellar magnetic field (black arrows) with it. The wind is deflected around the heliosphere, pressing the magnetic field strongly inward in the southern hemisphere and pushing the shock closer to the Sun at Voyager 2. SOURCE: Courtesy of Merav Opher, George Mason University Department of Physics and Astronomy.

It was predicted that some of the interstellar ions would bounce back and forth like cosmic ping pong balls between the magnetic fields on opposite sides of the shock, becoming low energy cosmic rays as they slowly gained speeds up to half the speed of light. As a result, it was expected that the intensity of these cosmic rays would be highest at the shock. However, neither Voyager found an intensity maximum at the shock. Instead, the intensity is higher further out in the heliosheath away from the shock, indicating that the source of these low energy cosmic rays is not the shock regions crossed by the two spacecraft. Future observations may reveal whether anomalous cosmic rays originate at remote regions of the shock or in the outer regions of the heliosheath.

As the two Voyagers continue to explore this new region of the solar system, the Interstellar Boundary Explorer (IBEX) launched in 2008 will make a two-dimensional map of the heliosheath as viewed from Earth orbit. Measuring the intensity of neutral atoms streaming toward Earth from the heliosheath, IBEX will provide a new estimate of the distance across the heliosheath to the edge of interstellar space.

Although uncertain, current indications are that Voyager 1 may cross the heliosheath and enter interstellar space by 2015. Surrounded for the first time by matter from other stars, it will measure the direction and strength of the local interstellar magnetic field draped around the heliosphere and the intensity of low energy cosmic rays from the galaxy that are blocked from entering the heliosphere. If the spacecraft remains healthy, there is enough electrical power from its radioisotope thermoelectric generators to last well beyond 2020 when Voyager 1 will be more than 150 AU from the Sun.

A NEW VIEW OF THE SOLAR SYSTEM AND THE HELIOSPHERE

The International Geophysical Year ushered in the Space Age. The subsequent era of space science has given us a new view of the solar system, revealing dozens of unexpectedly diverse worlds enveloped in a giant heliospheric bubble created by the Sun. It has also given us our first journey to interstellar space, setting the stage for even more distant journeys. But most importantly, it has given us the both knowledge that there is much more to be discovered and the impetus to launch future journeys of exploration.



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