THE SUN’S GALACTIC ENVIRONMENT

THE OUTER LIMITS AND BEYOND

Where does the heliosphere end and the Sun’s galactic environment—the local interstellar medium (LISM)—begin? The inner boundary of the heliosphere is a shock wave, the so-called termination shock, that forms where the supersonic solar wind is slowed to subsonic speeds by its encounter with the LISM. The heliosphere’s outer boundary is the heliopause. Between it and the termination shock is the heliosheath—a region of solar wind that has been slowed and heated by its passage through the shock. The thickness of the heliosheath and the location of the heliopause are not known. However, recently published data suggest that the Voyager 1 deep-space probe may have encountered the termination shock at a distance of some 86 AU—more than 12.5 billion kilometers—from the Sun.

The heliosphere’s boundaries are not static, but instead move with changes in the solar wind’s density and speed, and vary in location with the solar cycle. The size and structure of the heliosphere depend not only on the properties of the solar wind but also on those of the LISM, and these change during the course of the Sun’s journey around our galaxy. (The Sun orbits the galactic center at a speed of some 250 kilometers per second.) For the past few thousand years, for example, the Sun has been immersed in a low-density (0.2 protons per cubic centimeter) bubble within the local interstellar cloud with a temperature of about 7,000 K. At some point in the future, however, it will enter a different region of our galaxy, perhaps one of greater density. Recent computer simulations suggest that an encounter with an interstellar cloud whose density is 10 protons per cubic centimeter could push the termination shock as close to the Sun as 10 to 14 AU, well inside the orbit of Uranus. Such a dramatic change would increase the number of cosmic rays reaching Earth and could alter the interaction of our planet’s magnetosphere with the solar wind.

The boundaries of the heliosphere are not completely impermeable to the LISM. Interstellar plasmas and magnetic fields are excluded from the heliosphere,



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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics THE SUN’S GALACTIC ENVIRONMENT THE OUTER LIMITS AND BEYOND Where does the heliosphere end and the Sun’s galactic environment—the local interstellar medium (LISM)—begin? The inner boundary of the heliosphere is a shock wave, the so-called termination shock, that forms where the supersonic solar wind is slowed to subsonic speeds by its encounter with the LISM. The heliosphere’s outer boundary is the heliopause. Between it and the termination shock is the heliosheath—a region of solar wind that has been slowed and heated by its passage through the shock. The thickness of the heliosheath and the location of the heliopause are not known. However, recently published data suggest that the Voyager 1 deep-space probe may have encountered the termination shock at a distance of some 86 AU—more than 12.5 billion kilometers—from the Sun. The heliosphere’s boundaries are not static, but instead move with changes in the solar wind’s density and speed, and vary in location with the solar cycle. The size and structure of the heliosphere depend not only on the properties of the solar wind but also on those of the LISM, and these change during the course of the Sun’s journey around our galaxy. (The Sun orbits the galactic center at a speed of some 250 kilometers per second.) For the past few thousand years, for example, the Sun has been immersed in a low-density (0.2 protons per cubic centimeter) bubble within the local interstellar cloud with a temperature of about 7,000 K. At some point in the future, however, it will enter a different region of our galaxy, perhaps one of greater density. Recent computer simulations suggest that an encounter with an interstellar cloud whose density is 10 protons per cubic centimeter could push the termination shock as close to the Sun as 10 to 14 AU, well inside the orbit of Uranus. Such a dramatic change would increase the number of cosmic rays reaching Earth and could alter the interaction of our planet’s magnetosphere with the solar wind. The boundaries of the heliosphere are not completely impermeable to the LISM. Interstellar plasmas and magnetic fields are excluded from the heliosphere,

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Understanding the Sun and Solar System Plasmas: Future Directions in Solar and Space Physics Artist’s concept showing the heliosphere and its immediate galactic environment. Distances are indicated on a logarithmic scale. Earth’s nearest stellar neighbor, α Centauri, is 4 light-years away. but the electrically neutral, unmagnetized gases and large dust grains of the interstellar medium are not, and flow unimpeded across the heliopause and into the heliosphere, where they interact with the solar wind. A fascinating aspect of this interaction is the creation of anomalous cosmic rays (ACRs), a population of cosmic rays different in origin and composition from galactic cosmic rays. ACRs are produced from interstellar neutral atoms that are turned into ions by solar ultraviolet radiation, or by loss of an electron to a solar wind ion. These newly created ions are then “picked up” by the solar wind plasma and swept along in its outward flow to the termination shock, where they are accelerated to extremely high energies. Scientists have long dreamed of sending a spacecraft to the boundaries of the heliosphere and into the interstellar medium beyond, of setting sail into the cosmic ocean. The twin Voyager spacecraft may provide a tantalizing first glimpse of this exotic environment; however, they are more than 25 years old, some of their instruments are no longer functioning, and the veteran explorers may run out of power before crossing the heliopause. What is needed is an Interstellar Probe, a mission specifically designed for the comprehensive investigation of the boundaries of the heliosphere and for the exploration of our local galactic environment. To reach its destination within a reasonable time (~15 years), an Interstellar Probe will have to travel much faster than any existing spacecraft. The Survey Committee urges that development of the needed advanced propulsion technology be given high priority. Although the LISM remains beyond reach for the present, interstellar material that finds its way into the inner heliosphere provides scientists with tantalizing samples of the stuff of our galactic environment. The Survey Committee endorses the concept of an Interstellar Sampler mission, which will measure interstellar neutrals and associated pickup ions (extrasolar ions picked up and carried along by the solar wind) in the inner heliosphere (1 to 4 AU) and provide energetic neutral atom and extreme-ultraviolet images of the heliospheric boundaries.