PLANETARY SCIENCE:
Enhanced: News from the Edge of Interstellar Space

Auroral activity and magnetic storms that occasionally disrupt electric power grids are caused by the supersonic solar wind [HN1] that sweeps past Earth as it blows radially away from the Sun. This wind creates the heliosphere [HN2] , a bubble of magnetized plasma that surrounds the Sun and includes the orbits of all known planets. Two spacecraft, Voyager 1 and 2 [HN3] , are approaching the edge of the heliosphere [HN4]--the heliopause--where the radially decreasing pressure of the expanding solar wind balances the inward pressure of the local interstellar medium [HN5]. En route, the spacecraft are sending back important information about the far reaches of the solar system.

The size of the heliosphere varies as the solar wind pressure changes with the 11-year solar cycle [HN6], with maximum size at the time of minimum solar activity (1). Currently, the heliosphere is shrinking because solar activity is near its maximum. Furthermore, it is distorted into a cometlike shape by the motion of the interstellar medium relative to the Sun. A long tail extends in the downwind direction. The two Voyager spacecraft are headed in the opposite, upwind direction, where the heliopause is closest to the Sun (see the first figure). 

SOURCE: ADAPTED FROM (18 )

The exact location of the heliopause remains uncertain because the strength and direction of the interstellar magnetic field and the densities of ionized H and He, which contribute to the interstellar pressure, are not precisely known. If we knew the size of the heliosphere, we would thus also have a measure of the properties of the local interstellar medium.

The Voyager spacecraft are still too far from the heliopause to provide direct information about its location. But they may soon encounter another feature of the outer solar system, the termination shock [HN7], which provides an important clue to the overall size of the heliosphere.

As the solar wind approaches within ~40 astronomical units (AU) (1 AU = Sun-Earth distance) of the heliopause, it slows abruptly to a hot, subsonic flow that gradually turns in the tailward direction. This results in a termination shock front, the location of which is a direct indication of the size of the heliosphere and the pressure of the local interstellar medium. The location of the termination shock has not yet been determined directly. But there have been recent estimates based on five distinct approaches (see the second figure).

The first approach is based on the dynamic pressure balance between the solar wind and the interstellar magnetic field. Using Voyager 2 measurements for the solar wind pressure and assuming an interstellar magnetic field of 0.5 nanotesla [HN8], Belcher [HN9]et al. (2) calculated that for 1990 through 1993, the termination shock was on average located between 78 and 108 AU. Although based on a highly uncertain estimate of the interstellar magnetic field, this interstellar pressure is very similar to a more recent assessment that included uncertainties in the various contributions to the interstellar pressure and led to a distance of 88 ± 22 AU ( 3, 4).

Ionized interstellar H and He are deflected around the heliosphere. In contrast, neutral interstellar atoms flow deep into the heliosphere, where they are ionized and picked up by the interplanetary magnetic field in the outward flowing solar wind. Using Ulysses [HN10] observations of such interstellar pickup ions [HN11] within 5 AU of the Sun, Gloeckler [HN12] et al. ( 5) derived an upper limit of ~85 AU for the shock distance. Ulysses observations of increased solar wind dynamic pressure at high solar latitudes have also been incorporated into models of the interaction of the heliosphere and the interstellar medium, yielding shock locations of 88 AU (6 ), 95 AU (7), 80 ± 10 AU (8 ), and 88 to 103 AU (9).

Low-frequency radio emissions [HN13] thought to be excited in the local interstellar medium by the arrival of a large interplanetary shock complex are exploited in the second approach. Gurnett and Kurth [HN14] have combined the observed onset times of these radio emissions with a magnetohydrodynamic model for the propagation of such shocks outward from the Sun (10 ). They estimated a heliopause located at 110 to 160 AU, corresponding to a termination shock at 80 to 115 AU. In a recent shock propagation model that includes the presence of interstellar neutral atoms and pickup ions in the heliosphere, Zank [HN15]et al. ( 11) estimated a termination shock distance of less than 85 to 90 AU.

The backscattering of solar H Lyman-a radiation [HN16] by neutral interstellar H provides a third route to estimating the size of the heliosphere. Differences in the backscattered ultraviolet intensities observed by Voyager (upwind) and Pioneer 10 [HN17] (downwind) in 1990 imply the existence of a termination shock between 70 and 105 AU (12).

The termination shock accelerates interstellar pickup ions to total energies of more than 250 MeV. These anomalous cosmic rays diffuse and drift inward from the shock, establishing an intensity gradient that provides a fourth method for estimating the shock distance. Extrapolating the gradients observed inside 49 AU outward to the shock source gave a best fit shock location of 84 ± 5 AU for the 1980s (13 ).

The final approach is based on the observation that outward propagating shock complexes cause transient decreases in the intensities of anomalous and galactic cosmic rays [HN18]. Model calculations indicate that the duration of such a decrease reflects the transit time of the interplanetary shock complex to the termination shock ( 14). Local temporal variations often perturb the observed recovery, but analysis of one such transient decrease in 1999 indicated that the termination shock was only 10 AU beyond Voyager 1 at 83 ± 1 AU ( 15). A related study used the duration of a transient inward flow of anomalous cosmic rays, leading to a shock distance of 88.5 ± 7 AU (16).

The assumptions and simplifications in each of the five methods introduce further uncertainties that are more difficult to quantify. Nevertheless, the models and observations are sufficiently different that the systematic uncertainties are unlikely to be correlated among the methods. The clustering of the estimates between 80 and 100 AU thus lends additional weight to the individual estimates.

The location of the termination shock is not fixed in time. Whang and Burlaga [HN19] have incorporated the temporal variations observed by Voyager 2 into a two-dimensional magnetohydrodynamic model (1). They find that the location of the termination shock varies by about 20 AU over the solar cycle. The distance is smallest following times of maximum solar activity.

The shock is presently moving inward (see the second figure). Within the next few years, wind speed and pressure will increase, and with the arrival of the increased pressure, the termination shock will begin moving outward. This will affect when the Voyager spacecraft encounters the termination shock.

Voyager 1 is currently at ~82 AU and moves outward at 3.6 AU per year, followed by Voyager 2, now at ~66 AU and moving outward at 3.3 AU per year [HN20]. Comparison with the Voyager 1 trajectory suggests the possibility of one or more encounters with the termination shock by 2005. If there has been no encounter by then, the shock will likely be moving outward again. It may then be 2 to 5 more years before it moves back into range for Voyager 1 to take a direct measure of the size of the heliosphere (17).

1. Y. C. Whang, L. F. Burlaga, Geophys. Res. Lett. 27, 1607 (2000) [ADS][fulltext]. 
2. J. W. Belcher et al., J. Geophys. Res. 98, 15177 (1993) [ADS]. 
3. R. von Steiger et al., Space Sci. Rev. 78, 1 (1996) [issue contents]. 
4. A. C. Cummings et al., personal communcation. 
5. G. Gloeckler et al., Nature 386 , 374 (1997) [ADS]. 
6. H. L. Pauls, G. P. Zank, J. Geophys. Res. 101, 17081 (1996) [ADS]. 
7. _------, J. Geophys. Res. 102 , 19779 (1997) [ADS]. 
8. T. J. Linde et al., J. Geophys. Res. 103, 1889 (1998) [ADS]. 
9. G. Exarhos, X. Moussas, Astrophys J. 542 , 1075 (2000) [ADS]. 
10. D. A Gurnett, W. S. Kurth, Space Sci. Rev. 78, 53 (1996) [ADS][abstract]. 
11. G. Zank et al., in preparation. 
12. D. Hall et al., J. Geophys. Res. 98, 15185 (1993) [ADS]. 
13. E. C. Stone, A. C. Cummings, Proc. 26th Int. Cosmic Ray Conf. 7, 500 (1999) [proceedings][fulltext]. 
14. J. A. le Roux, H. Fichtner, J. Geophys. Res. 104, 4709 (1999) [ADS]. 
15. W. R. Webber et al., J. Geophys. Res. 106, 253 (2001) [ADS]. 
16. F. B. McDonald et al., J. Geophys. Res. 105, 1 (2000) [ADS]. 
17. E. C. Stone and A. C. Cummings [Proc. 27th Int. Cosmic Ray Conf. (2001)] give further details on estimates of the termination shock location [conference Web site] . 
18. G. P. Zank, Space Sci. Rev. 89, 413 (1999) [ADS]. 

HyperNotes
Related Resources on the World Wide Web

The Academic Press Dictionary of Science and Technology is made available on the Web by its publisher Harcourt. 

Astronomy Unbound, a virtual astronomy text from the Department of Physical Sciences, University of Hertfordshire, UK, provides a glossary . 

The Stanford Solar Center provides a solar glossary. 

The Space Environment Center of the National Oceanic and Atmospheric Administration (NOAA) provides a glossary of solar-terrestrial terms and a primer on the space environment. 

AstroWeb, a collection of pointers to astronomy-related information available on the Internet, is maintained by the AstroWeb Consortium, an international collaboration involving seven institutions. 

Links ²Go provides links to Internet resources on space physics. 

The Space Physics and Aeronomy Section of the American Geophysical Union provides a collection of Internet resources, as well as a selection of educational Web sites. 

The NASA Astrophysics Data System makes available abstracts of astronomy and astrophysics journal articles, an archive of articles from some journals, and other resources. 

Windows to the Universe, sponsored by NASA, offers presentations about Earth and space science topics. 

Cosmic and Heliospheric Pages and Services is a browsable and searchable collection of Internet links provided by the National Space Science Data Center (NSSDC) at NASA Goddard Space Flight Center (GSFC). 

The Cosmic and Heliospheric Learning Center is provided by GSFC's Laboratory for High Energy Astrophysics. A glossary and Internet links are provided. 

The Space Physics Group at the University of Oulu, Finland, provides a collection of links to Internet resources in space physics as well as the Space Physics Textbook. 

NASA's Jet Propulsion Laboratory provides the Voyager Project Home Page. JPL offers a tutorial (with a glossary) on the basics of space flight. A presentation on the environment of the solar system is included. 

Bill Arnett's Nine Planets provides a presentation on the interplanetary medium. 

Astronomy Notes is a Web textbook by N. Strobel, Physical Science Department, Bakersfield College, CA. 

J. Evans, Department of Physics and Astronomy, George Mason University, Fairfax, VA, provides lecture notes for an astronomy course. Lecture notes on solar activity and the interstellar medium are included. 

H. E. Smith, Center for Astrophysics and Space Sciences, University of California, San Diego, offers an astronomy tutorial. A presentation on the interstellar medium is included 

The Astronomica Web site makes available Astronomy: The Cosmic Journey, a Web textbook by W. Hartmann and C. Impey, Department of Astronomy, University of Arizona. 

S. Daunt, Department Physics and Astronomy, University of Tennessee, offers lecture notes for courses on the solar system and stars, galaxies, and cosmology. Presentations on the solar wind and the interstellar medium are included. 

S. Myers, National Radio Astronomy Observatory, Socorro, NM, provides lecture notes for an astrophysics course. 

I. Cairns, School of Physics, University of Sydney, Australia, provides lecture notes (see the course syllabus for the topics of the numbered lectures) for a course on solar and space physics. 

E. Möbius, Experimental Space Plasma Group, University of New Hampshire, provides lecture notes for a course on the solar wind and cosmic rays. 

A Science Strategy for Space Physics, a 1995 report from the Space Studies Board of the National Academy of Sciences, includes a chapter on the physics of the solar wind and heliosphere. 

The January-February 2000 issue of American Scientist had an article by P. Frisch about the heliosphere and interstellar medium titled "The galactic environment of the sun." 

1. Windows to the Universe provides an introduction to the solar wind. The Solar Physics Group, NASA Marshall Space Flight Center, provides an introduction to the solar wind and the heliosphere. The Solar-Heliospheric Research Group, University of Michigan, provides information on the solar wind and its effects in a virtual tour for nonscientists. The Cosmic and Heliospheric Learning Center offers a presentation on the solar wind. Exploration of the Earth's Magnetosphere, an educational presentation from GSFC by D. Stern and M. Peredo, includes information on the solar wind (and its history). A. Galvin , Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, makes available a presentation titled "Solar wind and particles." The Atmospheric Physics Laboratory, University College London, makes available lecture notes by A. Aylward on the solar wind and its interaction with the planets for a solar system science course. The U.S. National Report to International Union of Geodesy and Geophysics, 1991-1994 has a contribution by K. Ogilvie and M. Coplan titled "Solar wind composition." The 9 March 2001 issue of Science had an Enhanced Perspective by R. Lundin titled "Erosion by the solar wind." The 16 June 2000 issue had a review by J. Lyon titled "The solar wind-magnetosphere-ionosphere system." 
2. The Education and Outreach Center of GSFC's Laboratory for Extraterrestrial Physics defines heliosphere. The Cosmic and Heliospheric Learning Center offers a presentation on the heliosphere. The Ulysses mission Web site from the European Space Agency (ESA) provides an introduction to the heliosphere. H.-R. M¸ller, Bartol Research Institute, University of Delaware, offers a presentation on the heliosphere. The U.S. National Report to International Union of Geodesy and Geophysics, 1991-1994 has a contribution by M. Neugebauer titled "Charting the heliosphere in three dimensions." The Voyager Plasma Science Experiment Web site from the MIT Space Plasma Group offers a presentation on the heliosphere by W. Axford and S. Suess, as well as a prototype heliospheric animation. N. Schwadron, Space Research Laboratory, University of Michigan, offers a slide introduction to heliospheric physics. I. Cairns provides lecture notes on the outer heliosphere for a course on solar and space physics. 
3. NSSDC provides a Voyager Project Information Web site. JPL's Voyager Project Home Page offers a description of Voyager's interstellar mission. The MIT Space Plasma Group maintains a Voyager Plasma Science Experiment Web site. JPL issued a 2 September 1997 press release titled "Two Voyager spacecraft still going strong after 20 years" and an 18 December 2000 press release titled "Most distant spacecraft may reach shock zone soon." Discovery.com offers a 20 December 2000 article by L. O'Hanlan titled "Voyager 1 heads for interstellar space." 
4. The Cosmic and Heliospheric Learning Center provides a definition of heliopause. Stanford Solar Center's glossary defines heliopause. The Encyclopædia Britannica, provided by Britannica.com, includes an introduction to the heliopause . The Voyager Plasma Wave Investigation Web site at the University of Iowa makes available a 1993 NASA press release about Voyager 1 and 2 spacecraft discovering evidence of the heliopause. The 15 April 1998 issue of Geophysical Research Letters had an article by M. Gruntman and H. Fahr titled "Access to the heliospheric boundary: EUV-echoes from the heliopause" (made available on the Web by the Space Physics Group at the University of Washington). 
5. The Astronomy Unbound glossary defines interstellar medium. N. Strobel's Astronomy notes includes a section on the interstellar medium. The Education and Outreach Center of GSFC's Laboratory for Extraterrestrial Physics provides an introduction to the interstellar medium. The Experimental Space Plasma Group, University of New Hampshire, provides a tutorial on the interstellar medium; a presentation on the local interstellar medium is included. J. Schombert, Department of Physics, University of Oregon, provides lecture notes on the interstellar medium for a course on the birth and death of stars. J. Hawley, Department of Astronomy, University of Virginia, provides lecture notes on the interstellar medium for an astronomy course. E. Möbius provides lecture notes (in Adobe Acrobat format) and review notes on the interstellar medium for an astronomy course. A. Goodman, Department of Astronomy, Harvard University, provides lecture notes (in Adobe Acrobat format) for a course on the physics of the interstellar medium. JPL's Interstellar Probe Web site offers a presentation on the interaction of the interstellar medium with the solar wind. S. Suess, Solar Physics Group, NASA Marshall Space Flight Center, provides a presentation on the scientific objectives of the interstellar probe mission. The online proceedings Science with the Hubble Space Telescope - II offers a section of papers on the interstellar medium that includes a contribution by R. Lallement et al. titled "The GHRS and the heliosphere." T. Glover, Department of Physics and Astronomy, Rice University, offers a presentation on the 3D mapping of the local interstellar medium. 
6. Windows to the Universe provides an introduction to the solar cycle. The Space Physics Textbook includes a section on the solar cycle. The Encyclopædia Britannica article on the sun includes a section on sunspots and solar activity. Sunspots, a presentation of San Francisco's Exploratorium, includes a section on the sunspot cycle. Sunspots and the Solar Cycle is an educational Web site sponsored by Science@NASA. The Solar and Astrophysics Laboratory at the Lockheed Martin Advanced Technology Center presents an introduction to solar cycles. The Solar Physics Group, NASA Marshall Space Flight Center, provides an introduction to the sunspot cycle. Science@NASA provides a 15 February 2001 article titled "The sun does a flip." 
7. I. Cairns includes a section on the termination shock in his lecture notes on the outer heliosphere. The Proceedings of the Grand Challenge Computing Science Fair, presented by the Center for Advanced Computing Research, California Institute of Technology, includes a presentation by P. Liewer, N. Omidi, and Bruce Goldstein titled "Particle simulations of the solar wind termination shock." Axford and Suess's presentation on the heliosphere has a section on the distance to the termination shock. A. Eriksson, Swedish Institute of Space Physics, Uppsala, offers a presentation on shock waves titled "The heliosphere in the kitchen sink" for a course on space physics. 
8. The online American Heritage Dictionary defines nanotesla and tesla. Nanotesla and tesla are defined in the Dictionary of Units of Measurement, provided by R. Rowlett, Center for Mathematics and Science Education, University of North Carolina. 
9. J. Belcher is in the Space Plasma Group, MIT Center for Space Research, and at the Department of Physics, Massachusetts Institute of Technology. 
10. NSSDC provides information about Ulysses, a joint ESA/NASA mission to study the Sun and solar wind at high latitudes. JPL provides a Ulysses Web page. ESA provides information about the Ulysses mission; a presentation about Ulysses' findings on the heliosphere from the first orbit is provided. The November 1997 issue of the ESA Bulletin had an article by R. Marsden, K.-P. Wenzel, and E. Smith titled "The heliosphere in perspective -- Key results from the Ulysses mission at solar minimum." The 21 June 1997 issue of the Economist had an article (made available by Britannica.com) on the Ulysses mission titled "Space odyssey." 
11. Pickup ion is defined in the glossary of the interstellar medium tutorial. The glossary of the Cosmic and Heliospheric Learning Center includes a definition of pickup ions. The U.S. National Report to International Union of Geodesy and Geophysics, 1991-1994 has a contribution by P. Isenberg titled "Interstellar pickup ions: Not just theory anymore." The Solar and Heliospheric Research Group at the University of Michigan offers a presentation on cosmic ray transport theory and interstellar pickup ions. I. Cairns provides a section on interstellar pickup ions in lecture notes on kinetic and small-scale solar wind physics. 
12. G. Gloeckler is in the Space Physics Group , Institute for Physical Science and Technology, University of Maryland. The Solar and Heliospheric Research Group at the University of Michigan makes available a preprint of a 1998 paper by G. Gloeckler and J. Geiss titled "Interstellar and inner source pickup ions observed with SWICS on Ulysses." 
13. I. Cairns includes a section on radio emissions from the outer heliosphere in his lecture notes on the outer heliosphere. Electronic Publications of the Astronomical Society of Australia had an October 1999 article by I. Cairns, P. Robinson, and G. Zank. with a section on outer heliospheric radio emissions. 
14. D. Gurnett and W. Kurth are in Space Plasma Wave Group, Department of Physics and Astronomy, University of Iowa. 
15. G. Zank is at the Bartol Research Institute, University of Delaware. 
16. Lyman-alpha radiation is defined in the Academic Press Dictionary of Science and Technology. The xrefer Web site provides information about the Lyman series and Theodore Lyman. The NIGHTGLOW mission Web site provides a definition of Lyman alpha radiation. The Geophysical Research Web page of the Finnish Meteorological Institute provides a section about measuring scattered Lyman-alpha radiation in a presentation about SOHO's SWAN (Solar Wind ANisotropies) instrument. T. Herter, Department of Astronomy, Cornell University, provides lecture notes on the hydrogen atom's spectral lines for an astronomy course. 
17. NSSDC provides information on the Pioneer 10 mission. The Pioneer Project Home Page provides an illustration of the flight path of Pioneer 10. NASA issued a 1 May 1999 press release titled "Data from Pioneer 10 may determine if it is still interior the heliopause" as well as a 3 June 2001 Pioneer 10 update. Science@NASA provides a 3 May 2001 article about Pioneer 10 titled "Seven billion miles and counting...." 
18. R. Mewaldt , Space Radiation Laboratory, California Institute of Technology, presents an introduction to cosmic rays. The Space Physics Textbook includes information on cosmic rays with a section on anomalous cosmic rays. The Cosmic and Heliospheric Learning Center provides a definition of anomalous cosmic rays as well as a presentation on cosmic rays with a section on anomalous cosmic rays. For an online high school course on cosmic rays, the Centre for Subatomic Research, University of Alberta, makes available a presentation by J. Guercio titled "Cosmic rays, high energy phenomena" which has a section on anomalous cosmic rays. A. Cummings, Space Radiation Laboratory, California Institute of Technology, makes available 1996 preprints of an article by E. Stone, A. Cummings, and W. Webber titled "The distance to the solar wind termination shock in 1993 and 1994 from observations of anomalous cosmic rays" and an article by Cummings and Stone titled "Composition of anomalous cosmic rays and implications for the heliosphere." 
19. Y. Whang is in the Department of Mechanical Engineering, Catholic University of America, Washington, DC. L. Burlaga is in the Interplanetary Physics Branch of the Laboratory for Extraterrestrial Physics at NASA GSFC. The article by Whang and Burlaga titled "Anticipated Voyager crossing of the termination shock" (1) appeared in the 1 June 2000 issue of Geophysical Research Letters. 
20. The Voyager Cosmic Ray Subsystem Web page at GSFC provides an image showing the location and trajectories of the Voyager spacecraft. The MIT Space Plasma Group provides a Voyager Trajectory Page. The Heavens-Above Web site, hosted by the German Space Operations Center, shows the current positions of the Pioneer and Voyager spacecraft. 
21. E. C. Stone is at Jet Propulsion Laboratory and the Space Radiation Laboratory , California Institute of Technology.