Exploration of the Outer Heliosphere and the Local Interstellar Medium

A WORKSHOP REPORT

Committee on Solar and Space Physics

Space Studies Board

Division on Engineering and Physical Sciences

NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES

THE NATIONAL ACADEMIES PRESS
Washington, D.C.
www.nap.edu



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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report Exploration of the Outer Heliosphere and the Local Interstellar Medium A WORKSHOP REPORT Committee on Solar and Space Physics Space Studies Board Division on Engineering and Physical Sciences NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES THE NATIONAL ACADEMIES PRESS Washington, D.C. www.nap.edu

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report THE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington, DC 20001 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the panels responsible for the report were chosen for their special competences and with regard for appropriate balance. Support for this project was provided by Contract NASW 01001 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsors. Cover: Results from a self-consistent numerical simulation of the interaction of the solar wind with the partially ionized local interstellar medium (LISM). Shown in color is the temperature distribution (plotted on a logarithmic scale) of the solar wind and interstellar plasma for the two-dimensional, steady-state, two-shock heliosphere. The plasma boundaries—termination shock, heliopause, and bow shock—are labeled. The heliosheath comprises solar wind material, both supersonic and subsonic, and extends into the heliotail (labeled). The solid lines show the stream lines of the plasma. A key feature of the interaction of the solar wind with the LISM is the ability of neutral hydrogen to enter the heliosphere, where it can then couple to the solar wind by the process of charge exchange, depicted schematically by the arrow. The distances along the x and y axes are measured in astronomical units (AU). Image courtesy of H.R. Mueller and G.P. Zank, University of California, Riverside. International Standard Book Number 0-309-09186-1 (Book) International Standard Book Number 0-309-53140-3 (PDF) Copies of this report are available free of charge from Space Studies Board National Research Council The Keck Center of the National Academies 500 Fifth Street, N.W. Washington, DC 20001 Additional copies of this report are available from the National Academies Press, 500 Fifth Street, N.W., Lockbox 285, Washington, DC 20055; (800) 624-6242 or (202) 334-3313 (in the Washington metropolitan area); Internet, http://www.nap.edu. Copyright 2004 by the National Academy of Sciences. All rights reserved. Printed in the United States of America

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Wm. A. Wulf is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National Research Council. www.national-academies.org

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report OTHER REPORTS OF THE SPACE STUDIES BOARD Issues and Opportunities Regarding the U.S. Space Program: A Summary Report of a Workshop on National Space Policy (2004) Plasma Physics of the Local Cosmos (2004) Steps to Facilitate Principal-Investigator-Led Earth Science Missions (2004) “Assessment of NASA’s Draft 2003 Earth Science Enterprise Strategy” (2003) “Assessment of NASA’s Draft 2003 Space Science Enterprise Strategy” (2003) Satellite Observations of the Earth’s Environment: Accelerating the Transition of Research to Operations (2003) The Sun to the Earth—and Beyond: Panel Reports (2003) Assessment of Directions in Microgravity and Physical Sciences Research at NASA (2002) Assessment of the Usefulness and Availability of NASA’s Earth and Space Science Mission Data (2002) Factors Affecting the Utilization of the International Space Station for Research in the Biological and Physical Sciences (2002) Life in the Universe: An Examination of U.S. and International Programs in Astrobiology (2002) New Frontiers in the Solar System: An Integrated Exploration Strategy (2002) Review of NASA’s Earth Science Enterprise Applications Program Plan (2002) “Review of the Redesigned Space Interferometry Mission (SIM)” (2002) Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface (2002) The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics (2002) Toward New Partnerships in Remote Sensing: Government, the Private Sector, and Earth Science Research (2002) Using Remote Sensing in State and Local Government: Information for Management and Decision Making (2002) Assessment of Mars Science and Mission Priorities (2001) The Mission of Microgravity and Physical Sciences Research at NASA (2001) The Quarantine and Certification of Martian Samples (2001) Readiness Issues Related to Research in the Biological and Physical Sciences on the International Space Station (2001) “Scientific Assessment of the Descoped Mission Concept for the Next Generation Space Telescope (NGST)” (2001) Signs of Life: A Report Based on the April 2000 Workshop on Life Detection Techniques (2001) Transforming Remote Sensing Data into Information and Applications (2001) U.S. Astronomy and Astrophysics: Managing an Integrated Program (2001) Limited copies of these reports are available free of charge from: Space Studies Board The National Academies 500 Fifth Street, NW, Washington, DC 20001 (202) 334-3477 ssb@nas.edu www.nationalacademies.org/ssb/ssb.html NOTE: Listed according to year of approval for release.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report COMMITTEE ON SOLAR AND SPACE PHYSICS JAMES L. BURCH, Southwest Research Institute, Chair CLAUDIA J. ALEXANDER, Jet Propulsion Laboratory VASSILIS ANGELOPOULOS, University of California, Berkeley ANTHONY CHAN, Rice University ANDREW F. CHENG, Johns Hopkins University JAMES F. DRAKE, University of Maryland JOHN C. FOSTER, Massachusetts Institute of Technology STEPHEN A. FUSELIER, Lockheed Martin Advanced Technology Center SARAH GIBSON, National Center for Atmospheric Research CRAIG KLETZING, University of Iowa GANG LU, National Center for Atmospheric Research BARRY H. MAUK, Johns Hopkins University FRANK B. McDONALD, University of Maryland EUGENE N. PARKER, University of Chicago, Professor Emeritus ROBERT W. SCHUNK, Utah State University GARY P. ZANK, University of California, Riverside ARTHUR CHARO, Study Director THERESA M. FISHER, Senior Program Assistant

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report SPACE STUDIES BOARD LENNARD A. FISK, University of Michigan, Chair GEORGE A. PAULIKAS, The Aerospace Corporation (retired), Vice Chair J. ROGER P. ANGEL, University of Arizona ANA P. BARROS, Harvard University RETA F. BEEBE, New Mexico State University ROGER D. BLANDFORD, Stanford University JAMES L. BURCH, Southwest Research Institute RADFORD BYERLY, JR., University of Colorado HOWARD M. EINSPAHR, Bristol-Myers Squibb Pharmaceutical Research Institute (retired) STEVEN H. FLAJSER, Loral Space and Communications, Ltd. MICHAEL H. FREILICH, Oregon State University DON P. GIDDENS, Georgia Institute of Technology/Emory University DONALD INGBER, Harvard Medical Center RALPH H. JACOBSON, The Charles Stark Draper Laboratory (retired) TAMARA E. JERNIGAN, Lawrence Livermore National Laboratory MARGARET G. KIVELSON, University of California, Los Angeles CALVIN W. LOWE, Bowie State University BRUCE D. MARCUS, TRW, Inc. (retired) HARRY Y. McSWEEN, JR., University of Tennessee DENNIS W. READEY, Colorado School of Mines ANNA-LOUISE REYSENBACH, Portland State University ROALD S. SAGDEEV, University of Maryland CAROLUS J. SCHRIJVER, Lockheed Martin Solar and Astrophysics Laboratory ROBERT J. SERAFIN, National Center for Atmospheric Research MITCHELL SOGIN, Marine Biological Laboratory C. MEGAN URRY, Yale University J. CRAIG WHEELER, University of Texas, Austin JOSEPH K. ALEXANDER, Director

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report Foreword The outward expansion of the solar wind carves out a region from the local interstellar medium (LISM) known as the heliosphere. We live in the inner part of this region, which is dominated by the Sun. Far beyond the orbit of the known planets, in the outer heliosphere, a complex interaction occurs between the solar wind and the LISM. The observation and study of this interaction region, and of the pristine interstellar medium that lies beyond, will be exploration in the truest sense, an epic journey that helps define our place in the universe. As this report went to press in mid-2004, the Voyager spacecraft appeared to be approaching, or perhaps even encountering, the termination shock of the solar wind—the region where the supersonic flow goes subsonic. Already, the Voyagers’ observations have demonstrated that the interaction region will be complex, surprising, and fascinating. Theories and models of the interaction region will have to be modified to account for the observed complexity. Interstellar neutral gas penetrates far into the heliosphere and can be observed in its recently ionized form as pickup ions in the solar wind, in particular by Ulysses. These two missions, Voyager and Ulysses, represent our best immediate hope to increase our understanding of the properties of the LISM and its interactions with the heliosphere. Real progress in this field, however, demands new observations and improved theories. It is possible to make far better and more insightful observations of neutral interstellar gas in the inner heliosphere, and to exploit other remote sensing techniques that may be demonstrated to provide important information on the termination shock and the interaction region beyond. Theories and models always can be improved, incorporating and explaining current and future observations. The real journey will occur when we embark on an interstellar probe, with sufficient instrumentation and the capability to rapidly access the distant heliosphere. This journey will be one of the great explorations of humankind, when we leave the safety of our solar system and venture forth into interstellar space. This report provides a strategy to prepare for this exploration. Lennard A. Fisk, Chair Space Studies Board

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report Preface In 2003, the National Research Council (NRC) produced The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics.1 In addition, an internal advisory committee at the National Aeronautics and Space Administration (NASA) completed a roadmap for the Office of Space Sciences’ Sun-Earth Connection Theme.2 Recognizing that exploration of the outer heliosphere was the least thoroughly developed aspect of either document and that planning and technology development for such an effort would require a long lead time, NASA requested that the NRC conduct a workshop that would further investigate the challenges and opportunities for space missions to explore this region (see Appendix A, Statement of Task). The workshop, which was organized by the Space Studies Board’s Committee on Solar and Space Physics (CSSP), took place May 6-7, 2003, at the National Academies’ Beckman Center, located at the University of California at Irvine (see Appendix B for the agenda). Gary Zank, a member of the CSSP, led the committee’s effort in developing the following report, which summarizes the discussions and conclusions of workshop participants. 1   National Research Council, 2003, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, National Academies Press, Washington, D.C. 2   See NASA, 2003, The Sun-Earth Connection Roadmap 2003-2028: Understand the Sun, Heliosphere and Planetary Environments as a Single Connected System, NASA, Washington, D.C. Available at http://sec.gsfc.nasa.gov./sec_roadmap.htm.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report Acknowledgment of Reviewers This report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council’s Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their review of this report: Leonard F. Burlaga, NASA Goddard Space Flight Center, George Gloeckler, University of Maryland, Michael Gruntman, University of Southern California, Les Johnson, NASA Marshall Space Flight Center, J.R. (Randy) Jokipii, University of Arizona, Martin Lee, University of New Hampshire, Paulett C. Liewer, Jet Propulsion Laboratory, David J. McComas, Southwest Research Institute, and Ralph McNutt, Jr., Johns Hopkins University. Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Donald J. Williams, Johns Hopkins University. Appointed by the National Research Council, he was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report Contents     SUMMARY   1 1   INTRODUCTION   4 2   SCIENCE SUMMARY: THE INTERACTION OF THE SOLAR WIND AND THE LOCAL INTERSTELLAR MEDIUM   7      Global Structure,   9      Neutral Interstellar Atoms and Pickup Ions,   15      Heliospheric Lyman-Alpha Absorption Toward Nearby Stars,   16      Particle Acceleration in the Heliosphere and at the Termination Shock,   17      Galactic Cosmic Rays: Entry into the Heliosphere,   19      Radio Emissions from the Outer Heliosphere,   21      Energetic Neutral Atoms in the Heliosphere,   22 3   THE NEXT LOGICAL STEPS   24      Probing the Local Interstellar Medium in the Inner Heliosphere,   25      Remote Sensing of the Heliospheric Boundary Regions,   27 4   AN INTERSTELLAR PROBE TO THE BOUNDARIES OF THE HELIOSPHERE AND NEARBY INTERSTELLAR SPACE   29 5   STRATEGIC DIRECTIONS   34     BIBLIOGRAPHY   37

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report     APPENDIXES         A   Statement of Task   49     B   Workshop Agenda and Participants   51     C   Acronyms and Abbreviations   55