Readiness for the Upcoming Solar Maximum [1 ed.] 9780309583794

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Readiness for the Upcoming Solar Maximum

Committee on Solar and Space Physics Space Studies Board Commission on Physical Sciences, Mathematics, and Applications Committee on Solar-Terrestrial Research Board on Atmospheric Sciences and Climate Commission on Geosciences, Environment, and Resources National Research Council

NATIONAL ACADEMY PRESS Washington, D.C. 1998

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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 committees responsible for the report were chosen for their special competences and with regard for appropriate balance. 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 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. William 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. Kenneth I. Shine 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 Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council. Support for this project was provided by Contract NASW 96013 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the organizations or agencies that provided support for this project. Cover: Solar x-ray images of the Sun's atmosphere from the Yohkoh mission of ISAS, Japan. Obtained between 1991 (bottom) and 1995 (top) at regular intervals, they provide a dramatic view of how the corona changes during the waning portion of the solar cycle. The Sun's atmosphere, heated to millions of degrees, is hot enough to emit x rays, while its much cooler surface (at about 6,000 °C) is not. As a result, an x-ray image of the Sun will display a bright glow for the corona and a black disk for the surface. The images in this figure also show that as the solar activity cycle progresses from maximum to minimum, the Sun's magnetic field changes from a complex structure to a simpler configuration with fewer fields. Image credit: “The Changing Sun,” Lockheed Martin Palo Alto Research Laboratory, G.L. Slater and G.A. Linford. Copies of this report are available from Space Studies Board National Research Council 2101 Constitution Avenue, N.W. Washington, D.C. 20418 Copyright 1998 by the National Academy of Sciences . All rights reserved. Printed in the United States of America

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COMMITTEE ON SOLAR AND SPACE PHYSICS GEORGE L. SISCOE, Boston University, Chair JANET G. LUHMANN, * University of California, Berkeley, former Chair SPIRO K. ANTIOCHOS, * Naval Research Laboratory. CHARLES W. CARLSON, University of California, Berkeley ROBERT L. CAROVILLANO, Boston College TAMAS I. GOMBOSI, University of Michigan RAYMOND A. GREENWALD, Applied Physics Laboratory JUDITH T. KARPEN, Naval Research Laboratory ROBERT P. LIN, * University of California, Berkeley GLENN M. MASON, University of Maryland MARGARET A. SHEA, Air Force Phillips Laboratory HARLAN E. SPENCE, * Boston University KEITH T. STRONG, Lockheed Palo Alto Research Center MICHELLE F. THOMSEN, * Los Alamos National Laboratory RICHARD A. WOLF, Rice University ARTHUR A. CHARO, Senior Program Officer CARMELA J. CHAMBERLAIN, Senior Program Assistant

* Term ended in 1997.

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SPACE STUDIES BOARD CLAUDE R. CANIZARES, Massachusetts Institute of Technology, Chair MARK R. ABBOTT, Oregon State University JAMES P. BAGIAN, * Environmental Protection Agency DANIEL N. BAKER, University of Colorado LAWRENCE BOGORAD, Harvard University DONALD E. BROWNLEE, University of Washington JOHN J. DONEGAN, * John Donegan Associates, Inc. GERARD W. ELVERUM, JR., TRW Space and Technology Group ANTHONY W. ENGLAND, University of Michigan MARILYN L. FOGEL, Carnegie Institution of Washington MARTIN E. GLICKSMAN, * Rensselaer Polytechnic Institute RONALD GREELEY, Arizona State University BILL GREEN, former member, U.S. House of Representatives ANDREW H. KNOLL, Harvard University JANET G. LUHMANN, * University of California, Berkeley ROBERTA BALSTAD MILLER, CIESIN BERRIEN MOORE III, University of New Hampshire KENNETH H. NEALSON, * University of Wisconsin MARY JANE OSBORN, University of Connecticut Health Center SIMON OSTRACH, Case Western Reserve University MORTON B. PANISH, AT&T Bell Laboratories (retired) CARLÉ M. PIETERS, Brown University THOMAS A. PRINCE, California Institute of Technology MARCIA J. RIEKE, * University of Arizona PEDRO L. RUSTAN, JR., U.S. Air Force (retired) JOHN A. SIMPSON, Enrico Fermi Institute GEORGE L. SISCOE, Boston University EDWARD M. STOLPER, California Institute of Technology RAYMOND VISKANTA, Purdue University ROBERT E. WILLIAMS, Space Telescope Science Institute JOSEPH K. ALEXANDER, Director (as of February 17, 1998) MARC S. ALLEN, Director (through December 12, 1997)

* Term ended in 1997.

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COMMISSION ON PHYSICAL SCIENCES, MATHEMATICS, AND APPLICATIONS ROBERT J. HERMANN, United Technologies Corporation, Co-chair W. CARL LINEBERGER, University of Colorado, Co-chair PETER M. BANKS, Environmental Research Institute of Michigan WILLIAM BROWDER, Princeton University LAWRENCE D. BROWN, University of Pennsylvania RONALD G. DOUGLAS, Texas A&M University JOHN E. ESTES, University of California at Santa Barbara MARTHA P. HAYNES, Cornell University L. LOUIS HEGEDUS, Elf Atochem North America, Inc. JOHN E. HOPCROFT, Cornell University CAROL M. JANTZEN, Westinghouse Savannah River Company PAUL G. KAMINSKI, Technovation, Inc. KENNETH H. KELLER, University of Minnesota KENNETH I. KELLERMANN, National Radio Astronomy Observatory MARGARET G. KIVELSON, University of California at Los Angeles DANIEL KLEPPNER, Massachusetts Institute of Technology JOHN KREICK, Sanders, a Lockheed Martin Company MARSHA I. LESTER, University of Pennsylvania NICHOLAS P. SAMIOS, Brookhaven National Laboratory CHANG-LIN TIEN, University of California at Berkeley NORMAN METZGER, Executive Director

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COMMITTEE ON SOLAR-TERRESTRIAL RESEARCH MICHAEL C. KELLEY, Cornell University, Chair MARVIN A. GELLER, * State University of New York at Stony Brook, former Chair GUY P. BRASSEUR, * National Center for Atmospheric Research JOHN T. GOSLING, * Los Alamos National Laboratory MAURA HAGAN, National Center for Atmospheric Research MARY K. HUDSON, Dartmouth College GORDON HURFORD, * California Institute of Technology NORMAN F. NESS, Bartol Research Institute THOMAS F. TASCIONE, Sterling Software H. FRANK EDEN, Senior Program Officer DORIS BOUADJEMI, Administrative Assistant TENECIA A. BROWN, Administrative Assistant

* Term ended in 1997.

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BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE ERIC J. BARRON, Pennsylvania State University, Co-Chair JAMES R. MAHONEY, International Technology Corporation, Co-Chair SUSAN K. AVERY, CIRES LANCE F. BOSART, State University of New York at Albany MARVIN A. GELLER, State University of New York at Stony Brook DONALD M. HUNTEN, University of Arizona JOHN IMBRIE, Brown University CHARLES E. KOLB, Aerodyne Research Inc. THOMAS J. LENNON, WSI Corporation MARK R. SCHOEBERL, NASA Goddard Space Flight Center ELBERT (JOE) W. FRIDAY, JR., Director (as of July 20, 1998) WILLIAM A. SPRIGG, Director (through March 31, 1998)

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COMMISSION ON GEOSCIENCES, ENVIRONMENT, AND RESOURCES GEORGE M. HORNBERGER, University of Virginia, Chair PATRICK R. ATKINS, Aluminum Company of America JERRY F. FRANKLIN, University of Washington B. JOHN GARRICK, St. George, Utah THOMAS E. GRAEDEL, Yale University DEBRA KNOPMAN, Progressive Foundation KAI N. LEE, Williams College JUDITH E. McDOWELL, Woods Hole Oceanographic Institution RICHARD A. MESERVE, Covington and Burling, Washington, D.C. HUGH C. MORRIS, Canadian Global Change Program RAYMOND A. PRICE, Queen's University at Kingston, Ontario H. RONALD PULLIAM, University of Georgia THOMAS C. SCHELLING, University of Maryland VICTORIA J. TSCHINKEL, Landers and Parsons, Tallahassee, Florida E-AN ZEN, University of Maryland MARY LOU ZOBACK, U.S. Geological Survey ROBERT M. HAMILTON, Executive Director (as of December 31, 1997) STEPHEN RATTIEN, Exective Director (through August 1997)

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FOREWORD

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Foreword

Studies of the Sun have both scientific and practical benefits. Being 100,000 times closer and 10 billion times brighter than any other star, the Sun is a unique laboratory for gaining deep understanding of stellar astrophysics. The Sun is also Earth's one and only external source of energy. Changes in its electromagnetic and particle output affect the structure of our atmosphere, as well as the radiation environment encountered by orbiting satellites. The next maximum in the roughly dozen-year cycle of solar activity is now only a few years away. This report takes a broad look at how the nation is preparing to observe and understand solar phenomena from space during this peak period. The Committee on Solar and Space Physics together with the Committee on SolarTerrestrial Research considered the plans of the National Aeronautics and Space Administration, National Oceanic and Atmospheric Administration, National Science Foundation, Department of Defense, and Department of Energy, all of which have some relevant interest and involvement. There are specific findings and recommendations for each agency. The nation seems well positioned to make the best of the coming swell of solar activity. Many spacecraft are already in place or under development that will measure different aspects of the event. With the appropriate coordination and supporting research, this phase of the Sun's cycle could yield important results for science and society. Claude R. Canizares, Chair Space Studies Board

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FOREWORD x

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ACKNOWLEDGMENTS

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Acknowledgments

This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's (NRC's) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the 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 contents of 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 participation in the review of this report: Loren Acton, Montana State University, Christopher F. McKee, University of California, Berkeley, P. Buford Price, University of California, Berkeley, Patricia H. Reiff, Rice University, Christopher T. Russell, University of California, Los Angeles, and John R. Winckler, Professor Emeritus of Physics, University of Minnesota. Although the individuals listed above have provided many constructive comments and suggestions, responsibility for the final content of this report rests solely with the authoring committees and the NRC.

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ACKNOWLEDGMENTS xii

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CONTENTS

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Contents

Executive Summary

1

1

Introduction

6

2

Scientific Background Earth's Space Environment Response Earth's Climate Response

8 9 10

3

Technology Considerations Potential Technological Concerns During the Solar Maximum Existing Resources Summary

15 15 17 24

4

Agency Activities and Related Recommendations National Aeronautics and Space Administration National Oceanic and Atmospheric Administration National Science Foundation Department of Defense Department of Energy

25 25 27 30 32 37

5

Concluding Observations and Recommendations

39

A B

Appendixes Biographical Sketches of Committee Members Acronyms and Abbreviations

43 51

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CONTENTS xiv

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EXECUTIVE SUMMARY

1

Executive Summary

Our Sun undergoes activity cycles characterized by increases in its output of electromagnetic and particle radiation over a broad range of energies every 9 to 13 years. The number of sunspots, recorded since the 1600s, shows that these cycles have occurred regularly (albeit with varying intensity) at least 22 consecutive times. The approaching maximum of cycle 23 (expected to occur between 1999 and 2002) represents an unprecedented opportunity to understand the physics of the solar cycle and its effects on Earth. Knowledge has advanced to the point that researchers are able to investigate specific atmospheric and Earth-space responses, experimental capabilities have greatly improved, and models and laboratory tools make possible controlled simulations of cause and effect. There are also technological motivations for learning more about the solar cycle control of “space weather” as society becomes increasingly dependent on systems (e.g., distributed power grids, satellite-based communications, and navigation networks) sensitive to space environment disturbances. At the request of the Sun-Earth Connection science program director at the National Aeronautics and Space Administration (NASA), the Space Studies Board's Committee on Solar and Space Physics (CSSP), working jointly with its federated Committee on Solar-Terrestrial Research (CSTR) of the Board on Atmospheric Sciences and Climate, reviewed the nation's preparedness for the solar maximum. 1 This consideration of readiness concerns both the unique research opportunities presented by the upcoming solar maximum, as well as our capability to mitigate technological problems that might result from the effects of the active Sun. NASA, the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), the Department of Defense (DOD) (the Air Force and the Navy), and the Department of Energy (DOE) provided information for this review, in part during invited agency briefings to the CSSP/CSTR at their meeting in Washington, D.C., on February 26-28, 1997. The committees' assessment is based on the following assumptions regarding the various agencies' roles: • NASA is the nation's space agency responsible for solar and geospace exploration as well as the human use of space; • NOAA is the major provider of civilian solar and space environment information; • NSF is a major sponsor of basic solar-terrestrial research and the lead agency for the National Space Weather Program;

1The

committees' review of preparedness for the upcoming solar maximum does not include an analysis of the current capabilities of the nation's ground-based optical and radio solar observatories. This assessment is being performed as part of an ongoing National Research Council study by the Space Studies Board's Task Group on Ground-based Solar Research (TGGSR). The task group study is sponsored by the NSF and NASA, which requested a broad examination of the “health” and future prospects of ground-based solar research. In addition, the agencies requested a focused examination of issues related to the future of the National Solar Observatory. The TGGSR is expected to release its findings in early summer 1998. The CSSP/CSTR emphasize that the present report's lack of recommendations that are specific to ground-based solar facilities is a direct consequence of the ongoing TGGSR study and should in no way be construed as a lack of concern by the committees over the future of these facilities.

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EXECUTIVE SUMMARY

2

• DOD (and in particular the Air Force and the Navy) is both a user of space environment information and a sponsor of related research and monitoring; and • DOE is an additional user and sponsor concerned with national security aspects of the space environment. AGENCY-SPECIFIC RECOMMENDATIONS National Aeronautics and Space Administration NASA has built an excellent multisatellite observatory to explore the Sun-Earth Connection; to yield maximum dividends, this investment should be exploited through the upcoming solar maximum. Moreover, continuing this observatory will aid construction of the International Space Station, which will require many extravehicular activities throughout the period of the solar maximum when the space environment will be disturbed. It is unlikely that a Sun-Earth Connection “great observatory” like the current one will be available in the foreseeable future, and NASA's projected budgets and plans do not include such an observatory for the solar maximum of cycle 24. Thus, the timing is right for the current observatory to have its maximum impact on the science issues it was designed to address. • The committees recommend that, at a minimum, NASA continue the existing International SolarTerrestrial Physics (ISTP) program and related operating missions (ACE, Ulysses, Yohkoh, FAST, SAMPEX, and the Voyagers) through the upcoming solar maximum. This includes acquiring highquality data (e.g., through the Deep Space Network) and then validating, archiving, interpreting, and publishing them. • The committees also recommend the timely launches of TRACE, TIMED, and IMAGE and encourage U.S. participation in Equator-S and Cluster, so that spacecraft capable of making unique contributions will be available during this unprecedented solar maximum observational campaign. • Finally, the committees recommend that a dedicated guest investigator program be initiated to complement the existing program during the solar maximum. Such a program would allow all selected investigators to have full use of the collected Sun-Earth Connection data to address the problems of the origin of solar activity and its effects in the solar system, especially its effects on Earth. National Oceanic and Atmospheric Administration NOAA is the leader of the nation's space environment monitoring program and a cornerstone of the interagency National Space Weather Program (NSWP). The agency has the unique responsibilities of distributing high-quality geophysical data to a broad-based national and international community and providing reliable space weather forecasts to the civilian sector. It is also the agency responsible for improving an operational space weather monitoring and forecasting system. NOAA played a key role in arranging for the research community to receive real-time data transmissions from ACE. However, NOAA resources have not been available for translating modern data-based or theoretical research models into improved monitoring and forecasting tools. The absence of a NOAA commitment to this unique and critical role will have a fundamental impact on the success of the NSWP.

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EXECUTIVE SUMMARY

3

• The committees recommend that NOAA, through its Space Environment Center, develop and execute a plan to fulfill its responsibilities within the National Space Weather Program during the coming period of enhanced demand for space environment forecasting services. • The committees also recommend that NOAA ensure the certification and prompt dissemination of space environment and geophysical databases through its National Geophysical Data Center. National Science Foundation Overall, NSF appears well prepared to face the scientific opportunities and technological challenges of the upcoming solar maximum. The committees thus primarily encourage NSF to continue its efforts and to supplement them as much as possible. • The committees recommend that NSF continue its leadership role in the National Space Weather Program and champion stronger interagency involvement in the NSWP to maximize the nation's benefit from the program during the solar maximum. • The committees also recommend that NSF consider initiating interagency discussion of a specific solar maximum campaign similar to that developed for the comet Shoemaker-Levy 9 event. Department of Defense Although DOD preparations and activities are notable for their breadth and forethought, several areas might benefit from reassessment. In particular, the Air Force has invested primarily in space hardware at the expense of basic research and analysis. Like NOAA, the DOD in general has not recognized the critical need for investment aimed at making data-based and theoretical research models operational. The committees' findings and recommendations include two issues relating to both the Air Force and Navy: • Although the continued operation of Yohkoh, SOHO, and the other ISTP experiments through the solar maximum is NASA's responsibility, the committees recommend that DOD make its reliance on these missions (especially for solar and and interplanetary observations) known to NASA. • The continuing participation in and support for the National Space Weather Program on the part of both the Air Force and the Navy are critical to that program's success. The committees recommend that this participation be strengthened through joint endeavors such as the development of rapid prototyping systems for space environment forecasting. Specific recommendations for the Air Force programs during the solar maximum include the following: • Further integrate the Air Force efforts with the National Space Weather Program, both to take advantage of the NSWP products and to provide insight on tools useful to the NSWP. This involvement would also provide ongoing peer review of those DOD efforts that can

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EXECUTIVE SUMMARY

4

be discussed in an open forum, ensuring that DOD's investment will result in the greatest possible benefits. • Reassess the support and plans for the 55th Space Weather Squadron to ensure that the squadron will be well prepared for the demands of the upcoming solar maximum. This includes provisions for access to state-of-the-art knowledge and forecasting tools. The committees' recommendations for the Navy solar maximum program include the following: • Consider an accelerated research initiative in solar physics to take advantage of the large data sets expected from the Yohkoh and SOHO experiments during the solar maximum, so that knowledge gained can be rapidly put to use. • Sponsor or cosponsor a community guest investigator program for collaborations on analysis and interpretation of the data from the Navy solar and upper-atmosphere experiments. By enhancing the productivity of those experiments and bringing in useful external expertise, such a program would help speed the National Space Weather Program's rapid application of new knowledge. Department of Energy Although DOE has reasons for its highly targeted commitment to the space environment endeavor, the committees believe that with minimal disruption of the status quo, DOE's contribution to the solar maximum activities described herein can be magnified. • The committees recommend that DOE participate in the dialogue of the interagency coordinating committee for the National Space Weather Program and reassess its own role in that activity (e.g., in the area of power transmission). • The committees also recommend that DOE continue its support for the flight of space radiation monitors, together with support for making its data available to the community at large, with special expediency during the solar maximum. CONCLUDING OBSERVATIONS AND RECOMMENDATIONS In addition to agency-specific recommendations, the committees offer the following general observations and recommendations on the nation's readiness for the upcoming solar maximum: • For this solar maximum, an unprecedented solar-terrestrial spacecraft “armada” will be in orbit to use in studying the active Sun as well as Earth's responses. These spacecraft must remain operational (to the maximum extent possible) with sufficient supporting research to exploit the opportunity they afford— an opportunity unlikely to be equaled in the foreseeable future. • NSF and DOD have shown their support for the National Space Weather Program, but to realize the goals of the NSWP, NOAA should work to translate research models of the

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EXECUTIVE SUMMARY 5

solar-terrestrial system to operational uses, perhaps through the creation of the proposed rapid prototyping center. • Increasing effects of solar and geospace environment disturbances on human activities are expected during the period of the solar maximum. The committees recommend that an interagency workshop (or summit) involving scientists, agency representatives, and industrial administrators and engineers be held to improve their state of preparedness through sharing of information.

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INTRODUCTION

6

1 Introduction

A solar maximum is a cosmic “event” in the same sense as the periodic return of a comet or the episodic burst from an exotic astrophysical source. Records of various kinds indicate that the Sun has repeated its version of celestial fireworks in the form of flares, radio bursts, and coronal eruptions on a roughly 11-year cycle throughout geologic and human history. The predictability of a solar maximum's occurrence is a great boon to scientists because it allows advanced preparation for detailed study of primary astrophysical processes as well as their interplanetary and planetary consequences. During a solar maximum, processes generating energetic particles and photons can be observed on a large scale, and scientists can extrapolate these observations to try to characterize the physical processes in less easily investigated regions of the cosmos, in particular, the neighborhoods of other stars. Changes in solar activity also have direct relevance to humans. The increased emissions of solar particles and radiation and the enhanced interplanetary magnetic fields associated with a solar maximum cause measurable changes on Earth and in its space environment. A solar maximum thus allows the analysis of the response of Earth's atmosphere and climate to extreme and variable solar conditions of the kind believed to have prevailed early in the solar system's history. The solar maximum period is thus highly pertinent to the search for life's origins, a quest that recently has captured the public's and NASA's attention. Solar activity also has technological impacts. Historically, the solar maximum has been the time when problems related to short-term variations in the Sun's radiative and particle outputs occur most frequently. As our nation increases its investment in commercial space applications, we become more vulnerable to these effects. For example, the increased use of space-based communication and navigation systems and the scheduled construction of the International Space Station (in an orbit subject to hazards arising from solar events and their effects) bring a greater urgency to the need to understand how the active Sun affects geospace, or “space weather.” Before anyone can embark on long-term crewed flight beyond the protection of Earth's magnetosphere (such as manned exploration of the Moon or Mars), the likely extent and effects of solar variability must be known. The coming solar maximum offers an unprecedented opportunity to make significant advances in this challenging endeavor. In this short report, the Committee on Solar and Space Physics (CSSP) and the Committee on SolarTerrestrial Research (CSTR) assess the extent to which the United States is prepared to take advantage of the approaching solar maximum, expected to occur between 1999 and 2002. This readiness consists of preparation to take advantage of research opportunities unique to the solar maximum, and a capability for mitigating problems that might result from the effects of the active Sun. The committees' assessment consists primarily of a review of the relevant activities of the agencies most concerned with the use or investigation of space: the National Aeronautics and Space Administration (NASA), the nation's “space agency” responsible for solar and geospace exploration as well as the human use of space; the National Oceanic and Atmospheric Administration (NOAA), the major provider of civilian solar and space environment information; the National Science Foundation (NSF), a major sponsor of basic solar-terrestrial research and the lead agency for the National Space Weather Program

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INTRODUCTION

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(NSWP);1 the Department of Defense (DOD), in particular the Air Force, which has its own space environment monitoring and research program in support of DOD, and the Navy, which historically has been a major participant in solar and ionospheric research; and the Department of Energy (DOE), whose support of space activities related to defense and other national security needs includes a group at Los Alamos National Laboratory that analyzes aspects of the the space environment that affect national security. The report begins with a brief description of the considerations that led to this assessment, followed by summaries of each agency's activities and programs, including any special plans adopted in anticipation of the solar maximum. These descriptions were drawn in part from information obtained during agency briefings for the CSSP/CSTR at their February 26-28, 1997, meeting in Washington, D.C. It then offers an assessment of these various programs regarding their optimal use of national resources to both learn from and protect against the events of the rapidly approaching solar maximum. It concludes with recommendations regarding important additional benefits that could be derived, in many cases with existing resources or minimal additional investment. .

1The

committees' review of agency activities does not include an assessment of the current capabilities and future options for ground-based optical and radio solar observatories. In particular, the present report does not make recommendations related to the NSF-funded National Solar Observatory at Sacramento Peak, New Mexico, and Kitt Peak, Arizona. Such an assessment is being performed as part of an ongoing National Research Council study by the Space Studies Board's Task Group on Ground-based Solar Research (TGGSR).

Copyright © 1998. National Academies Press. All rights reserved.

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SCIENTIFIC BACKGROUND

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2 Scientific Background

Although the Sun has been studied for centuries, it is only since the advent of the space age that its complexity and the full range of its effects on Earth and other planets in our solar system have been fully appreciated. For example, sunspots can be seen with the naked eye and were observed in ancient times, but only since the advent of the telescope are the 11-year cycles accurately chronicled in human records, and it has been only 3 or 4 cycles since the discovery that Earth is continuously bathed in a supersonic “wind” of ions, electrons, and a magnetic field streaming incessantly from the Sun. At about the same time, researchers became aware of the magnetic bubble of the magnetosphere surrounding Earth, which deflects most of the solar wind around our atmosphere. The highly variable solar wind compresses, stretches, and buffets this magnetic shield constantly. In the early 1970s, Skylab images of the Sun revealed both x-ray dark regions (“coronal holes”) that give rise to high-speed streams, as well as huge eruptions traveling outward through the solar atmosphere. Measurements from the Solar Maximum Mission showed that even the output of the Sun is not constant. Although the strongest variations occur in the normally weak invisible parts of the emitted electromagnetic spectrum, Earth's atmosphere still “detects” and responds to these. The nature and extent of that response remain subjects of investigation. One of the most striking features of our Sun is its cycle of “activity.” The numbers of sunspots, solar flares, solar radio bursts, and coronal disturbances increase and then decrease again every 9 to 13 years. Only during the last three cycles has it been generally appreciated that the underlying driver of this cycle is the solar magnetic “dynamo,” a convecting layer of electrically conducting material inside the Sun that produces a surface magnetic field of changing character with nearly periodic behavior. These types of cycles have now been remotely sensed on other solar-type stars in our galaxy. The solar cycle seems to be linked with a wide range of cyclic changes in the geospace environment—for example, changes in the upper atmosphere, the ionosphere, the cosmic-ray-related radiation levels, Earth's surface magnetic field, and the Van Allen radiation belts. Geospace is coupled to the Sun, and as inhabitants of Earth and members of a technologically advanced society, we are compelled to understand the causes, modes, extent, and consequences of that coupling. The coming maximum in solar activity raises two central questions with basic scientific and applied aspects: 1. How does the Sun undergo its dramatic changes during the rise to solar maximum? 2. What are the practical consequences for Earth and our society? Magnetic fields generated by the solar dynamo can only be directly observed at and above the visible surface where sunspots contain the strongest concentrations of field. The solar activity cycle was first noticed in changes of the number, pattern, and magnetic polarity of the sunspots over the average 11-year cycle. Even early observations hinted that the fusion processes producing radiant energy deep within the Sun were quite steady, although the interaction of the solar field with its outer layers was not. Images of the Sun taken through filters that isolate narrow ultraviolet (UV), extreme ultraviolet (EUV), soft x-ray, and optical

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SCIENTIFIC BACKGROUND

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wavelength bands show that the fields extend far above the sunspots, producing sometimes complex patterns of loops and arcades. These images also indicate the presence of weaker magnetic fields between the sunspots that underlie larger-scale systems of loops and arcades visible in the corona during solar eclipses. Most frequently at solar maximum, the interactions between the magnetic fields and the moving, ionized solar atmosphere can twist strong fields above sunspot groups into unstable configurations storing substantial amounts of energy. This energy can be suddenly released in solar flares, which temporarily enhance the solar radiant output at radio and x-ray wavelengths by several orders of magnitude. The larger-scale magnetic structures also undergo frequent eruptions during solar maximum. The ejecta from these coronal mass ejections (CMEs) can travel outward through the background solar wind at speeds of up to 2,000 km/s. These fast CMEs cause interplanetary shocks as they move, thereby greatly enhancing the density and fields in the solar wind. The large-scale character of solar magnetic fields over the cycle also determines the large-scale pattern of coronal fields carried by the solar wind. Although the solar magnetic field is considered ultimately responsible for all solar variability, the physics underlying its character is not yet understood. Models of the solar dynamo are able to simulate a cyclical behavior, but the changes they predict are inconsistent with observations. This problem seems to stem in large part from an an inaccurate description of the dynamics of the solar interior. Helioseismology techniques now allow researchers to probe the interior structure and dynamics of a star. In just the last few years it has been learned that the differential solar rotation observed on the surface persists inside the visible Sun to about twothirds of its radius from the center, where there is a strong, thin shear layer. Inside that radius, the Sun rotates almost as a solid body. The implications of this new knowledge are just beginning to be explored but raise even more important questions: Does the situation change in any way as the solar activity cycle progresses? Are the origins of sunspots and CMEs apparent beneath the surface? Do the magnetic fields ever become dynamically important in the solar interior? Is this a critical factor in explaining the Sun's activity cycle? The intensity of the solar maximum varied dramatically over time. What aspect of the solar dynamo determines what the solar maximum intensity will be? What are the consequences for Earth? EARTH'S SPACE ENVIRONMENT RESPONSE Only since the last solar maximum have researchers begun to understand the fundamental geospace responses to the maximum and minimum of solar activity. Even the limited spacecraft capabilities that were available for solar and space environment observations during the last solar maximum resulted in a revolution of scientific understanding of the causes and consequences of geomagnetic activity. It is now clear that conditions in interplanetary space and geospace can become extreme and highly dynamic. In particular, the Van Allen radiation belts, previously regarded as slowly varying and predictable features, were found to be much more dynamic and transient than any statistical model would imply. The bulk of the major geospace disturbances were a consequence of the CMEs, which on occasion compress the magnetosphere to altitudes well inside the geosynchronous orbit (at 6.6 Earth radius [RE equatorial). Those CMEs that travel fast enough with respect to the low-speed (~400 km/s) ecliptic solar wind, moreover, produce leading shock waves that were a major cause of solar energetic proton events. These events can contribute to

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SCIENTIFIC BACKGROUND

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the radiation belt for weeks or months, adding to other transient populations that follow the passage of the shock front through the magnetosphere. Moreover, by virtue of their disturbance of the entire magnetosphere, it is the CMEs that cause widespread effects including auroral activity, ionospheric disturbances, and induced ground currents. Further discoveries have resulted from the observational capabilities that became available during the present solar minimum period. These illustrated the special role of high-speed solar wind streams emanating from coronal holes during solar inactive periods. These high-speed wind streams somehow control the intensity of energetic (>1-MeV) electrons in the radiation belts with a fair degree of predictability. We are now witnessing a transition from the high-speed solar wind stream activity characteristic of a solar minimum to the less predictable eruptive events that characterize a solar maximum. This change in behavior manifested itself most spectacularly with the event of January 6-11, 1997, when a CME produced an enormous magnetic cloud that expanded toward Earth. The CME produced an interplanetary shock that hit the magnetosphere on January 10, shortly after midnight Universal Time. The shock was followed roughly 24 hours later by an interval of unusually high-density solar wind (150 cm−3, roughly 15 times the average solar wind density) that compressed the magnetosphere on the dayside within the geosynchronous orbit. This strong pressure pulse was followed by a stream of higher than average speed (~600 km/s). A rapid buildup within the inner magnetosphere of energetic (>1 MeV) electrons by 3 to 4 orders of magnitude was under way before arrival of the high-density solar wind impulse. Figure 1 illustrates the connections that can now be made between solar, interplanetary, and geospace observations during such events. What was particularly significant about this January 6-11, 1997, event was the first-ever opportunity to observe the effects of a CME from cradle to grave: Observations from the Solar and Heliospheric Observatory (SOHO) and Yohkoh showed the changes in the photosphere and corona that preceded the eruption. The coronagraph on SOHO then imaged the dense plasma accompanying the erupting fields, the CME, as it moved away from the Sun. Radio waves emitted from the approaching shock front were detected on board the Wind spacecraft (as was the passage of the leading interplanetary shock and the trailing density pulse), while the Polar spacecraft and other magnetospheric satellites and ground-based observatories measured the shock's effects on geospace. Theory teams sponsored by the International Solar-Terrestrial Physics (ISTP) program produced a global numerical simulation of the event, the longest to date, to capture the entire period of magnetic cloud passage and to analyze the physical causes of the energetic particle increases and atmospheric effects. Researchers have never been so well prepared to study the complete life cycle of this type of complex and important natural event. EARTH'S CLIMATE RESPONSE The past 15 years have also produced explosive growth in our knowledge of our Sun's outputs and the responses of Earth's atmosphere to them. Accumulating records of the total solar radiative output1 are finally yielding accurate measures of the degree to which the “solar constant” actually varies (by ~0.1%) as the contemporary Sun goes through the extremes of its ~11-year activity cycle. In addition, images of the Sun in wavelengths over a broad range of the

1Board

on Global Change, National Research Council, Solar Influences on Global Change, National Academy Press, Washington, D.C., 1994.

Copyright © 1998. National Academies Press. All rights reserved.

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SCIENTIFIC BACKGROUND

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electromagnetic spectrum (of which SOHO's new full-disk images, shown by examples in Figure 2, are especially remarkable) allow for determination of what very different regions of the solar surface dominate the various spectral lines and bands as well as how historical proxies (such as sunspot number) can be used to infer much longer-term solar radiative behavior. As noted above, significant progress has been made in understanding the solar cycle and shorter-term variations of the particle and field outputs of the Sun, including the solar wind and solar energetic particle fluxes. The different consequences of flares and CMEs on the Sun, in interplanetary space, and at Earth are beginning to be understood.2 Yet the physical means by which these different aspects of solar variability affect Earth's atmosphere are as diverse as their own underlying solar sources. Some remain controversial and others await validation at a time when an unprecedented combination of state-of-the-art models and instrumentation can be used for the first time during a period of maximum solar activity. Several examples serve as the best illustration. Research using less comprehensive and sensitive data sets than are available for the upcoming solar maximum has established that the magnitude of total solar irradiance variations is determined largely by a combination of the brightening of faculae (bright, patchy areas seen in monochromatic light that surround sunspots and active regions) partially canceled by sunspot blocking. However, how the Sun's magnetic field establishes the degree of imbalance of these two magnetically controlled features remains unknown. Moreover, another significant contribution may come from the “active network,” a region on the visible Sun's surface that has been inadequately characterized. These total irradiance variations are the solar variations most likely to directly affect Earth's climate over long time frames.3 If it can be learned how solar magnetic fields modify the solar constant by quantifying the amount of sunspot blocking compared to facular and network brightening (e.g., with the tools that SOHO and Yohkoh provide), researchers may be able to interpret features in Earth's climate record such as the Little Ice Age, which coincided, remarkably, with a period of low sunspot numbers known as the Maunder Minimum. It might also be possible to make more extensive estimates of such effects over history, to better use solar-type star observations to infer Earth's distant past and its destiny, and to better extrapolate to conditions on the other bodies in the solar system and around other Sun-like stars. A second example relevant to the relationship between the Sun and Earth's climate concerns the possible role of the ionization state of Earth's atmosphere, which is known to affect its chemistry, its response to solar wind disturbances of the magnetosphere, and other processes that depend on the presence of free charges. Although this state is mainly controlled by the solar UV and EUV flux responsible for the ionosphere proper, some deeper ionization is produced by solar x-rays, solar energetic protons, and galactic cosmic rays. The flux of galactic cosmic rays at Earth and other solar system bodies is known to be reduced during periods of high solar activity. At the same time, the more variable fluxes of generally less energetic particles from solar flares and from CME-driven interplanetary shock acceleration of some solar wind particles increase greatly. The flares also produce impulsive bursts of energetic electromagnetic radiation (UV, x-rays, and sometimes gamma rays). These different energetic emissions all increase the amount of ionization in the atmosphere.

2Space

Studies Board, National Research Council, Space Weather: A Research Perspective, 1997. This report is not available in hard copy; it may be viewed on the World Wide Web at the following address: