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1 Report of the Panel on the Sun and Heliospheric Physics
Pages 1-46

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From page 1...
... Requiring Technology Development 36 1.6 NEW RESEARCH OPPORTUNITIES (NOT PRIORITIZED) 39 Instrumentation to Observe the Solar Atmosphere at 300 to 1,000 Angstroms AlGaN Solid-State Detectors for Solar Ultraviolet Observations 40
From page 2...
... 2 THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS Low-Frequency Helioseismology 40 Radar Studies of the Quiet and Active Solar Corona 41 Instrumentation and Techniques for Imaging and Mapping the Global Heliosphere 41 Spectral-Spatial Photon-Counting Detectors for the X-ray and EUV Regions 41 Miniaturized, High-Sensitivity Instrumentation for In Situ Measurements 42 1.7 CONNECTIONS TO OTHER PHYSICS DISCIPLINES 42 Atomic Physics 42 Nuclear Physics 43 Plasma Physics 43 1.8 RECOMMENDATIONS (NOT PRIORITIZED) 43 Policy and Education 43 Other 45 ADDITIONAL READING 45
From page 3...
... In particular, it is essential that NASA maintain capabilities to image the corona at x-ray and extreme ultraviolet (EUV) wavelengths, to image coronal mass ejections in white light, to do helioseismology, and to measure the solar wind plasma, magnetic field, and energetic particle variations in near-Earth interplanetary space.
From page 4...
... However, to understand how the solar wind originates and evolves in the inner heliosphere requires direct in situ sampling of the plasma, energetic particles, magnetic field, and waves, as close to the solar surface as possible the panel's top science priority for the coming decade. Such measurements will determine how energy flows from the interior of the Sun through the surface and into the solar atmosphere, heating the corona and accelerating the wind, and will also reveal how the wind evolves with distance in the inner heliosphere.
From page 5...
... and the thermal structure of the solar atmosphere in three dimensions. Moreover, radio imaging spectroscopy provides a range of powerful techniques for measuring magnetic fields in the corona.
From page 6...
... an interstellar probe, to pass through the boundaries of the heliosphere and penetrate directly into the interstellar medium with state-of-the-art instrumentation; (2) a multispacecraft mission to obtain a global view of the Sun, to reveal the Sun's polar magnetic field and internal flows, to provide three-dimensional views of coronal mass ejections, and to observe internal flows, surface magnetic fields, and the birth of active regions everywhere; and (3)
From page 7...
... 1.1 INTRODUCTION The Sun is a magnetic star, while the solar wind is both the prototype stellar wind and the only stellar wind we can hope to sample directly with in situ measurements. Solar, hel iospheric, geomagnetic, and ionospheric activity are all linked via the solar wind to the variability of magnetic fields that pervade the solar atmosphere.
From page 8...
... Exploring the solar interior, · Understanding the quiet Sun, · Exploring the inner hel iosphere, · UnderstandingtheactiveSunandheliosphere, and · Exploring the outer heliosphere and the local interstellar medium. Within these themes the panel has identified the outstanding science questions that currently are at the cutting edge of solar-heliospheric physics, and at the end of Section 1.3 it prioritizes those questions requiring new initiatives to continue present progress across a broad front.
From page 9...
... · Co-rotating energetic particle events were discovered at very high solar latitudes near the solar activity minimum, suggesting a new model for the heliospheric magnetic field. · A new source of pickup ions, thought to be solar wind deposited on and re-emitted from interplanetary dust grains, was discovered in the inner heliosphere.
From page 10...
... · Two classes of solar energetic particle events were recognized in the heliosphere: impulsive events accelerated during flaring activity and the much larger gradual events accelerated in the solar wind by CMEdriven shocks. · Trans-iron nuclei with 36 < Z < 83 were found to be overabundant in some impulsive solar energetic particle events by a factor of ~1,000, an important clue for understanding acceleration processes at the Sun.
From page 11...
... · The discovery that suprathermal ions are always present in the slow solar wind provided direct evidence for persistent ion acceleration in the inner heliosphere. · The discovery of rapid intensity variations in small solar energetic particle events provided direct evidence for the random walk of open field lines on the solar surface.
From page 12...
... · A hydrogen "wall" was predicted and detected at the nose of the heliosphere in the direction of the solar system's motion through the local interstellar medium. 1.3 SCIENCE THEMES FOR THE COMING DECADE The panel's recommendations for new initiatives in solar and heliospheric physics in the coming decade are based on science questions arising in five basic research themes; these research themes and the underlying science questions are discussed below in some detail.
From page 13...
... Hence, d inferential rotation, cyclonic turbulence, and magnetic buoyancy al I must play important roles in the generation of the observed large-scale magnetic fields associated with the 22-year activity cycle. Rapid advances in massively parallel computer architectures are now enabling detailed studies of the processes thought to be crucial to the global dynamo.
From page 14...
... The magnetic field inside and outside active regions emerges through the photosphere, spreads, and cancels constantly, accompanied by ceaseless dynamic and radiative manifestations of ongoing energy release on a wide range of spatial and temporal scales. Even long-lasting structures like prominences are far from static.
From page 15...
... Progress has been made on all fronts over the last decade, due to new observations, ideas, and models as well as to advances in computer capabilities, but more work in this area is needed. The panel recommends two pertinent observational quests for the coming decade: measurements of the time-varying magnetic field from photospheric to coronal heights and detai led measurements of the spatial and temporal scales on which magnetic flux emerges and dissipates through the solar cycle (see subsections "Frequency Agile Solar Radiotelescope," "Focused Theory/Modeling/Simulation Mission: A Virtual Sun," and "A Reconnection and Microscale Probe," in Section 1.5, and "Instrumentation to Observe the Solar Atmosphere at 300 to 1,000 Angstroms," in Section 1 .6~.
From page 16...
... These observations have improved dramatically over the past decade and provide tantal izing gl impses of the physical nature of the innermost heliosphere. To discriminate among THE SUN TO THE EARTH AND BEYOND: PANEL REPORTS competing theories and to make a major breakthrough in our understanding of the coronal origins of the solar wind, however, we need to know physical parameters that are difficult or impossible to measure remotely, including the structure, strength, and fluctuation spectrum of the ambient magnetic field, the hydromagnetic waves/turbulence down to scales comparable with the thermal ion gyroradi i, phase-space distribution functions of both thermal and suprathermal particle populations, and the manner in which these quantities evolve with distance from the Sun in the innermost heliosphere.
From page 17...
... These secondary photons and particles provide high-resolution information on solar energetic particles trapped in solar active regions that are not otherwise di rectly observable. UNDERSTANDING THE ACTIVE SUN ANDTHE HELIOSPHERE Stressed magnetic fields are thought to provide the free energy for a wide range of transient energetic phenomena on the Sun, ranging from the smallest microjets and microflares to the largest flares and CME/filament eruptions.
From page 18...
... Because competing CME models assume distinctly different initial conditions, a pressing observational issue for the coming decade is to determine the pre-eruption magnetic field and plasma conditions. We now know from in situ observations that at least 30 percent of CMEs in the solar wind have a magnetic flux rope structure, yet it remains uncertain how and where these flux ropes are generated.
From page 19...
... Missions such as STEREO should begin to bridge this gap and will help provide a comprehensive picture of the entire disturbance process from birth at the Sun to heliospheric response. However, full understanding of disturbance propagation in the solar wind, an essential component of both the National Space Weather and the Living With a Star programs, will require multipoint observations at different heliocentric distances and longitudes (see subsection "A Multispacecraft Hel iospheric Mission," in Section 1 .5~.
From page 20...
... will play a critical role in deciphering the physics of shock acceleration. EXPLORING THE OUTER HELIOSPHERE AND THE LOCAL INTERSTELLAR MEDIUM As the solar wind expands through the solar system it eventual Iy reaches a point where it can no longer hold off the pressure of the interstellar medium, and it undergoes a shock transition, forming the solar-wind termination shock (Figure 1.8~.
From page 21...
... Although high-energy cosmic rays, neutral interstellar gas, and large interstellar dust grains are able to cross the heliopause, the solar system is effectively shielded from the interstellar plasma, magnetic fields, low-energy cosmic rays, and small dust grains. Thus the outer heliosphere and its dynamics affect the space environment of Earth.
From page 22...
... Ne, and Ar) is able to penetrate the heliosphere, where some of the material is ionized by solar UV or charge exchange with the solar wind and become pickup ions.
From page 23...
... or by the new initiatives (Section 1.5~. These capabilities include the ability to obtain continuous and redundant solar irradiance measurements, to image the corona in x rays and the EUV, to image CMEs in white light, to do helioseismology, and to measure the solar wind plasma, magnetic field, and energetic particle variations in near-Earth interplanetary space.
From page 24...
... through the solar wind. Solar Terrestrial Relations Twin spacecraft that provide stereo imaging and in situ measurements of coronal mass ejections 3, 4 Observatory (STEREO)
From page 25...
... understanding the active Sun and heliosphere, and (5) exploring the outer heliosphere and the local interstellar medium.
From page 26...
... · Global Oscillations Network Group. The group operates identical Michelson Doppler imaging instruments at six sites around the world to allow nearly uninterrupted full-disk observation of solar oscillations and magnetic fields.
From page 27...
... the processes that reconfigure magnetic fields in the solar atmosphere while releasing energy gradually or explosively, (5) the magnitude and cause of both short- and long-term variations in the full-disk solar irradiance spectrum, and (6)
From page 29...
... Remote sensing observations and in situ sampling of the solar wind far from the Sun have provided tantalizing glimpses of the physical nature of this region. However, to understand how the solar wind originates and evolves in the inner heliosphere, the panel's top science priority for the coming decade, we need direct in situ sampling of the plasma, energetic particles, magnetic field, and waves as close to the solar surface as possible.
From page 30...
... and the thermal structure of the solar atmosphere in three dimensions. Moreover, radio imaging spectroscopy provides a variety of powerful techniques for measuring magnetic fields in the corona.
From page 31...
... FASR will be a versatile and powerful instrument, providing unique data to solar and space physicists for studying basic physical processes operating in the solar atmosphere such as reconnection, plasma heating and acceleration, and electron transport. FASR will address aspects of the panel's top three science priorities by providing the following: · Measurements of coronal magnetic fields and currents; · Observations of nonthermal particle emission associated with flares, CM Es, and coronal shocks; · Observations of the three-dimensional structure of the solar atmosphere from chromospheric to coronal heights; · Observations of sites and mechanisms of coronal heating and solar wind acceleration; and · Observations of coherent emission mechanisms and plasma wave-particle interactions.
From page 32...
... However, the Virtual Sun is envisioned to have a far more extensive scope than those efforts. Examples of Virtual Sun components include models of the emergence of magnetic loops in the photosphere, the evolution of MHD turbulence in the solar wind, the evolution of CME-driven disturbances, including the acceleration of energetic particles, and the interaction of the solar wind with the LISM.
From page 33...
... The images shown are isosurfaces where the magnetic field strength ABE equals ~B~max/2.The tubes are initially oriented perpendicular to each other and are pushed toward each other at a small fraction of the Alfven speed (panels (a)
From page 34...
... A mission of this kind will illuminate the connections between solar activity, heliospheric disturbances, and geomagnetic activity, will directly addresses the panel's third science priority, and will be an essential element of NASA's Living With a Star program. The main scientific issues that should be addressed by a mu Itispacecraft hel iospheric mission i ncl ude determination of the following: · How CME-driven disturbances evolve as functions of heliocentric distance and longitude in a structured ambient solar wind; · The internal structure and magnetic topologies of CMEs as well as effects of external field draping; · How energetic particle populations accelerated by CME-driven shocks vary as functions of heliocentric distance and longitude; and · How the large-scale structure of the solar wind evolves in longitude and with distance in the inner heliosphere in the ecliptic plane.
From page 35...
... The scientific return from a multispacecraft heliospheric mission wi l l be considerably enhanced if it is timed to be concurrent with coronagraph observations of CME disturbances departing from the solar atmosphere. A multispacecraft mission of this nature is of paramou nt i mportance i n u nderstand i ng and pred icti ng the form and intensity of heliospheric disturbances impinging on Earth's magnetosphere and hence should be an essential part of NASA's Living With a Star program.
From page 36...
... ; · Simultaneous imaging of plasmas from transitionregion to flare temperatures; · Sufficient i magi ng sensitivity to detect emission from wave and shock compressions; and · Ful l-Sun context imaging of the surrounding magnetic field and plasma conditions. By deciphering the evolving dynamics and energetics of fine-scale coronal plasmas, a RAM probe will make major breakthroughs on several of the outstanding problems in solar physics, including coronal heating, CME initiation, and solar wind acceleration, thus addressing different aspects of the panel's top three science priorities.
From page 37...
... A new mission, an interstellar probe, is needed to carry instruments specifically designed for comprehensive study of the heliospheric boundaries and exploration of our local galactic environment (Figure 1 .12~. Once beyond the heliopause, an interstellar probe will discover the properties of interstellar gas, dust, the interstel lar magnetic field, and low-energy cosmic rays unaffected by the heliosphere.
From page 38...
... Among the accompl ishments expected from such a comprehensive mission are these: · Measuring for the first time the Sun's evolving polar magnetic field and subsurface polar motions; · Probing three-dimensional structures deep beneath the surface using two-position helioseismology techniques, and predicting when and where active regions will emerge over the entire Sun; · Probing coronal magnetic fields with x-band and Ka-band Faraday rotation measurements; · Examining three-dimensional thermal and magnetic structures from a polar perspective; · Tracking the complete life cycle of active regions and coronal holes; · Linking variations in the high-latitude heliosphere to surface conditions; and · Measuring the global effects of dynamic events with complementary stereoscopic imaging and in situ observations. These observations would address the panel's second and third science priorities by exploring the role of polar convection in solar magnetic field evolution, understanding the mechanisms by which magnetic field reversal occurs, exploring the azimuthal and latitudinal structure of the corona and streamer belt, and understanding the three-dimensional structure of CMEs and polar plumes.
From page 39...
... While it is now recognized that solar energetic particles (SEPs) are accelerated both in solar flares and at CME-driven shock waves, the details of these particle acceleration processes remain elusive.
From page 40...
... The time is ripe to take advantage of the enormous worldwide investment being made in AlGaN materials research to develop UV image senTHE SUN TO THE EARTH AND BEYOND: PANEL REPORTS sors that meet the unique set of requirements for solar and hel iospheric appl ications. LOW-FREQUENCY HELIOSEISMOLOGY During the past decade, acoustic helioseismology was highly productive as a probe of the solar interior.
From page 41...
... The same imaging principles should apply to the remote sensing of energetic particle acceleration processes on the Sun, in interplanetary space, and at the outer boundaries of the hel iosphere. Although current techniques have been limited by very small geometric factors, more sensitive designs using new detection methods and background-suppression techniques appear promising.
From page 42...
... Suitably modified, they will be able to detect and measure the highly time-dependent and directional flows that are the hallmarks of magnetic reconnection in the solar corona, both along and transverse to the line of sight and to record the associated thermalization of the ambient plasma. Experiments employing these detectors can provide the first fully three-dimensional picture of reconnection in energetic solar phenomena, from coronal heating to coronal mass ejections, thus answering some of the most fundamental questions in solar and heliospheric physics (themes 2 and 41.
From page 43...
... Accurate modeling of these processes, including the effects of charge-to-mass ratio dependent fractionation during particle acceleration and transport, is essential for interpreting the measured ionic charge state and elemental composition of thermal, suprathermal, and more energetic particles in the solar wind. For example, comparisons between ionic charge-state observations and models of shock acceleration near the Sun, i n interplanetary space, and at the termination shock have been used recently to interpret the time scale for such acceleration processes.
From page 44...
... · The panel strongly encourages NASA, NSF, and other agencies that fund solar and heliospheric physics to continue interagency planning and coordination activities that will optimize the science return of groundand space-based assets. It encourages a similar high level of planning and coordination between the AST and ATM divisions of NSF.
From page 45...
... The panel strongly recommends that NSF continue its support for these groups, in particular SHINE. ADDITIONAL READING A strategy for the conduct of space physics research has been set down in a number of reports by the NRC's Space Studies Board and its predecessor the Space Science Board.
From page 46...
... 2001. Physics of the Solar Corona and Transition Region.


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