The Sun to the Earth -- and Beyond A Decadal Research Strategy in Solar and Space Physics (2003) / Chapter Skim
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1. Solar and Space Physics: Milestones and Science Challenges
1. Solar and Space Physics: Milestones and Science Challenges
Pages 22-52
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From page 22... ...
Intellectual inquiry into fundamental processes often yields utilitarian benefits of considerable value to society.4 In the case of solar and space physics, utility is found principally in three areas. First, solar activity produces disturbances in Earth's space environment that can adversely affect certain important technologies and threaten the health and safety of astronauts.
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From page 23... ...
more than 12.5 billion kilometers from the Sun.3 The size and structure of the heliosphere are determined by the relative pressures of the solar wind and the interstellar medium and will change as these pressures vary. At this point in the history of the solar system, most of the changes in the dimensions of the heliosphere probably result from variations in solar wind ram pressure over the course of the 11-year solar cycle and are likely to be comparatively minor (a few percent)
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From page 24... ...
The Sun's total irradiance (the solar"constant") varies on time scales at least as long as the 1 1 -year solar activity cycle, but the variations directly observed have been too small (0.1 percent of the total irradiance)
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From page 25... ...
Schatten, A discussion of plausible solar irradiance variations, 1 700-1992, Journal of Geophysical Research 98, 18895-18906, 1993) uses a long-term variation based on the length of the 1 1 -year cycle rather than its amplitude, together with other parameters of solar variability.
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From page 26... ...
However, important questions remain for example, about the distribution, emergence, and evolution of magnetic flux on the Sun; about the storage and release of energy in solar magnetic fields; and about the detailed workings of the solar dynamo and the origins of the solar cycle. In particular, the astonishing fibril state of the magnetic field at the visible surface needs to be understood, along with the degree to which the field is in a fibril state far below the surface.
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From page 27... ...
A shock wave may also form upstream of the heliosphere if the motion of the heliosphere through the interstellar medium is supersonic. Shown in the figure are the trajectories of the four deep-space probes Pioneers 10 and 1 1 and Voyagers 1 and 2 that are headed out of the solar system and that may soon encounter the termination shock.
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From page 28... ...
The emissions are from Fe IX/X at a temperature of ~1,000,000 K The magnetic field emerging from the photosphere structures the coronal plasma in an intricate and dynamic architecture of loops, arcades, and filaments.
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From page 29... ...
The solar wind's interaction with the magnetosphere, effected principally through a temporary merging of the interplanetary and terrestrial magnetic fields, stirs and energizes the magnetospheric plasmas and leads to periodic explosive releases of magnetic energy. In these events, known as magnetospheric substorms, powerful electrical currents flow between the magnetosphere and the ionosphere, injecting several billion watts of power into the upper atmosphere and producing often quite spectacular displays of the aurora borealis and australis the Northern and Southern Lights (Figure 1.~.
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From page 30... ...
The ionized gases that populate the magnetosphere are remarkably dilute: The densest magnetospheric plasma is 10 million times less dense than the best laboratory vacuum! Nevertheless, the motions of these highly tenuous plasmas drive powerful electrical currents, and during disturbed periods, Earth's magnetosphere can dissipate well in excess of 100 billion watts of power a power output comparable to that of all the electrical power plants operating in the United States.
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From page 31... ...
During the second half of the century, correlations reported between solar activity (as manifested in the changing sunspot number and in flares) , disturbances in the Earth's magnetic field, and auroral activity clearly suggested the existence of a physical connection between the Sun's activity and terrestrial magnetic and upper atmospheric phenomena.
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From page 32... ...
. During the ensuing decades, space physicists made significant progress in understanding this link, which involves the merging of the interolanetarv and terrestrial magnetic fields and ~ ~ .
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From page 33... ...
Early direct balloon- and ground-based measurements had shown the occasional existence of energetic particles of solar origin in addition to the everpresent flux of galactic cosmic rays. In the late 1950s and early 1960s, measurements of cosmic radio noise absorption at high geomagnetic latitudes revealed that these events were more frequent than had been real ized, and subsequent in situ measurements from balloons, rockets, and satellites have provided a wealth of information on the properties of these energetic particle fluxes, which form an important component of space weather and its human consequences.
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From page 34... ...
plasma that corotates with Earth. Visible in the image is a faint tail of plasma that is being eroded from the plasmasphere and transported to the dayside magnetopause, where it is lost into the solar wind.
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From page 35... ...
Coronal holes, low-density regions of open magnetic field, were shown to be the source of the high-speed solar wind, which dominates the heliosphere near and at solar minimum. As noted above, coronal mass ejections are powerful eruptions of coronal plasma and magnetic fields.
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From page 36... ...
Research topics in heliospheric physics on which sign if icant advances were made duri ng the 1 970s and 1 980s i ncl uded the propagation of transient disturbances and turbulence through the heliosphere, the corotating interaction regions (ClRs) formed by alternating high- and low-speed solar wind streams during the declining phase of the solar cycle and at solar minimum, the acceleration of energetic particles at heliospheric shocks, and the distribution and modulation of cosmic rays in the outer heliosphere (see sidebar, "Galactic and Anomalous Cosmic Rays".
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From page 37... ...
The sound speed is now known to about one part in 104 through most of the solar interior. A major advance in our ability to perform helioseismic studies occurred in the mid1990s, with the organization of global networks of dedicated observing sites, preeminent among which are the six sites of the Global Oscillations NetworkGroup (GONG)
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From page 38... ...
Upon ionization, they are picked up by the solar wind's magnetic field and carried outward, toward the termination shock. Some are accelerated to higher energies by interplanetary shocks and may experience further acceleration to energies in excess of 1 GeV at the termination shock.
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From page 39... ...
. The data, which cover a period of more than five decades, from the declining phase of solar cycle 18 through the peak of solar cycle 23, clearly illustrate the anticorrelation between galactic cosmic ray intensity and solar activity.
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From page 40... ...
In heliospheric physics, theory also predicted the existence of the spiral interplanetary magnetic field; the interplanetary magnetic field's role in modulating cosmic ray intensity at Earth; the overall large-scale structure of the heliosphere, including the termination shock (not yet directly observed) ; the corotating interaction regions between fast and slow solar wind streams; the development of forward and reverse shocks bounding those interaction regions at large heliocentric distances; and transient shock wave disturbances driven by the episodic ejection of high-speed plasma from the Sun.
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From page 41... ...
Fortunately, however, none of these impediments is insurmountable, and the solar and space physics communities look forward to the implementation, during the first decade and a half of the new millennium, of missions, facilities, and programs that will equip them to respond to the five scientific challenges set forth below and finally to answer some of the outstanding questions about the Sun and the objects i mmersed i n and i nteracti ng with its atmosphere. The Sun's Dynamic Interior and Corona Challenge 1: Understanding the structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the origin of the solar cycle, the causes of solar activity, and the structure and dynamics of the corona.
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From page 42... ...
The concentrated fibril bundles of emerging subsurface solar magnetic flux are mysterious and far from fully explored. Periodic reversals of magnetic field, variability of the level of activity, and the nature and origin of coronal holes, all features of the solar cycle, are fundamental drivers of the state of the entire heliosphere.
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From page 43... ...
How and where are particles accelerated at the Sun? The Heliosphere and Its Components Challenge 2: Understanding heliospheric structure, the distribution of magnetic fields and matter throughout the solar system, and the interaction of the solar atmosphere with the local interstellar medium.
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From page 44... ...
How are plasma, neutrals, heavy ions, turbulent fluctuations, solar energetic particles, and galactic cosmic rays distributed throughout the entire heliospheric volume? · How do the solar wind plasma and magnetic field interact with the electromagnetic field, plasma, and neutrals in the nearby region of the galaxy?
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From page 45... ...
Nonetheless, important gaps in scientific understanding remain concerning, for example, the configuration and dynamics of the magnetosphere under extreme solar wind conditions (i.e., during strong geomagnetic storms) ; many aspects of magnetic reconnection at the dayside magnetopause and in the magnetotail; particle energization in the inner magnetosphere; the complex structure of the magnetotail as it absorbs and releases energy extracted from the solar wind; and the temporal and spatial scales of the electrodynamical processes by which some of this energy, along with momentum, is transferred to and redistributed within the ionosphere-thermosphere system.
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From page 46... ...
Owing to the presence of localized crustal remanent magnetic fields, Mars's interaction with the solar wind is expected to be more complicated than that of Venus with the solar wind and that of Titan with Saturn's magnetospheric plasma. The smallest of the planetary magnetospheres is Mercury's, for which only limited data, acquired by Mariner 10 during two passes in the mid1 970s, are avai lable.
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From page 47... ...
As these processes largely involve matter in the plasma state, solar physics and space physics can both be considered branches of plasma electrodynamics, to which field they have contributed significantly over the years. The heliosphere is a natural laboratory for the study of plasma physics, and the next decade of research can be expected to lead to advances in our understanding of such fundamental plasma physical processes as magnetic reconnection; turbulence; charged particle acceleration and scattering; generation, transport, and damping of plasma waves; and magnetic dynamo action.
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From page 48... ...
Much remains to be learned, however, about processes such as radiation belt enhancements that affect the environment in which many satellites operate; about the variations in the properties of the ionosphere-thermosphere system that can adversely affect Global Positioning System navigation systems and high-frequency radio wave propagation; and, finally, about the solar drivers of space weather. During the coming decade these problems will be the focus both of pure space physics research and of targeted basic research activities such as those envisioned in NASA's Living With a Star initiative, the National Science Foundation's National Space Weather Program, and the Department of Defense's Space Weather Architecture Study.45 An important aspect of space weather-related research is the development of specification and predictive models that can be used for system design, space operations, and both now-casting and forecasting.
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From page 49... ...
We also observe space plasma physics processes in the solar system much more closely than in more distant astrophysical objects. The detailed observations and models developed in space physics can therefore help to develop physically motivated explanations of less-well-constrained astrophysical phenomena.
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From page 50... ...
Finally, distinct plasma regions and regimes are coupled across boundaries in a highly nonlinear, dynamical fashion. Such cross-system coupling is exemplified by the coupling that occurs between the solar wind and Earth's magnetosphere as a result of the merging of interplanetary and geomagnetic field lines and by the electromagnetic coupling of the magnetosphere and the ionosphere.
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From page 51... ...
6. Partial protection of the martian atmosphere from the solar wind is provided by the strong, localized remnant crustal magnetic fields recently discovered by the Mars Global Surveyor magnetic field experiment.
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From page 52... ...
14. Magnetohydrodynamic theory incorporates the effects of magnetic fields in the hydrodynamic description of ionized gases (plasmas)
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