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Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
55
the upper atmosphere. Much work is now focused on understanding the impact of climate change on the
thermosphere and ionosphere and on identifying signatures in the thermosphere that can be used to assist
in monitoring and clarifying the sources and mechanisms of climate change.
KEY SCIENCE CHALLENGES
The advances over the past decade focus attention on the challenges that are most pressing, both
scientifically and practically. Significantly, recent progress includes the greatest advances to date toward
achieving the predictive capability needed to safeguard the global technological infrastructure. Distilled
from the science goals presented by the survey’s three interdisciplinary panels, the challenges identified
below by the survey committee are major areas of ongoing inquiry that provided the context for development of the program of research advocated by this survey for the coming decade. Scientifically important
in their own right, the frontiers of heliophysics are also important as a source of practical knowledge for
maintaining the operability of the assets of our increasingly technological society. Despite the challenges
in studying these systems, the experience of the past decade demonstrates that scientists and engineers in
the field have achieved dramatic progress in advancing the state of knowledge, and progress in the areas
outlined below can reasonably be expected to be equally impressive. Even though the committee anticipates that key components of these challenges will be resolved in the coming decade, some questions will
undoubtedly remain open, and other challenges are likely to emerge.
Challenges Related to the Sun and Heliosphere
The magnetic field of the Sun is, directly or indirectly, the driver of much of the dynamics of the
heliosphere. Thus, understanding how the solar magnetic field is generated—the dynamo problem—is
a key challenge. Despite the complexity of the solar magnetic field at multiple scales, as is evident, for
example, in Figure 2.1, the global solar magnetic field exhibits an approximately 22-year periodicity, with
the polarity of the field reversing every 11 years. Various solar phenomena, of which number of sunspots
is the most familiar, exhibit this same 11-year cycle. Indeed, evidence for decadal-scale periodicities has
been found in the luminosities of other Sun-like stars. Scientists know that the twisting and amplification
of seed magnetic fields in the Sun’s convective zone, the outer one-third of the Sun, are the source of
the solar magnetic field. The resulting solar dynamo has therefore become a prototype for understanding
how magnetic fields are generated throughout the universe. Although researchers have shown that quasiperiodic reversals are a natural consequence of dynamo action, they have not yet established why the
solar dynamo produces a field that reverses with a nearly regular 11-year period. Moreover, the deep and
prolonged minimum in the present cycle, cycle 24, was totally unexpected, demonstrating that understanding of the solar magnetic field has not yet risen to the state of a predictive science. The following remains
a primary challenge: SHP-1. Understand how the Sun generates the quasi-cyclical magnetic field that
extends throughout the heliosphere.
The complex structure of the light emitted from the corona as shown in Figure 2.1 reflects the corresponding structure of the magnetic field since hot plasma making up the corona very quickly spreads out
along lines of force of the magnetic field. Scientists can now calculate this complex magnetic structure
with computational models using the ground-based measurements of the magnetic field at the Sun’s visible
surface, or photosphere. However, the time variation of the complex magnetic field in the Sun’s tenuous
outer atmosphere or corona, which often takes the form of explosive events, is not fully understood and
remains at the frontier of heliophysics research. Probing the details of the solar magnetic field at multiple
heights in its atmosphere, and at very high temporal and spatial resolution, is the goal of NSF’s Advanced
Technology Solar Telescope (ATST).
Solar and Space Physics: A Science for a Technological Society
SOLAR AND SPACE PHYSICS: RECENT DISCOVERIES, FUTURE FRONTIERS
55
the upper atmosphere. Much work is now focused on understanding the impact of climate change on the
thermosphere and ionosphere and on identifying signatures in the thermosphere that can be used to assist
in monitoring and clarifying the sources and mechanisms of climate change.
KEY SCIENCE CHALLENGES
The advances over the past decade focus attention on the challenges that are most pressing, both
scientifically and practically. Significantly, recent progress includes the greatest advances to date toward
achieving the predictive capability needed to safeguard the global technological infrastructure. Distilled
from the science goals presented by the survey’s three interdisciplinary panels, the challenges identified
below by the survey committee are major areas of ongoing inquiry that provided the context for development of the program of research advocated by this survey for the coming decade. Scientifically important
in their own right, the frontiers of heliophysics are also important as a source of practical knowledge for
maintaining the operability of the assets of our increasingly technological society. Despite the challenges
in studying these systems, the experience of the past decade demonstrates that scientists and engineers in
the field have achieved dramatic progress in advancing the state of knowledge, and progress in the areas
outlined below can reasonably be expected to be equally impressive. Even though the committee anticipates that key components of these challenges will be resolved in the coming decade, some questions will
undoubtedly remain open, and other challenges are likely to emerge.
Challenges Related to the Sun and Heliosphere
The magnetic field of the Sun is, directly or indirectly, the driver of much of the dynamics of the
heliosphere. Thus, understanding how the solar magnetic field is generated—the dynamo problem—is
a key challenge. Despite the complexity of the solar magnetic field at multiple scales, as is evident, for
example, in Figure 2.1, the global solar magnetic field exhibits an approximately 22-year periodicity, with
the polarity of the field reversing every 11 years. Various solar phenomena, of which number of sunspots
is the most familiar, exhibit this same 11-year cycle. Indeed, evidence for decadal-scale periodicities has
been found in the luminosities of other Sun-like stars. Scientists know that the twisting and amplification
of seed magnetic fields in the Sun’s convective zone, the outer one-third of the Sun, are the source of
the solar magnetic field. The resulting solar dynamo has therefore become a prototype for understanding
how magnetic fields are generated throughout the universe. Although researchers have shown that quasiperiodic reversals are a natural consequence of dynamo action, they have not yet established why the
solar dynamo produces a field that reverses with a nearly regular 11-year period. Moreover, the deep and
prolonged minimum in the present cycle, cycle 24, was totally unexpected, demonstrating that understanding of the solar magnetic field has not yet risen to the state of a predictive science. The following remains
a primary challenge: SHP-1. Understand how the Sun generates the quasi-cyclical magnetic field that
extends throughout the heliosphere.
The complex structure of the light emitted from the corona as shown in Figure 2.1 reflects the corresponding structure of the magnetic field since hot plasma making up the corona very quickly spreads out
along lines of force of the magnetic field. Scientists can now calculate this complex magnetic structure
with computational models using the ground-based measurements of the magnetic field at the Sun’s visible
surface, or photosphere. However, the time variation of the complex magnetic field in the Sun’s tenuous
outer atmosphere or corona, which often takes the form of explosive events, is not fully understood and
remains at the frontier of heliophysics research. Probing the details of the solar magnetic field at multiple
heights in its atmosphere, and at very high temporal and spatial resolution, is the goal of NSF’s Advanced
Technology Solar Telescope (ATST).