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Principal Science Opportunities and
Initiatives for Ground-Based
Solar Research
Advances in experimental and observational techniques now make it
possible to observe aspects of the Sun that were previously unknown or
unappreciated. The observations reveal a star of complex and mysterious
behavior. Neutrino astronomy; helioseismology; high-resolution observa-
tions of the solar surface; radio, infrared, UV, X-ray, and gamma-ray
observations of the outer atmosphere; vector magnetic field observations;
and spacecraft observations of the secular changes in the solar luminosity
have all uncovered new and puzzling aspects of the Sun. These fundamen-
tal investigations have been possible only because of the proximity of the
Sun. One may infer that other stars are equally mysterious, but they cannot
be resolved in the telescope and are too far away for the necessary close
scrutiny.
In this chapter the committee explores in greater detail the principal
needs and most promising opportunities for investigation over the coming
5 years in the four research areas at the forefront of solar physics today:
(1) probing the solar interior, (2) the physics at small spatial scales, (3) the
mechanisms underlying the solar cycle, and (4) the physics of transients.
The committee interviewed leading solar physicists from all major solar
physics research centers in the United States and solicited oral and written
comments from the solar community at large.
16
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17
PROBING THE SOLAR INTERIOR
The Basic Issues
Information from the interior of the Sun is needed to understand
fluctuations in the Sun's radiative and nonradiative outputs, to verify the
theory of stellar structure and evolution, to help develop an understanding
of fluid motions in realms beyond laboratory and theoretical modeling,
and to advance several areas of basic physics. Recent work suggests that
significant revisions are required in our current concepts of all these topics
and that the ramifications may extend far beyond the traditional range of
solar physics.
One of the triumphs and major foundations of astrophysics is the
theory of stellar structure and evolution. Much of what we understand
about the universe derives from this theory. It is now possible to critically
test the predictions of the theory for the case of the Sun, and the results
are disturbing. The flux of neutrinos produced in the solar core has
been measured since 1968 in a celebrated experiment located deep in the
Homestake mine in South Dakota. Only one-third the flux of neutrinos
predicted by the best models of the solar interior has been measured. A
new experiment located at Kamioka, Japan, was started in 1987; the first
results confirm that the neutrino flux is less than that predicted by solar
models. This "neutrino problem" is larger than can be explained by current
understanding and uncertainties of the relevant physics.
Another prediction from the theory of stellar structure concerns the
frequencies of the normal modes of oscillation of the Sun. Helioseismolog-
ical observations have measured these frequencies with a precision of a few
parts per hundred thousand. There is a systematic discrepancy between the
observations and the predictions of a few parts per thousand. Again, this
discrepancy is larger than can be explained by current understanding of the
relevant physics.
The theory of stellar evolution predicts that the Sun should have
brightened by about 30 percent since the formation of the solar system.
Geological and climatological evidence suggests that the change in solar
luminosity has been much smaller. One proposed solution to this problem
is to mix the solar interior to provide fresh fuel to the energy-generating
core. Mixed models seem to be ruled out by current helioseismology results.
Evolutionary theory also suggests that the interior of the Sun should be
rotating much more rapidly than the surface layers, which have been braked
by angular momentum transfer to the solar wind. Instead, helioseismology
indicates that the interior is rotating very much as the surface rotates.
Theoretical understanding of circulation and convection inside the
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Sun is not well advanced because of the intrinsic difficulty of the relevant
physics, an inability to construct and run realistic numerical models, the
large extrapolations required from laboratory experience, and the relative
lack of observations of the solar interior to provide guidance. Existing
models of motions within the convection zone have not been confirmed
by observations. Predictions of a polar vortex, giant circulation cells, and
strong variations in rotation rates with depth and latitude in the convection
zone have not been supported by observation.
Evidence from a variety of observations suggests that nearly all stars
with a mass of less than about 1.5 times the solar mass (and this means most
stars) exhibit activity of the type that we observe in the Sun. We do not yet
have a good understanding of how magnetic activity is produced even within
the best observed star- the Sun. Much has been learned from observations
of other stars that have a range of physical parameters different from
those of the Sun. Probing the interior of the Sun can provide additional
information about how stellar and solar activity is generated. Initial results
from helioseismology indicate that the subsurface structure of sunspots and
active regions does not agree with that described by current models. New
models of the solar magnetic dynamo, which is thought to generate the
solar activity cycle, are under development based on helioseismology.
The discrepancies between current models and current observations
listed above have challenged many researchers to suggest innovative solu-
tions. Some of these suggestions extend into the realm of exotic physics.
A typical example is the hypothesis that there may exist weakly interacting
massive particles (WIMPS or cosmions) within the solar interior (and else-
where). Such cosmions could reduce the central temperature of the Sun
and thereby explain the neutrino deficit. It is worth noting that a model of
the Sun that includes cosmions predicts p-mode oscillation frequencies that
are significantly closer to observations than are those predicted by stan-
dard solar models. It has also been suggested that neutrinos have a small
rest mass, and even a magnetic moment, that could explain aspects of the
neutrino problem. Laboratory results on this important physics question
are conflicting, but future solar observations should help to verify or deny
these suggestions.
Initiatives and Impacts
Researchers in the United States have led or participated in most
investigations involving the solar interior. The United States has been
particularly strong in observational work and can maintain a leading role
in some areas and a presence in others by balancing support for continuing
facilities and programs, initiating selected new programs, and collaborating
with international partners where appropriate.
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19
Theory and Modeling
The study of the solar interior depends intimately on predictions
from theory and modeling. It is essential that support for this activity
be accorded as much priority as observational programs. The United
States can continue among the world leaders in this field by initiating and
supporting collaborative as well as domestic work. A good example is a
6-month workshop planned for 1990 at the Institute for Theoretical Physics
of the University of California, Santa Barbara.
Neutrino Obse~vaiions
A survey of recent publications and plans for future projects clearly
shows that the United States is heavily involved in neutrino observation,
although not always in a leadership role. The committee urges that the
United States maintain its presence in the field by continuing a few key
experiments and supporting U.S. participation in international projects.
Leading opportunities for initiatives include the following:
1. Continue operation of the 37C1 experiment through the next solar
maximum expected in 1991, and continue support of the Kamiokande II
experiment, whose results are an important consistency check for the 37C1
experiment. This will allow tests of suggested correlations of neutrino
flux with the solar activity cycle and, more speculatively, with the Earth's
heliocentric latitude. A confirmation of modulation of the neutrino flux will
have a profound impact on solar physics, astrophysics, and particle physics.
2. Support U.S. participation in additional new international mea-
surements of neutrinos from the Sun. Leo experiments may be considered
as examples. The first is the proposed 2H experiment (Sudbury Solar Neu-
trino Observatory), a Canadian, U.S., and U.K experimental collaboration,
which will measure a variety of FIB neutrino properties, including their
spectrum; the second is a 40A experiment, led by the Italians, which will
provide an independent measurement of the SIB neutrino properties. In
addition to determining the production rates and spectra of the neutrinos,
these experiments will address the question of the mass of the neutrino and
the hypothesis that the neutrino problem is due to a change of one type of
neutrino to other types in transit to the earth.
3. Support U.S. participation in international measurements of neu-
trinos from the main nuclear reaction that produces solar energy, although
given the already existing strong international support for these projects,
support might be more modest than the support for the preceding ex-
periments. This next generation of experiments will enlarge the scope
of neutrino measurements beyond that of the current experiments, which
sample neutrinos from a minor nuclear reaction in one region of the solar
core. Ibro experiments using 7iGa are in preparation to do this. One is
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primarily a European experiment, and the other is a Soviet experiment
with limited U.S. participation. These experiments will detect neutrinos
from the most common reaction that produces energy in the solar core.
Results will indicate whether the neutrino problem originates in physics or
astronomy.
4. Complete an experiment to deduce the average neutrino flux over
the last several million years. Sometime in 1989, results are expected from
981t extracted from about 20 boxcars of molybdenum ore mined from the
Henderson mine in Colorado. I-his isotope is produced by absorption of
neutrinos that have penetrated the 1500-m depth of the mine. Since the
half-life of the isotope is a few million years, this difficult experiment may
be able to measure the constancy of neutrino flux over the past several
million years. If evidence for a changing flux is found, orthodox views of
stellar evolution will need to be changed.
Helioseismology Projects
The United States is the world leader in helioseismology observations
utilizing solar imagery. Europe leads in helioseismology of the Sun ob-
served as a star. Both approaches enjoy unusually strong and stimulating
international contributions and cooperation by observers and theoreticians.
As a result, the field of helioseismology has expanded rapidly since its
beginning in 1975. The literature comprised about 700 papers in mid-1987
and has doubled every 3 years. Work in this field (see, for example, Figure
3.1) has already answered some long-standing questions about the solar
interior but has raised new questions of potentially wide-ranging signif-
icance throughout astrophysics and physics. fib maintain leadership and
momentum in this field, the United States should pursue a number of
initiatives:
1. Support exploration of new observational methods and techniques.
Groups at the California Institute of Technology; Stanford University; the
Universities of Arizona, Delaware, Hawaii, and Southern California; the
National Solar Observatory (NSO); the High Altitude Observatory (HAO);
National Aeronautics and Space Administration (NASA)/Goddard; and
elsewhere are advancing state-of-the-art observational helioseismology. A
good example is the tomography of sunspot structure developed recently by
researchers from NASA and the University of Hawaii using NSO/Kitt Peak
facilities. The result of supporting innovative observational helioseismology
will be the development of new and improved methods for probing the solar
interior.
2. Support the Global Oscillations Network Group (GONG) project.
This is a community project initiated by the NSO to provide continuous
solar oscillation data for a period of 3 years. It was a response to an
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21
~7,:
, 35 days
30 days
\
)
/
-
-
/
~ ~26 days
'`l
~ \
\
MY
(
/ 25 days
FIGURE 3.1 Contour plot: cross section through the Sun, with contoum of constant
rotation period as a function of latitude and depth in the solar interior. The dashed
line marks the base of the solar convection zone. The picture is based on measurements
of oscillations of the Sun's surface, which are a manifestation of sound waves traveling
through the solar interior. The rotation rate is determined by comparing waves that travel
east-to-west and west-tc'+ast at different depths inside the Sun. Because of limitations in
the current measurements, the results here are only accurate for radii larger than 0.4 solar
radii. The results indicate that the Sun's surface rotation persists throughout the outer
30 percent of the Sun, where it is probably driven by large-scale convection. Below the
convection zone the Sun appears to rotate nearly rigidly (with the possible exception of the
deep intenor) at a period of about 27 days.
This picture is based on helioseismology data obtained by K. Libbrecht at Big Bear
Solar Observatory and on inversions of the data by J. Christensen-Dalsgaard and J. Schou,
as well as by P. Goode and W. Dziembowski. (Reproduced flay permission of the California
Institute of Technology.)
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22
obvious scientific need for such data and to an invitation by NSF for
innovative projects. Motivation for the project is reduction of the noise
and confusion introduced by nightly gaps in solar oscillation data obtained
from single observatories. Continuous observation by means of a network
of six sophisticated instruments around the world promises to reduce this
problem by at least an order of magnitude. The impact of this project will
be great improvements in the precision of p-mode oscillation frequencies
and amplitudes for degrees up to about 300. This will permit definitive
determinations of the temperature stratification and large-scale motions of
most of the solar interior. It is important that funding also be provided
to assist the helioseismology community to analyze and interpret the data
from the GONG project.
3. Support the U.S. helioseismology experiment on the European
Space Agency's (ESA) SOHO spacecraft. This experiment was selected
by NASA and ESA as one of the major tasks for the SOHO spacecraft
expected to be launched in 1995. Aside from the important advantage of
continuous sunlight afforded by an orbit around the Ll Lagrangian point,
the lack of atmospheric distortion will present unique opportunities to
study oscillations of both high degrees and long periods. The impact of
these observations will be a definitive determination of the stratification
and motions of the upper layers of the convection zone, where our current
understanding of the physics is quite uncertain.
Investigation of the Interiors of Other Solarlike Stars
The study of the solar interior gives us information about one star. It
would be naive to think that we can safely-extrapolate that information to
other stars without some verification. Similarly,-comparison of some of the
characteristics of the solarlike stars, such as age, chemical composition, and
rotational velocity, would provide a considerably sharper test of the theory
of both solar and stellar structure and evolution. For example, the study
of the depletion of light elements in a wide range of stars is a sensitive
indicator of the maximum temperature to which convecting material is
exposed in the outer layers of a star. In the Sun and several other stars,
the outer layers appear to have been exposed to higher temperatures than
can readily be explained by standard theory. While neutrino radiometry of
other stars is currently beyond the capabilities of foreseeable technology,
the prospects are good for seismic probing of solarlike stars. Already the
first steps have been taken on both observational and theoretical fronts
and have shown considerable promise. On the observational side, what
is needed is a highly stable echelle spectrograph, fed by a several-meter-
aperture telescope, and large blocks of contiguous night scheduling. A
recent experiment involved 2 weeks of observing time with the Soviet 6-m
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23
telescope. A dedicated facility would be optimum because of the peculiar
requirements of large amounts of observing time to do seismology of other
stars, but a facility shared with other observing programs is also a feasible
solution.
Another way of approaching this problem is to attempt precise photom-
etry of members of stellar clusters. Although such work is best done from
space, it may be possible to obtain sufficient accuracy with ground-based
equipment. Such experiments should be supported.
The impact of work in this area will be to allow confident application of
what we learn about the solar interior to other stars. The theory of stellar
structure and evolution will be tested over a broader range of parameters
than can be done using the Sun alone. There will also be feedback of
information about other stars into the total picture of the solar interior.
THE PHYSICS AT SMALL SPATIAL SCALES
The Basic Issues
It is now well known that magnetic fields play a central role in the
dynamics of the solar surface layers (for example, by ordering local trans-
port coefficients such as thermal conductivity in an anisotropic fashion, by
blocking convective transport, and by carrying the "mechanical" energy and
momentum flux required for coronal plasma heating and acceleration of
the solar wind); hence solar magnetic activity largely defines the interaction
between the Sun's interior and atmosphere, and between the Sun's atmo-
sphere and the heliosphere and terrestrial magnetosphere. The detailed
physics by which the magnetic activity both arises in the solar interior and
ultimately couples to the outer solar atmosphere and heliosphere remains
a matter of active research. It is nevertheless clear that the answers lie
in an understanding of the interaction between magnetic fields and tur-
bulent conducting fluids and of the equilibrium and stability properties of
magnetized plasmas, and in the realm of collective plasma behavior.
These issues of physics are intimately connected and are, furthermore,
of great interest both to space physicists and terrestrially bound plasma
physicists. Thus issues of plasma confinement (and their attendant problems
of magnetohydrodynamic equilibrium and stability) and plasma heating
(by wave and/or particle beam and plasma interaction) and transport are
central to fusion plasma efforts. It should therefore not be surprising that,
for example, current models for solar plasma heating borrow heavily from
recent advances in the laboratory domain, and that, conversely, some of
the early work on plasma confinement schemes grew out of work originally
carried out in the astrophysical domain.
Because the phenomenolo~ of the solar surface layers is so rich, one
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cannot hope to summarize fairly the entire range of current theoretical
and observational work; hence the following represents an outline of what
the committee perceives as the most exciting current research directions,
with an emphasis on those that exemplify various aspects of the interaction
between solar magnetohydrodynamics and plasma and space physics, plasma
astrophysics in general, and the terrestrial fluid dynamics and laboratory
plasma domains.
Magnetic Field Generation and Intermittency
Solar magnetic fields are striking in two very distinct respects: they
persist in spite of the observed rapid diffusion of surface magnetic fields,
and they are whenever observed spatially highly concentrated and in-
homogeneous. It is commonly believed that these circumstances can be
understood by appealing to the interaction between magnetic fields and
turbulent shear flows. Thus much of the observed phenomenology associ-
ated with the solar magnetic cycle can be reproduced by kinematic magnetic
dynamo models, and spatial intermittence is thought to result from "sweep-
ing" initially homogeneous magnetic fields into regions of stagnant flow by
organized cellular flows (viz, classic Benard convection cells).
Unfortunately, solar magnetic fields are relatively strong, so that it is
dubious whether kinematic theories are an appropriate description of the
physics underlying the solar dynamo; furthermore, the solar convection zone
is far from laminar in behavior (the Rayleigh number is far above critical,
and the Reynolds number exceeds unity by many orders of magnitude), so
that it is unclear whether results from laminar theory can be immediately
adopted. It is therefore not surprising that these issues are currently being
attacked via sophisticated numerical simulation schemes, which include the
effects of magnetoconvection and buoyancy. What is particularly fascinating
about this work for (solar) fluid dynamicists and plasma physicists is that
the Sun at present provides the only "laboratory" for testing theories of
flux concentration and enhanced (turbulent) diffusion of magnetic fields.
Equilibnum and Stability Theory
Because magnetized plasma structures in the outer solar atmosphere-
ranging from cool prominences to million-degree coronal "loops"~an
show both periods of great quiescence and intervals of highly intermittent
activity, there has been a concerted effort to understand the equilibrium
configurations and stability properties of magnetic-pressure-dominated plas-
mas. There are of course obvious parallels to related work in the plasma-
fusion community, and indeed the early solar studies anticipated related
laboratory plasma studies. Stability calculations are being actively pursued
today in the solar context, with substantial input from the now classic work
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from the laboratory domain. This includes use of the Bernstein "energy
principle" and the concept of "line-tying" as applied to magnetic field lines
entering the high-density photosphere from the overlying tenuous chromo-
sphere and corona; application of helicity conservation in construction of
equilibria; studies of the existence of equilibria under specified (realistic)
boundary conditions; and study of field line stochasticity.
Rapid Magnetic Field Reconnection
The role played by collective effects in the solar atmosphere was first
appreciated in the impulsive phenomenon known as the solar flare, com-
monly believed to occur when oppositely directed magnetic fields in the
solar corona "reconnect," thereby releasing energy in the form of heat,
particle acceleration, and induced rapid flows. It has long been evident
that the observed short time scale of impulsive energy release demands a
breakdown of the classical (high electrical conductivity) magnetohydrody-
namic picture normally used to describe the solar outer atmosphere. As
a result, a blossoming of interest in magnetic reconnection (driven also by
observations of related impulsive phenomena in the terrestrial magnetotail)
has occurred: steady-state fluid theory has been placed on a robust, for-
mal footing; calculations have been extended to the collisionless domain;
and extensive efforts at numerical simulation and laboratory modeling of
reconnection are currently being conducted.
From the solar perspective, one needs to understand the geometric
configuration of the reconnection site; to understand the conditions under
which sudden energy release occurs; and to be able to estimate the energy
released into fast particles, direct plasma heating, and flow acceleration.
These questions are indeed common to the various disciplines in which
field reconnection plays a role; the contribution of solar studies will be to
extend significantly the parameter regimes in which reconnection can be
studied.
The Physics of Thermal Heat; Conduction
The rapid rise of the gas temperature above the solar photosphere
to several million degrees within a few thousand kilometers has raised
many questions, not the least of which is how one is to calculate the
thermal transport coefficients properly. The classical Spitzer-Harm thermal
conduction is inherently a linearization, entailing asymptotic expansion
in the ratio of the thermal mean free path to the temperature gradient
scale length. This has been shown to fail in laboratory studies of heat
transport in hot plasmas for very small values of this ratio. In addition,
inertial confinement studies suggest that microturbulent effects may also
come into play. These terrestrial laboratory results are only now finding
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their way into the solar plasma physics domain, and it seems inevitable
that rather significant changes in our understanding of the interchange of
energy between the solar corona and the underlying photospheric gas will
result.
The impact of these applications is in our understanding of the fol-
lowing: previous calculations of the (transition region) thermal heat flux
may be in error; the large mean free path of coronal electrons may signif-
icantly alter the ionization balance of cooler, lower-lying layers (and thus
upset standard plasma diagnostic techniques); and the nonlocal character of
heat transport by long-ranging suprathermal electrons may vitiate previous
hydrodynamic studies based on local theory.
Plasma Diagnostics, Heating, and Monons
The current' state of the art in remote-sensing plasma diagnostics finds
solar plasma physics at the forefront. From the astronomical perspective,
this is by design, for' the Sun provides physical conditions that are not unlike
those encountered in much of the rest of the universe Tut at inaccessible
distances) and reduces demands on instrumentation (because its proximity
leads both to the availability of copious numbers of photons throughout the
electromagnetic spectrum and to some useful degree of spatial resolution of
the activity itself). Thus the Sun has been studied not only for its own sake
but also as a test case for exploring new instrumentation and diagnostic
concepts in a more familiar and accessible context. day's frontiers of
solar plasma diagnostics lie in the direction of nonequilibrium studies and
in the exploitation of high-spectral-resolution observations, combined with
high spatial and temporal resolution (particularly in wavelength domains
heretofore relatively poorly explored with spectroscopic tools). This frontier
area includes efforts to diagnose departures from ionizational equilibrium
(using, for example, satellite lines of strong resonance lines) by observing
detailed line profiles formed at transition-region and coronal temperatures
(which allow one to test for Doppler broadening from the systematic motion
of hot plasma associated either with' flows or with quasi-periodic motions
resulting from propagating or standing waves3.
The latter studies have particular relevance to tests of theories for
atmospheric plasma heating, to studies of mass exchange between the solar
photosphere and the hotter overlying layers (as can occur during the course
of solar flares), and to the classic problem of solar wind heating and accel-
eration in the immediate solar vicinity. High-resolution spectroscopy, when
combined with high spatial resolution and the ability to measure polariza-
tion states (i.e., the Stokes parameters), also allows direct measurement
of vector magnetic fields in the solar atmosphere and hence determination
of the magnetic field topology in the solar corona. At very high photon
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Representative terms from entire chapter:
magnetic fields
31
C`2
\
it, 1368
v
32
TABLE 3.2 Identified Components of Solar Luminosity Vanability
Cause Time Scale Amplitude
p-modes 5 min a few ppm (rms) per mode
Granulation 1 hr 0.05% rms broadband
Sunspots a few days < 0.2% peak to peak
Faculae a few weeks c 0.05% peak to peak
Long term ll years? : about O.l~o peak to peak
In terms of solar luminosity variability, the solar cycle appears to produce
a variation of some 0.1 percent, due to effects of solar magnetism that are
at present poorly understood.
The Nature of Solar Magnetism
As time extends the record of variability, its interpretation becomes
steadily more important in studies of solar interior dynamics. The mech-
anisms that create the solar magnetic field and distribute it through the
interior and atmosphere present some of the most fascinating challenges
of astrophysics; the solar dynamo, if understood quantitatively, might have
analogs in regions as exotic as accretion disks around black holes. Obser-
vations of the solar global structure and its evolution, on active-region and
solar cycle time scales, represent an observational prerequisite to solving
this problem.
The Influences of the Sun on the Earth
Solar magnetic activity produces hard radiation that affects the Earth's
atmosphere and has significant social and economic consequences. These
effects include the inflation of the Earth's upper atmosphere in proportion
to the degree of solar activity, with attendant orbital and pointing disrup-
tions of low-altitude satellites, the disruption of electrical power distribution
caused by ionospheric surges, disturbances of navigation systems, and haz-
ards for spacecraft and astronauts via solar flare energetic particles. Also,
solar variability must be studied in the context of its linkage to climate and
climate change.
Much of the interest in applied solar physics centers on the need
to predict solar activity for applications in the communications, naviga-
tion, electrical power, pipeline, oil exploration, and space industries. This
problem is reminiscent of weather forecasting, but there are certain simpli-
fications that might make the prediction of solar activity easier. In principle,
we can obtain solar data of uniform quality across an entire hemisphere
33
with good calibration and regular, frequent sampling. However, there are
intangible uncertainties connected with the unknown physics of the sub-
photospheric magnetic fields; this latter problem impels us to study basic
solar physics as vigorously as possible.
Initiatives and Impacts
Solar global observations include synoptic data, in which various tracers
of solar activity are followed through the years in a semiquantitative manner.
The conduct of such observations tends not to interest research-oriented
solar physicists (nor most astronomers), in part because very long- time scales
are necessary to achieve results. Perhaps for a similar reason, potential
commercial users have not stepped forward to support the creation of
improved data bases.
In spite of this lack of attention, the U.S. program of synoptic solar
data compilation and distribution by the National Oceanic and Atmospheric
Administration's (NOAA) National Geophysical Data Center is the world's
finest, presenting a large body of useful data ranging from white-light
to cosmic-ray observations. Unfortunately, many of these data are of
the qualitative classical type, benefiting little from recent technological
developments in detectors and data-handling systems and reflecting an
inadequate degree of access to the stable observing conditions of space.
A modern program of synoptic solar observation and data management
is long overdue. Such a program would have interdisciplinary consequences,
linking stellar, solar, and terrestrial researchers and applications users. It
would also represent an interagency effort, since components of the current
synoptic data come from the Department of Defense, NOAA, NSF, NASA,
and other sources, none of whose mission responsibilities specifies an
adequate program of solar data management.
The components of a new, comprehensive program of quantitative data
management should conform to the policy and guidelines of the National
Research Council's Committee on Geophysical Data report titled Geophys-
ical Data: Policy Issues (National Academy Press, Washington, D.C., 1988)
and the Committee on Solar-Terrestnal Research report titled Long-Term
Solar-Terrestrial Observations (National Academy Press, Washington, D.C.,
1988~. The synoptic solar observations would include, at a minimum, solar
imagery at moderate spatial resolution in a number of key wavelengths,
with a network of automated and carefully calibrated telescopes situated
so as to minimize gaps (synoptic observations from space would also be an
extremely attractive possibility). The time resolution of the observations
should be high enough to permit characterization of rapid fluctuations (e.g.,
flares and p-modes). Grants for theoretical work, to be carried out hand-
in-hand with the observational programs, should focus on solar interior
34
dynamics, including dynamo problems, which include the processes that
lead to the variability of the solar constant.
THE PHYSICS OF TRANSIENTS
The Basic Issues
A large solar flare releases as much as 1032 ergs in times as short
as 100 to 1000 s. Much of this energy appears in the form of high-
energy particles and hot plasma. It is believed that the flare energy comes
from the dissipation of- the nonpotential components of strong magnetic
fields~oronal current systems in the solar atmosphere, possibly through
magnetic reconnection, but the details of the energy release process as well
as the mechanism of particle acceleration are still only poorly understood.
The interactions in the solar atmosphere of accelerated electrons produce
radio emissions, and gamma-ray and hard X-ray bremsstrahlung, and the
interactions of protons and nuclei produce gamma-ray line and neutron
emissions. The combined observation of the time profiles of these emissions
is one of the best-known tools for studying the temporal development of
acceleration processes in astrophysics. For example, flares on the Sun are
one of the very few astrophysical sites where it has been possible to study
simultaneously the acceleration of electrons and protons. Solar flares are
also among the few astrophysical sites from which the escaping accelerated
particles can be directly detected. Furthermore, many closely correlated
lower-energy phenomena (soft X-ray, EUV, UV, and radio emissions),
some of which are the direct consequence of the interactions of accelerated
particles, can be observed as well. These lower-energy observations reveal
the properties (e.g., temperature, density, and magnetic configuration) of
the ambient plasma prior to, during, and after the flare.
The imaging of flares in hard X rays, the detection of gamma-ray
lines and continua from many flares, and the direct detection of solar
neutrons are the particularly significant results obtained from NASAs Solar
Maximum Mission satellite and the Japanese Hinotori satellites. It has
been known for some time that the hallmark of impulsive energy release
in flares is the acceleration of electrons to tens of keV, as evidenced by
hard X-ray emission. These nonrelativistic electrons probably contain a
large fraction of the total flare energy. The hard X-ray images have shown
that at least some of this energy is-deposited at the footpoints of magnetic
loops. The simultaneous brightening of distant footpoints suggests that
energy released in the loops is transported to the footpoints by electron
beams. The observation of impulsive gamma-ray emission from many flares
has shown that the acceleration of protons and relativistic electrons is also
a common properly of the impulsive energy release. The gamma-ray and
35
neutron observations have provided independent (albeit indirect) evidence
that the accelerated particles interact at the footpoints. Both the hard X-ray
and gamma-ray observations show that the acceleration is very impulsive.
Gamma-ray observations have demonstrated that protons are accelerated
to GeV energies and electrons to energies of tens of MeV in less than a few
seconds. Furthermore, the acceleration of the protons and the electrons is
practically simultaneous.
Particles in solar flares could be accelerated by shocks, turbulence,
and large-scale electric fields, as well as by a variety of other possible
mechanisms. Although it is quite clear that acceleration phenomena on
the Sun occur -at many sites and produce particle populations of widely
different observational characteristics, it is not known whether a single
mechanism is responsible for all of the observed acceleration phenomena
or whether different mechanisms operate at different sites. Furthermore, it
is not known whether particles are accelerated directly from the ambient
plasma by a single mechanism or whether the particles are preaccelerated
in a process that is distinct from the acceleration mechanism. And perhaps
most importantly, it is not clear at all how any of the above mentioned
mechanisms can accomplish the very rapid and efficient acceleration that is
indicated by the observations.
Particle transport in magnetic flare loops is an interesting and exciting
problem. The preferential detection of high-energy gamma-ray continua
from flares close to the solar limb suggests that the angular distribution
of relativistic electrons in the interaction region is anisotropic, peaking at
directions tangential to the photosphere. The required anisotropy could
result from magnetic mirroring and losses in a convergent chromospheric
magnetic flux tube, provided that the awns of the tube is perpendicular to
the photosphere. There is as yet no information on the angular distribution
of the ions. Such information could be obtained by observing the shapes
of gamma-ray lines with detectors having good energy resolution and by
observing neutrons from flares at different locations on the Sun. Many of
these observations remain to be carried out.
Abundances
Observations of X-ray and gamma-ray lines from solar flares have pro-
vided new techniques for determining abundances in the solar atmosphere.
The time-dependent flux of the 2.223-MeV line can provide information on
the abundance of 3He in the photosphere, where it has not been obtained
by any other method. This line results from neutron capture of hydrogen
in the photosphere; 3He is an important neutron sink, and therefore the
observed 2.223-MeV line intensity and time profile depend on the 3He/iH
ratio in the photosphere.
36
Nuclear deexcitation line fluences are directly proportional to the
abundance of elements in the interaction region of the accelerated particles.
This region is most likely located at chromospheric densities in flare loops.
The abundances obtained from nuclear line spectroscopy can be compared
with abundances obtained by atomic spectroscopy of the photosphere and
corona. The results indicate that the abundances of carbon and oxygen
relative to magnesium, silicon, and iron, as derived from the gamma-
ray data, are suppressed in comparison with the corresponding ratios in
the Photos there. It has been suggested that this suppression could be
due to charge-dependent mass transport from the photosphere to the
chromosphere. In addition to these elements, the abundance of neon has
also been determined by gamma-ray spectroscopy. A surprising result Is
that the neon/oxygen ratio deduced from the gamma-ray data is significantly
higher than the corresponding ratio in the corona or in the solar wind. The
origin of this difference is not yet understood. It should be noted, however,
that the photospheric neon abundance has not yet been measured.
Ra~liophysics and Plasma Dynamics
Electromagnetic radiation in the vast domain of radio astronomy has
demonstrated great potential for diagnostic characterizing of solar struc-
tures and dynamics, especially in the corona. In addition to pure elec-
tromagnetic waves, the corona generates several other types of radiation,
including hydromagnetic waves, Langmuir waves, and whistler waves. These
radiations, although not propagating to Earth, still have important roles
in energy transport and possibly in particle acceleration for many of the
phenomena of solar activity.
The electromagnetic radiation sources include continua from the free-
free, free-bound, gyroresonance, and synchrotron mechanisms; in addition,
there may also be weak emission lines formed by upper-level transitions in
hydrogen or other ions. The gyroresonance and synchrotron mechanisms
exhibit strong dependence on the magnetic field intensity and orienta-
tion; in general, at centimeter and longer wavelengths the corona may
become optically thick during flaring. For millimeter waves, unity optical
depth occurs in the upper photosphere in normal free-free opacity. The
submillimeter-far infrared spectrum then scans through the photospheric
layers down to the opacity minimum at about 1.6 microns.
Plasma waves in general have a much more complex physics and may
serve to couple particles and electromagnetic waves in coronal processes
such as the Type I-V radio bursts obseIved in meter-wave dynamic spectra
of the Sun. Further, masering cyclotron waves driven by anisotropic distri-
bution functions have been implicated in flare energetics. In general, the
numerous plasma wave modes provide a link to the distribution functions of
37
energetic particles, one of the key links between laboratory plasma physics
and astrophysical plasma applications.
Coronal Dynamics
The solar corona displays a wide variebr of transient phenomena, in-
cluding the radio bursts mentioned above. These constitute some of the
most dramatic forms of solar activity and have yet-unresolved associa-
tions with the physics of classical solar flares. In some cases, a powerful
flare will follow a white-light coronal transient and produce a clearly de-
fined blast wave that propagates into the interplanetary medium. In other
cases, transient phenomena occur in the corona without any soft X-ray or
H-alpha manifestation at all and sometimes from solar latitudes far above
the sunspot zones. Current observations are extremely deficient in both
synoptic and diagnostic leverage on these coronal phenomena, which is
unfortunate because of the rich physics they could reveal. This physics all
occurs in a region that is transparent to the observer, since the corona is
optically thin. Thus we are not afflicted with the subtleties of radiative
transfer theory; on the other hand, stereoscopic observations capable of to-
mographic reconstructions of the full three-dimensional geometry of these
regions are quite feasible in principle.
The coupling of more ordinary flares to the corona also remains a
frontier research area. We understand approximately what happens in
closed magnetic loops, with powerful energy release signaled by the hard
X-ray "impulsive phase," followed by the ablation of dense chromospheric
gas up into these loops. Less intelligible are the processes associated with
the open field lines known to exist from radio observations. Some flares,
often those relatively deficient in hard X-ray and gamma-ray production,
couple strongly into the interplanetary medium. Such flares are often
associated with one form of "solar cosmic ray" acceleration and coronal
transients responsible for geomagnetic perturbations. New observations
comparing white-light coronal data with impulsive X-ray signatures suggest
that the bunk coronal motions may often precede the impulsive burst, thus
raising the possibility that the origin of the flare resides in large-scale
magnetic field motions.
Initiatives and Impacts
Hard X rays and Gamma Rays
In the hard X-ray and gamma-ray area, observations with high spatial
and energy resolution are needed. Hard X-ray imaging spectroscopy of
sufficiently good sensitivity, energy coverage, and angular resolution will al-
low researchers to trace the evolution of the electron spectrum throughout
38
the source so that they can determine the accelerated electron distribution,
study the magnetic field geometry, and test theoretical transport models.
Hard X-ray and gamma-ray spectroscopy with the keV-energy resolution
now possible with high-purity germanium detectors will determine the
angular distribution of the accelerated ions and electrons, measure abun-
dances, and determine the temperature and density of the ambient plasma
in flares. There are no plans to carry out such high-resolution observa-
tions with satellite-borne detectors during the upcoming solar maximum.
Long-duration balloon flights therefore present an important alternative.
Neutron Monitors
Ground-based neutron monitors have turned out to be very useful for
observing the neutrons produced at the Sun by accelerated particle inter-
actions. Escaping neutrons have been directly detected from one flare by
several neutron monitors (e.g., monitors on Jungfraujoch in Switzerland).
Escaping neutrons were also detected by the Solar Minimum Mission satel-
lite; furthermore, the protons resulting from the decay of the neutrons
in interplanetary space were also observed. There are no U.S. plans to
operate neutron monitors during the upcoming solar maximum. However,
neutron monitors in Europe, the Soviet Union, Japan, and China will be
used for solar flare study during this solar maximum.
Radio Obse~vadons
Radio observations allow us to study the solar atmosphere from ap-
proximately the temperature minimum region out to 1 AU, roughly corre-
sponding to the wavelength range from 1 mm to 100 km. Observational
technique at these wavelengths has sharpened to the point that interfer-
ometry can produce images at milliarcsecond resolution on the Sun. One
milliarcsecond corresponds to only some 700 m!
At present, no large U.S. radio facilities are dedicated to solar obser-
vations, although glimpses with nonsolar instruments such as the very large
array (VLA) radio telescopes have produced wonderful results. These re-
sults, together with the innovative "frequency-agile" observations at Owens
Valley and U.S. and Japanese millimeter-wave work have shown that short-
wavelength radio astronomy should be considered seriously in the planning
of new observations both for the active and the quiet Sun. A strong case
can be made for a "mini-VLA" dedicated to frequency-agile microwave
observations at arcsecond resolution; such a facility would be well within
the technical state of the art.
MAX-91
MAX-91 is a coordinated program of great value for the study of so-
lar activity during the upcoming solar maximum. The core of the program
39
consists of Japan's Solar A satellite, NASAs Gamma Ray Observatory (with
limited solar observing time), and the Global Geospace Program's WIND
spacecraft. In addition, long-duration balloon flights will have a very impor-
tant role in this program. These flights could carry out the hard X-ray and
gamma-ray imaging and high-resolution spectroscopy observations that will
not be earned out by the satellite-borne instruments. In addition, a variety
of ground-based observations are planned to support these efforts. The sci-
entific objectives of the MAX-91 effort are the study of energy buildup and
flare onset and the characterization of energy release and transport mecha-
nisms. As indicated earlier, these issues involve fundamentally high-energy
phenomena intimately related to the problems of particle acceleration and
transport.
Fast Optical Spectroscopy
Flares, and perhaps other high-energy events, accelerate particles
downward into the chromosphere and lower regions of the solar atmo-
sphere with dramatic effect. Observations of these events provide important
information about energy and momentum balance in the flare process. The
physics of shock formation, explosive evaporation of the chromosphere, and
thermal conduction along magnetic flux tubes is vital to the flare process
but is still not well understood. If we had a more thorough understanding
of the physics of particle acceleration and propagation, we could interpret
the older, classic observations such as H-alpha images of flares in new and
more relevant ways.
The key observations are optical spectra of hare emission in Balmer
and other chromospheric lines with arcsecond spatial resolution and subsec-
ond time resolution. Only a few observations with relatively poor angular
resolution and especially poor time resolution have been obtained up to
the present time. These have been sufficient, nonetheless, to revolutionize
our understanding of the lower parts of flares. Many more observations
with better angular and temporal resolution are required. The develop-
ment of improved instruments to rapidly obtain optical spectra should be
supported. The impact of this initiative will be a better understanding of
the chromospheric and upper photospheric parts of flares, improved quan-
titative information about energy and momentum balance in flares, and
new diagnostics of thermal conduction along magnetic flux tubes.
Vector Magnetographs
It is evident that magnetic fields are responsible for nearly all solar
activity. As a result, a great deal of effort has been devoted to observing
magnetic fields in order to understand and predict solar activity. This effort
has centered largely on observation of the line-of-sight component of the
40
photospheric magnetic field because it is a fairly easy and robust measure-
ment to make. Such observations give only one component at one level
of a vector field that extends in three dimensions. Ideally, observations
that yield the full vector field measured in a three-dimensional volume as a
function of time are needed. This is a formidable observational task The
problem is complicated by requirements for excellent angular and spectral
resolution, high sensitivity to linear and circular polarization, and freedom
from polarization effects in telescopes. These requirements have frustrated
most earlier efforts to observe the vector magnetic field, in spite of the
importance of the task. Now we have improvements in detector technology,
new image stabilization techniques, and powerful analysis programs. Thus
the promise of obtaining useful vector field measurements, at least in the
photosphere, is brighter than ever before. This has led to development
activity at nearly every solar observatory. These activities should be sup-
ported, and the construction of a few of the most promising instruments
should be fully funded.
The impact of a successful vector magnetograph on the study of high-
energy solar phenomena will be profound. It will be possible to follow the
buildup and storage of magnetic energy, which is thought to power high-
energy events. It will also be possible to localize electric current systems,
which may trigger explosive energy release. These capabilities offer the
potential of predicting solar flare activity with far higher reliability than is
currently possible.'
Advanced Coronal Observations
As noted previously, it is now suspected that coronal transient'phe-
nomena result from the relaxation of stressed coronal magnetic fields.
The coupling between this relaxation and the occurrence of associated
solar flares or eruptive phenomena is currently unclear. Understanding
the transient process requires high-temporal-resolution observations of the
solar corona and a more precise understanding of the spatial and tem-
poral relationship between coronal activity and near-surface phenomena.
Current instrumentation is inadequate. Revolutionary new coronal obser-
vations will be carried out by instruments on board the SOHO spacecraft,
and it is within our technical capability to extend coronal remote-sensing
observations to 1 AU or beyond.
Limb observations of the innermost corona, with high spatial and spec-
tral resolution, can be obtained effectively with a ground-based
k-coronameter. A suitable k-coronameter will feature a sensitive system
capable of observing the corona to within 50,000 lan of the solar surface
with high spatial and temporal resolution, employing the polarization se-
lectivity necessary to observe the corona in the presence of sky light and
41
its fluctuations. Such an instrument is required to examine the nature of
the origin of coronal transient phenomena and the relation between tran-
sients and surface solar activity. The impact of these observations will be
to illuminate the nature of the evolution of the solar large-scale magnetic
field and the role of that evolution in generating solar activity.
