|
Items in cart [0] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 69
69
5. Air-Shower Observations
Two new installations, the Fly's Eye and the Homestake
underground detector, have recently become operational.
They offer the promise of significant advances in knowl-
edge of the composition and energy spectra of ultra-
high-energy cosmic rays. These measurements should be
continued. Additional surface arrays for measuring elec-
trons, muons, and hadrons may help in determining the
elemental composition of the primary particles, which is
a problem of fundamental importance.
VI I I . HIGH—ENERGY SOLAR ASTRONOMY
A. Introduction
Among all investigations in astrophysics the study of the
evolving and transient properties of the Sun has the most
direct and significant impact on our understanding of our
environment on Earth. Although the Sun emits only about
one millionth of its total luminosity in the form of
x- and gamma-ray photons and energetic particles, these
radiations carry much of the information we can obtain
about the complex process of the solar corona as well as
vital clues to the structure and dynamics of the under-
lying chromosphere and photosphere and the deeper, invis-
ible layers. Consequently, high-energy solar astronomy
has assumed a central role in the effort to understand
and predict the phenomena of the Sun on which the con-
ditions of life on Earth depend.
The Sun is the only star close enough to permit obser-
vation and measurement of its surface activity at all
wavelengths with spatial, temporal, and spectral resolu-
tions that could, in principle, reveal the finest signifi-
cant details. Moreover, the properties of matter ejected
from the corona as solar wind and energetic particles can
be determined directly by measurements in interplanetary
space. Thus, in the study of the Sun, astronomy can go
far beyond the limitations of other stellar observations,
in which the nature of stellar processes must be inferred
from analysis of spatially unresolved radiations and can
reveal in detail how an apparently typical main-sequence
dwarf star works. And in this study our observing sta-
tions can be located about as close as they can safely be
to a dense, hot cosmic plasma exhibiting many of the
phenomena of magnetohydrodynamics that must be understood
OCR for page 70
70
before other compact sources of x-ray photons and cosmic
rays, both Galactic and extragalactic, can be completely
comprehended.
The broad features of the Sun's structure are well
established. In particular the temperature profile of
the Sun along a radius from its center falls from an
estimated 15 X 106 kelvins in the core to a minimum
value near 4200 K at a height of 560 km above the photo-
spheric surface, the level at which the optical depth to
space is unity. Energy released by fusion of hydrogen
into helium in the core is transported outward by
radiative diffusion to the base of the convective zone,
which is believed to be at about 0.86 solar radius (1
solar radius is approximately 7 X 105 km). From there
the energy is carried primarily by turbulent convection
to the photosphere, the 400-km-thick layer from which
most of the solar power is radiated as visible photons
with a spectrum of which the continuum is like that of a
blackbody at 5770 K. Continuing outward through the
chromosphere, the temperature rises, at first gradually
to 9000 K at the top of the chromosphere, 2000 km above
the surface, then steeply to 105 K in a narrow transition
region with a radial thickness of only 700 km, and on to
values of several millions of degrees in the corona, which
extends to several solar radii during periods of moderate
solar activity. The solar spectrum is a composite of the
emissions from all the regions above the photospheric
surface.
Nonuniform rotation and convective motions of the
highly conductive material within the Sun generate mag-
netic flux, which is continually carried to the surface
by turbulent diffusion and magnetic buoyancy. From there
the field emerges into the solar atmosphere with a
strength that ranges from a few gauss in quiet regions to
100 gauss in active regions. Variations in this field,
caused by motions of its anchor points in the dense plasma
of the photosphere, give rise to a variety of phenomena
on size scales ranging from less than a few hundred kilo-
meters, the limit of currently available optical resolu-
tion, to millions of kilometers. The corresponding time
scales range from seconds in the case of explosive flares,
through days for major changes in specific active regions,
to years and decades for the general activity variations
that are caused by reconfigurations of the entire internal
solar magnetic field. On a scale of centuries the sunspot
activity cycle and the attendant coronal phenomena wax
and wane, sometimes ceasing almost completely for several
decades.
OCR for page 71
71
The corona is heated with energy supplied from the
interior region and transported across the cooler under-
layers against the temperature gradient by processes
other than diffusion or convection. It loses energy
steadily by emission of E W and soft x-ray photons, both
inward and outward by thermal conduction inward and by
expulsion of hot gas outward in the form of the solar
wind. The flux of this energy is small in comparison
with the luminosity of the photosphere. Nevertheless, the
coronal emission and the solar wind have profound effects
on the atmospheres and magnetospheres of the Earth and
other planets. The origin, transport, and conversion of
internal solar energy into coronal heat and kinetic energy
of the solar wind are central problems of solar astronomy.
With the recent discovery that hot coronas are present in
stars of all spectral types and nearly every luminosity
class the problem of coronal heating has become a general
problem of stellar astrophysics, and the results of
detailed studies of the solar corona have acquired a
broad significance for all of astronomy.
Solar flares are exDlORiOn-R in the mAan-hi~ nlA~mA
of the lower corona.
. .
~ ~ , . . _ _ ~ = ~
Their frequency and magnitude vary
with the sunspot activity that currently has two maxima
in a cycle of quasi-periodic magnetic field variations
with a period of about 22 years. Flares occur in the
vicinity of sunspots and last up to 20 min. They are
powered by 1028-1032 ergs of energy stored in unstable
configurations of the magnetic fields associated with
sunspots, and it is believed that they occur when this
energy is suddenly released by reconnection of field
lines at the boundaries of regions where they are oppo-
sitely directed. Flares result in acceleration of high-
energy particles, intense local heating of the chromo-
sphere, and in eruption of chromospheric material into
the corona. Their effects are manifest over the entire
electromagnetic spectrum from radio to hard x rays and
MeV gamma rays, in energetic electrons and nuclei of all
the elements in the corona, all ejected into interplane-
tary space, and detectable by suitable means. Undoubtedly
MeV neutrons are produced along with gamma rays when
energetic nuclei collide with coronal or photospheric
matter, but they mostly decay before they reach the
distance of the Earth. As the most readily observable
examples of the class of cosmic explosions resulting from
sudden conversion of magnetic energy stored at compara-
tively high concentrations into high energy particles and
heat, flares are of great interest both in themselves and
OCR for page 72
72
in the opportunity they present to develop a deeper under-
standing of plasma physics for application to other areas
of astronomy. The discovery of flares on other stars,
and the observation that some of them are enormously more
energetic than the largest solar flares, have given the
study of solar flares a broad significance for stellar
astronomy. Here again, as in the problem of stellar
coronal heating, systematic investigations of the cor-
relations between stellar flares and other stellar char-
acteristics have shed significant new light on the
mechanisms of flares on the Sun.
The focus of solar astronomy on the phenomena of a
single star, and the necessity of close coordination in
strategy and timing of solar observations over the entire
spectrum, make it desirable to consider the status and
future of high-energy observations in solar astronomy as
a whole rather than separately as parts of x-ray, E W.
gamma-ray, or cosmic-ray astronomy. Moreover, the bright-
ness of the Sun in the spectral range from the visible to
the x-ray region places special engineering requirements
on instrumentation that cannot be readily made compatible
with the requirements for extrasolar observations.
m e scientific status and future scientific objectives
of solar astronomy are discussed in detail in the report
of the Astronomy Survey Committee's Working Group on Solar
Physics. We present here only a brief summary of recent
progress and future prospects in high energy solar astron-
omy as background for our discussion of new programs.
Solar neutrino observations are considered separately in
the section on neutrino astronomy.
B. Progress during the 1970's
1. General Features of the Solar Atmosphere
Among the most important developments in understanding
the Sun's atmosphere during the decade of the 1970's was
the discovery that it is divided into regions of two
distinct kinds that are either magnetically "open" or
magnetically "closed." In closed regions the field lines
have the form of loops with both ends anchored in the
dense plasma of the photosphere; in open regions the
field lines are anchored at one end in the photosphere
and stream outward into interplanetary space.
Closed regions occupy typically 80 to 90 percent of
the solar surface. Magnetic force confines the plasma in
OCR for page 73
73
the inner corona in these regions to motions along the
loops. Maintained at comparatively high density by
magnetic confinement, the plasma is heated to tempera-
tures of millions of degrees by processes that derive
their energy from the bulk motions in the photosphere and
deeper layers. The relatively high density and high
temperature cause the plasma to be highly luminous in x
rays so that the closed regions appear in x-ray images as
bright arches of various sizes that trace the loop struc-
ture of the confining magnetic field.
The open regions are relatively low in density and
temperature and are correspondingly low in x-ray emis-
sivity. In x-ray images they appear as dark regions
called coronal holes. Spewing forth from these magneti-
cally open regions, which occupy only 10 to 20 percent of
the solar surface, coronal plasma spreads as it leaves
the Sun and within a few solar radii is moving radially
away over the entire solid angle to form the bulk of the
solar wind. Adjacent solar-wind streams from different
coronal holes may have magnetic fields with opposite
directions so that current sheets form on their bound-
aries, resulting in a sector structure of the solar wind
that has been measured by plasma detectors and magneto-
meters on satellites and space probes.
Coronal holes, first observed in rocket x-ray photo-
graphs, were studied in detail with instruments on the
Apollo Telescope Mount (ATM) of the Skylab mission in
1973. Thousands of high-resolution x-ray and W images
as well as x-ray and W spectra were recorded during a
period of many months when the Sun was in the low part of
its activity cycle. Many correlated ground-based obser-
vations were also made. Much still remains to be done
before the potential scientific value of the data archives
from Skylab and the correlated studies is fully realized.
X-ray bright points, discovered in the ATM x-ray
observations, are very small transient regions of intense
x-ray emission in the lower corona that are scattered
over the Sun's surface in both magnetically open and
closed regions. They subtend angles of only a few
arcseconds or less and last for several hours. Their
nature and cause are not well understood, though they
appear to be magnetic loops over miniature bipolar
features with diameters of the order of a few hundred
kilometers.
Ground-based spectrographic studies carried out with
very high angular resolution revealed a remarkable
property of the Sun's magnetic field in the photosphere
OCR for page 74
74
where the foot points of the coronal loops and the open
lines are anchored. In traversing the photosphere the
magnetic field is concentrated in isolated, widely
separated flux tubes with radii of the order of 100 km.
In these tubes the field intensity is of the order of
103 gauss, a statically unstable condition that is
apparently maintained by processes associated with the
turbulent convective motions.
Two central problems concerning the Sun's corona,
previously thought to have been solved, were cast in a
radically new light by high-energy observations during
the 1970's, and the inadequacies of the prevailing
theories were revealed. One of these problems is how th
corona is heated. In 1970 it was generally believed that
acoustic waves, generated in the turbulent convective
zone, traversed the chromosphere and transition zone and
dissipated their energy in the corona. ~ ~
On the basis of
this theory it was believed that stars of spectral type A
or earlier, being nonconnective, would not have coronas.
Then E W observations of the Sun by OS0-8 demonstrated
that the energy flux in such waves is inadequate by an
order of magnitude to supply the power radiated by the
corona. Ground-based optical observations and the IUE
found evidence for photoionized gas in the winds of O
stars, which do not have convective zones. And satellite
x-ray observations beginning with SAS-3 and culminating
with HEAD-1 and the Einstein x-ray observatory demon-
strated that million-degree, x-ray emitting coronas,
which may account for the photoionization in O stars, are
found among stars of all spectral types, including, in
particular, at' types ot nonconvect~ve stars. Emus the
acoustic wave theory of coronal heating was proved to be
inadequate to account for the Sun's corona and failed to
predict the coronal properties of other stars. New clues
to the nature of the heating mechanisms have since been
found in a correlation between stellar rotation and
coronal x-ray luminosities in late-type stars.
The second central problem of the corona is how it
gives rise to the solar wind.
Here, again, the conven-
tional view of the wind as a free expansion of plasma
heated by thermal conduction was shattered by the dis-
covery that the solar wind originates primarily in the
coronal holes. The thermal conduction mechanism was
already strained by an observed lack of an adequate
temperature gradient to sustain the energy flux, averaged
over the solar surface, that appears in the kinetic
energy of the wind. It failed by more than an order of
OCR for page 75
75
magnitude when confronted by the demands on the local
energy flux that are imposed by the fact that most of the
power of the solar wind comes from coronal holes that
occupy less than 20 percent of the total surface area.
2. Transient Events
Rapid changes in the magnetic field, such as occur in
flares, generate intense electric fields, which accel-
erate particles to suprathermal energies. These par-
ticles, confined to motion along magnetic field lines,
deposit a portion of their energy in Coulomb collisions
and thereby heat the plasma. In rarified regions the
energetic electrons emit synchrotron radiation, and some
particles may escape along open field lines into inter-
planetary space as solar cosmic rays. High-energy solar
charged particles accelerated in flares to several hundred
MeV were observed at Earth in the 1950's as well as bursts
of synchrotron radiation attributable to intense fluxes
of high energy electrons moving outward through the
corona. The Orbiting Solar Observatories (OSO's) of the
1960's and various rocket experiments recorded detailed
high-resolution spectra of the soft x rays emitted by
chromospheric plasma heated to temperatures of the order
of 107 K, apparently by the impact of flare-accelerated
particles traveling downward. Observations were also
made of hard x rays generated directly by the impact of
high-energy flare-produced electrons on the chromospheric
plasma. During the 1970's such observations were greatly
extended and refined by the ATM mission, which recorded
high-resolution images of the W and x-ray phenomena
associated with flares. Substantial progress was made in
relating features of the x-ray images to details of the
magnetic-field configuration and in utilizing W and x-ray
spectral diagnostics to determine the properties of the
flare-heated chromospheric plasma.
In many flares, protons and heavier ions are accel-
erated to high enough energies to produce gamma rays and
neutrons in nuclear collisions. The first astronomical
observations of nuclear gamma-ray line emission was
achieved by a detector on the OS0-7 in observations of
such solar flares, and further observations have been
made by HEAD-3 and the Solar Maximum Mission (SMM).
Gamma-ray observations made on the SMM satellite, with a
highly sensitive spectrometer, have shown that many
flares emit detectable gamma-ray line fluxes and that the
OCR for page 76
76
expected neutron emissions may also have been detected
following a giant limb flare in 1980. These results
opened a new approach to the study of the composition of
the solar atmosphere, which previously depended entirely
on direct measurements of the elemental and isotopic
composition of the solar wind and flare-accelerated
nuclei.
Observation of the solar cosmic rays have revealed
many cases of an abundance of 3He/4He, which is enhanced
by a factor of 103-104 above the solar abundance ratio.
This effect is not likely to be the result of spallation
of 4He since the observations are not accompanied by high
abundances of the expected companion spallation products
2H and 3H. The 3He-rich flare effect is likely due to a
selective plasma preheating or injection mechanism.
Events with 3He/4He enrichment may also be accompanied by
an enrichment of energetic Fe nuclei. mese and other
unexpected results from the study of the nuclear radiation
from solar flares, though not yet understood, illustrate
how the study of elemental and isotopic abundances in
solar cosmic rays yields unique information about the
conditions in the solar atmosphere and about the mech-
anisms of particle acceleration. It is expected that
correlation of the observations of particles and gamma
rays by the SMM will greatly clarify our understanding of
the high-energy aspects of solar flares.
3. Long-Term Variability
Historical research during the 1970's showed that the Sun
has been inactive--without spots or an extended corona--
for long periods of which the most recent was the last
half of the seventeenth century. The absence of solar
activity is correlated with periods of cold in the Earth's
climate. Thus the study of the nature and origin of solar
activity, and of the causes of long-term solar variabil-
ity, has significance not only for our understanding of
the structure and evolution of convective stars but also
for our understanding the evolution of life and human
society on Earth.
The solar output of high-energy photons and particles
changes dramatically with the number of Sun spots because
the latter, being major eruptions of magnetic flux gen-
erated inside the Sun, are conspicuous symptoms of the
conditions that produce magnetic variations and instabil-
ities in the solar atmosphere and consequent particle
OCR for page 77
77
acceleration and coronal heating. m e discovery of
coronal x rays from many nearby stars during the 1970's
has made it possible to undertake comparative studies of
how coronal activity is related to stellar structure and
rotation.
C. Scientific Objectives of High-Energy Solar Astronomy
Efficient progress in solar astronomy requires a broad
program of coordinated observations in many wavelength
bands as well as in situ measurements of particles and
fields within the heliosphere. Although we focus here on
the objectives for which high-energy observations are
specially important, we wish to stress the need for cor-
relative studies in the optical, W. and radio portions
of the spectrum.
The following questions define major problems of solar
astronomy that require high-energy observations:
1. What processes determine the structure and
dynamics of the solar chromosphere, corona, and solar
wind?
2. In solar flares and other transient solar phenom-
ena, what are the mechanisms of energy storage and
release, particle acceleration, and local heating?
3. How does the Sun influence the structure and
dynamics of the interplanetary medium?
4. What is the mechanism of the solar activity cycle,
and how can its future behavior be predicted?
5. Are the relative abundances of isotopes and
elements different in the various regions of the atmo-
sphere, and if so, what are the mechanisms of their
separation?
Within this broad context the specific objectives of high
energy observations during the next decade will include:
1. Detailed studies of the E W. x-ray, gamma-ray, and
neutron emissions from the solar atmosphere with the
highest attainable spatial and spectral resolutions;
2. Coordinated three-dimensional x-ray studies of
coronal structure and activity and simultaneous In situ
three-dimensional measurements of high-energy particles
in the interplanetary medium;
3. Synoptic studies of the long-term variations in
coronal activity and its effects on high-energy particles
OCR for page 78
78
in the solar system together with comparative analysis of
variations in coronal activity in other stars.
4. Observations of solar high-energy nuclei, elec-
trons, neutrons, plasma, and magnetic fields from
satellites and space probes.
These studies will be almost entirely dependent on obser-
vations from space vehicles, though new information about
particle acceleration in solar flares and the effects of
solar plasma on the flux of cosmic rays over long times
and at very high energies can be derived from ground-
based monitors that detect the secondary neutrons produced
by high-energy charged particles in the atmosphere.
D. Inventory of Present or Approved Resources
The inventory of resources for high-energy solar astronomy
must begin with the archives of data obtained by Skylab,
the OSO's, the SMM, and from such interdisciplinary mis-
sions as ISEE, IMP-8, and other deep-space probes (e.g.,
the Pioneers and Voyagers) that contained particle-and-
field experiments. These provide a rich data base for
the study of many aspects of solar physics. Much analyti-
cal and theoretical work remains to be done before the
scientific value of these archives is exhausted. It is
therefore essential that relevant studies based on these
archives be adequately supported.
A new facility in an advanced stage of development is
the International Solar Polar Mission (ISPM), which is
designed to obtain, for the first time, x-ray and E W
images of the solar atmosphere from above the Sun's poles.
The ISPM will also contain particle-and-field experiments.
With two spacecraft the mission would obtain critically
important "stereoscopic" observations of the optically
thin loop structures that appear to be the dominant
structural feature of the high-temperature corona.
Two solar E W experiments have been approved for
Shuttle missions. One is a stigmatic E W spectrohelio-
graph for study of the solar transition region. The
second is an E W resonance-line chronograph, which will
be used in combination with a conventional white-light
coronagraph for measuring the velocity, temperature, and
density of the corona within S AU of the Sun.
OCR for page 79
79
E. Capabilities of Present or Approved Resources
The SMM achieved major advances in the observation of
hard x rays and gamma rays associated with solar flares.
It is clear, nevertheless, that the solution of fundamen-
tal problems concerning the processes of coronal heating
and the phenomena of flares and other transient phenomena
will require facility-class instruments capable of spec-
trophotometric and line-profile studies with angular
resolutions that match the finest details of significant
structures that are known to exist, namely, 1 arcsec or
less. Such instruments will represent a considerable
advance over the capabilities of the pointed SMM
instruments.
m e ISPM will have only modest angular (approximately
4 arcsec) and spectral resolutions owing to the limits of
size and weight for interplanetary missions. Simultaneous
observations by an x-ray (X W) telescope in Earth orbit
would greatly enhance the scientific value of the ISPM
data.
Several experiments on the variation of the solar flux
at a number of wavelengths have been approved for Space-
lab. However, a comprehensive program for the study of
solar variability should be undertaken on a time scale
exceeding at least the fundamental 22-year cycle.
NASA supports basic theoretical studies in solar plasma
physics, which are essential to the interpretation of
observations and to the planning of future missions.
F. New Facilities and Programs for the 1980's
Recent advances in the techniques of high-energy observa-
tions have opened a broad range of opportunities for new
investigations that would assure a continuation of the
rapid growth in our understanding of the high-energy
phenomena of the Sun. Among these advances are improve-
ments in x-ray telescopes that make it possible to record
spectrally resolved soft x-ray images with angular resolu-
tions finer than 1 arcsec; coded aperture detectors for
hard x-ray image detection with resolutions of the order
of arcseconds; high spectral and spatial resolution x-ray
and E W spectrometers; highly sensitive scintillation
spectrometers and high-resolution nuclear gamma-ray solid-
state spectrometers; and cosmic-ray detectors capable of
resolving the isotopes of all the elements up to nickel
and the element distribution for Z up through 26. These
OCR for page 80
80
and other advanced technical capabilities would be
employed in the new programs and facilities of which the
following is a summary.
1. Shuttle Facilities
A comprehensive Solar Shuttle Observatory should be
developed to encompass a number of Principal Investigator
and/or facility-class instruments that provide major
improvements in angular resolution, spectral-resolving
power, and sensitivity over previous instruments. The
first component of this Observatory, the Solar Optical
Telescope (SOT) is already approved for construction.
The principal components required for high-energy
observations are the following:
Solar Soft X-Ray Telescope Facility (SSXTF): A large
soft x-ray telescope with interchangeable focal-plane
instruments that include high-resolution image detectors
and spectrographs. The SSXTF will be an 0.8-m nested
Wolter Type I telescope with 1200 cm2 effective collect-
ing area over most of the wavelength range from 1.7 to
300 A, and with an angular resolution of 0.5 arcsec.
Major emphasis will be placed on high-resolution spec-
troscopy in the focal-plane instrumentation.
~Pinhole" Telescope/Occulter: A coded-aperature
telescope employing a remote mask mounted on an extended
boom will be used in conjunction with a position-sensitive
detector to record images of hard x-ray events associated
with flares and other impulsive solar phenomena. The pin-
hole telescope will achieve subarcsec angular resolution
over the energy range from about 1 to 100 keV, with moder-
ate spectral resolution. The mask will also serve as an
occulting disk for coronal observations in the visible to
E W regions of the spectrum.
Grazing-Incidence Solar Telescope (GRIST): An X W
facility that may be developed by the European Space
Agency. Interchangeable focal-plane instruments would
include high-resolution image detectors and spectro-
graphs. The GRIST will provide (1) an order-of-magnitude
improvement in sensitivity compared with Skylab, (2) high
time resolution for the study of transient phenomena, and
(3) improved angular resolution (0.5-1.0 arcsec) necessary
for the study of fine structures known to be present in
the transition region. Spectroscopic observations with
the capability of measuring intrinsic line profiles will
OCR for page 81
81
be the major emphasis of the scientific program of the
GRIST.
Solar Shuttle E W Facility (SSEF): A normal-incidence
telescope of modest aperture but with a high-reflectivity
mirror for use in the spectral region from 360 to 1500
A. It will provide ultra-high angular resolution at
wavelengths shorter than can be efficiently observed with
the SOT. The E W facility will consist of a normal-
incidence telescope feeding stigmatic E W spectrohelio-
graphs operating between about 360 and 1500 A. The
design objective for spatial resolution is 0.1 arcsec and
that for spectral resolution is between 103 and 105.
This instrument will complement the SOT by extending
ultra-high-resolution observations to the spectral region
of radiation characteristic of temperatures as high as
2 X 106 K. It is intended to achieve an order-of-
magnitude improvement in angular resolution and spectral
resolution compared with any currently approved instru-
ment capable of observing material on the solar surface
hotter than 2 X 105 K.
Gamma-Ray Spectroscopy Facility (GRSF): A high-
. . .
resolution and high-sensitivity gamma-ray spectrometer
for the study of nuclear gamma-ray lines. Both scintil-
lation and solid-state detectors are needed to achieve
the desired performance over the entire range of energies.
This instrument and the "Pinhole" hard x-ray telescope
may also be useful in the study of transient phenomena in
extrasolar sources.
2. Solar Coronal Explorer
Investigations of the large-scale structure of the inner
heliosphere, begun on Skylab, would be extended with an
Explorer-class satellite carrying an x-ray/X W telescope
having an angular resolution finer than 1 arcsec and with
white-light and resonance-line coronagraphs. The purpose
of such a mission would be to measure the density,
composition, and flow velocities in the corona and inner
heliosphere and to explore the relations of these parame-
ters to the structure of the lower atmosphere. It would
also facilitate detailed investigations of the dynamics
of the coronal transients, which have an important effect
on the terrestrial environment.
OCR for page 82
82
3. Interplanetary Laboratory (IPL)
The proposed IPL Satellite in the Origin of Plasma in the
Earth's Neighborhood (OPEN) series of satellite missions
will carry advanced instrumentation for solar plasma
measurements, magnetic fields, and instrumentation to
measure the isotopes, especially the rare isotopes, of
elements ejected from solar flares.
4. Advanced Solar Observatory
m e Skylab results demonstrated the value of simultaneous
observations over a range of wavelengths with high spec-
tral and spatial resolutions over long periods of time.
The Skylab experience, together with the Shuttle solar
program, will provide a strong basis for development of
the central element of the strategy for solar astronomy
in the 1980's, namely the Advanced Solar Observatory
(ASO), which will contain facility-class instruments
operating with subarcsecond resolution at optical, X W.
soft x-ray, and hard x-ray wavelengths, together with
gamma-ray and neutron detectors with the highest attain-
able angular resolution. The ASO will be a long-lived
facility designed to be refurbished in orbit and periodi-
cally brought back to Earth for overhaul and replacement
of instruments. Since important solar problems can be
addressed effectively with facility-class instruments on
Shuttle sorties, the development of the instrument package
can be carried out stepwise by development and use of
individual facility-class Shuttle sortie instruments,
which will ultimately be integrated into the ASO, pos-
sibly aboard a Scientific Space Platform.
The x-ray components of the ASO will be the SSXTF and
the Pinhole telescope. The X W components of the ASO
will be the GRIST and the SSEF telescope facility. For
gamma-ray observations, the ASO will either incorporate
the GRSF or carry out studies in coordination with the
Gamma- Ray Transient Explorer.
5. Other Missions and Programs of Significance to Solar
Physics
The solar atmosphere and the interplanetary medium will
be studied by the Star Probe, which will approach close
to the Sun to measure directly conditions inside the
OCR for page 83
83
region of solar-wind acceleration and to carry out
ultra-high-resolution optical and x-ray observations.
Both gamma-ray and neutron measurements could, in
principle, be made on the Star Probe. The latter measure-
ments must be made as close to the Sun as possible to
enhance the low-energy neutron flux, which is greatly
depleted by decay. These combined measurements, which
are highly desirable at the next solar maximum, can
establish the spectral shape of both the energetic par-
ticles accelerated at the Sun and the products of their
nuclear interactions with solar matter (e.g., gamma rays
and neutrons). An alternative mission to accomplish this
important solar objective would a lightweight Mercury
orbiter. This mission would have the advantages not only
of long-term observations but would make possible the
observation of solar neutrons in the important low-energy
range before they decay and simultaneous gamma-ray
observations.
An important next step in high-energy stellar astronomy
will be detailed spectroscopic studies of stellar x-ray
emissions and their relations to surface temperature,
gravity, rotation, and magnetic fields. Such studies
will provide critical new tests of theories of stellar
activity and will therefore have important indirect
benefits for solar physics.
A strong program of supporting research and technology
will continue to be essential to the continued vitality
and health of solar physics. In this connection we note
that Shuttle-launched "Experiments of Opportunity," which
extend the observation time available to "rocket" payloads
to 24 hours, and longer-duration balloon flights offer
promising prospects for testing new instrumentation con-
cepts and for exploratory research. And we emphasize
once more that extended postmission data analysis and
exhaustive study of existing archival data must be
adequately supported in order to derive the full
scientific value from the missions that have been flown.
G. Summary and Principal Recommendations
High-energy solar astronomy has achieved fundamental
advances in our understanding of the Sun's outer atmo-
sphere. At present, high-energy solar astronomy stands
at the threshold of major new developments based on obser-
vation and analysis of the microstructure of the processes
that heat the corona and cause flares and other transient
Representative terms from entire chapter:
solar wind
