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OCR for page 112
Appendix D
Probing Fundamental Astrophysical
Scales with High-Resolution
Observations of the Sun:
Prospects for the
Twenty-first Century
INTRODUCTION
The past decouple has seen a great increase in the sophistica-
tion with which we are able to confront the physics of the Sun.
Physical theories have progressed from those that assume a simply
stratified, equilibrium atmosphere overlying a classical convection
zone, to those that recognize intermittent magnetic fields in the
convection zone and dynamical structures on Al spatial scales
throughout the atmosphere. In such situations, it Is possible that
there is no static equilibrium structure at ah. Furthermore, it is
believed that all of the observed structures in the Sun, even the
largest, are ultimately governed by small-scale processes associ-
ated with intermittent magnetic fields or turbulent stresses. For
example, electric currents on scales of order 10 km or less may well
be the fundamental entity giving rise to coronal heating.
Understanding the physics of the creation and decay of such
small-scale currents and their effects on mass and energy transport
is thus essential to a proper description of large-scale structures
such as coronal active regions, loops, flares, or mass loss in the
solar wind.
The interplay between processes occurring on vastly differ-
ent spatial scales is ubiquitous in astrophysics and heliospheric
112
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113
physics. Small-scale magnetohydrodynam~c turbulence ~ thought
to govern the accretion rate of accretion disks feeding compact
galactic x-ray sources and black holes in active galactic nuclei;
local instabilities such as Keivm-Helmholtz modes are thought to
control the coupling between the magnetospheres of neutron stars
and the surrounding matter. MagnetoLydrodynam~c turbulence
is also believed to suppress efficient heat conduction in the hot
halos of galaxies and galaxy clusters, thus controlling the rate of
accretion in these halos. Further, the same small-scale processes
that heat the solar corona are undoubtedly at work in the coronae
that are now known to surround many stars.
In all these cases, observations using even the most advanced
technology currently conceivable will not allow us to observe di-
rectly the controlling small-scale phenomena. In the case of the
Sun, however, we can indeed contemplate direct observations. The
Sun is therefore a unique too} for understanding a wide range of
astrophysical objects, by virtue of the opportunity it affords to
observe the underlying physical processes in some detail.
In this study, the workshop participants examine the scientific
rationale and technological bash for pushing the study of solar
magnetohydrodynamic processes well beyond the regime antici-
pated from solar space missions planned for the coming decade
(~100 km on the Sun, or ~0.! arceec at ~ AU). First, the scientific
issues involved are addressed, and it is concluded that observations
of solar structure on a spatial scale in some cases as small as 1
km would provide an enormous increase of our knowledge of basic
astrophysics. Next, practical limits imposed by the emitted flux
and the superposition of separate structures along the line of sight
are discussed. Finally, the workshop participants addre" some of
the technological challenges to be met if the objectives are to be
attainable early in the twenty-first century.
In this study, the workshop participants have adopted as a
baseline assumption and prerequisite the existence of a program
(broadly speaking, the Advanced Solar Observatory) that will,
over the next decade, result In the observational study of the
structure and dynamics of the solar atmosphere at a resolution
of about 100 km, in visible, ultraviolet, and x-ray wavelengths.
Thus, the welI-establ~shed rationale for observations at these spa-
tial scales is not discussed in any detail.
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114
TlIE ROLE OF SMALI-SCA[E PROCESSES IN
THE SO[AIt ATMOSPHERE
The solar atmosphere exhibits a vast range of spatial and
temporal scales, from coronal eruptions as large as the Sun itself,
to flare-related instabilities that involve scales of centimeters and
micro-seconds. Moreover, there is mounting evidence that the
large and small scales are inseparably linked, such that neither is
purely cause or effect; it is the combination that produces what
we observe. For example, very small-scale reconnection events in
the corona may suffice to destabilize a small portion of a highly
stressed magnetic arcade. The dynamic restructuring of the field,
and the fluid flows that go with it, may in turn drive further
reconnection and further instability, until the entire arcade blows
oE as a coronal mass ejection. In order to unravel such a synergistic
connection between large and small scales, we need to explore the
intermediate range of scales.
Some of the basic spatial scales of the solar plasma, such as
the ion gyroradius or the Debye length, typically fall in a range (on
the order of centimeters) that is not amenable to remote sensing
and Is unlikely to be explored in the forseeable future. However,
important physical processes can be studied on larger scales that
are still far below the current Innits of observation.
Solar activity and flares are prime examples. Present models
of the site of primary energy release and particle acceleration all
involve mechanisms—e.g., magnetic reconnection, plasma double
layers, strong shock ~raves, anomalous current dissipation that
require or generate gradients in magnetic field, temperature, pres-
sure, and velocity on scales in the range 0.1 to 10 km. We already
know, from indirect arguments, that flare kernels are inhomoge-
neous on scales of <100 km. Observations with spatial resolution
in the range 1 to 100 km thus offer our best hope for penetrating
to the heart of solar flares.
There is now good evidence that much of the flare energy ~
channeled into the production of beams of nonthermal electrons
and that the deposition of such beams in the chromosphere is
the principal means for creating the thermal flare plasma. The
detection of linearly polarized bremsstrahinng from the impact
of an electron beam would constitute a direct signature of this
process. Past attempts have been frustrated by the low efficiency
of soft x-ray polarimeters and especially by a complete lack of
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115
spatial resolution, which dilutes the signal and makes it difficult to
disentangle it from instrumental effects. If technological advances
make possible an efficient polarimeter with high spatial resolution,
it would be possible not only to demonstrate unequivocally the
existence of electron beams, but also to trace the beam (or beamed
to the site of primary energy release and to flare kerned in the
lower atmosphere.
Coronal loops, a fundamental building block of the solar atmo-
sphere, are now recognized to be fundamental to the understanding
of stellar coronae and stellar atmospheric activity In general. A
decade of intensive work has demonstrated that the gross proper-
ties of a loop (e.g., temperature, pressure, magnetic field strength)
do not allow us to decide whether it will be stable, let alone how it
is heated. We need to know the internal structure of the loop. For
example, what are the temperature and density profiles transverse
to the major axis of the loop? If the magnetic field ~ smooth,
add if only classical cornfield transport processes are at work,
flux tubes separated by only ~ to 10 km can have widely different
temperatures and pressures. If snowfield transport ~ enhanced
(due, for example, to a drift-wave instability), the characteristic
scale of the gradients may expand to 10 to 100 km. This first of
Al affects a directly observable quantity, the differential emission
measure, but it also bears on the structure of the loop as a whole.
Transport processes in a loop are only one aspect of an even
more basic question, the organization of the magnetic field. For
example, some theories of coronal heating postulate that intense
electric currents flow along coronal loops, and intense currents are
associated with smaD-scale twists in the magnetic field. To study
the internal magnetic structure of a loop, either by the use of
tracers or by direct measurement, we will require at least leant
perhaps as many as 10~resolution elements across the minor
axe, which extends a few arceeconde.
The structure of magnet+fluid turbulence in a gravitationally
bound system ~ a basic astrophysical problem that can probably
be investigated nowhere but in the Sun. Aside from its intrinsic
interest, turbulence in the photosphere concentrates and disperses
the magnetic field according to the (unknown) power spectrum of
the turbulence, and the photospheric magnetic field is the starting
point for creating or maintaining fine structure at higher leveb
in the atmosphere. Existing observations and theory already sug-
gest that most of the magnetic field in the photosphere exists as
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116
discrete flux tubes with characteristic diameters of <500 km. Cur-
rently planned observations with lO~km spatial resolution should
confirm or deny the existence of such flux tubes beyond doubt.
However, as in the case of coronal loops, we win almost certainly
need to resolve the internal structure of the tubes before we can
understand the physical basis for their formation and persistence.
The value of studying small-scale processes in the solar atmm
sphere can be illustrated by considering the development of our
understanding of the large-scale structure and dynamics of the
terrestrial magnetosphere, where we already know much about
the linkage between large- and small-scale processes. The early
observational and theoretical studies of the magnetosphere dealt
primarily with large-scale phenomena, and these studies led to a
good understanding of the gross, average properties of the mag-
netosphere. It was found, however, that a sound understanding of
the large-scale mass structure, electric current structure, and tem-
poral evolution of the magnetosphere required the detailed study
of microscopic plasma processes. Several examples come readily
to mund.
The onset of magnetospheric substorms is related to the initi-
ation of fast field-line reconnection in the geomagnetic tail, which
depends on microscopic plasma processes operating on spatial
scales comparable to the ion gyroradius. Magnetospheric current
systems, particularly during substorms, are controlled in part by
the field-aligned potential drop that ~ associated with double lay-
ers, anomalous resistivity, and other microscopic plasma processes
occurring on small spatial scales. The large-scale mass and energy
structure of the plasma sheet in the geomagnetic tad] is determined
in part by particle acceleration on microscopic scales and by parti-
cle precipitation into the ionosphere associated with wave-particle
interactions in the magnetosphere. Both the current structure and
the particle precipitation problems are intimately related to the
large-scale structure and evolution of the aurora.
EXAMPLES ON TEE SUN OF SMA[I~SCALE PROCESSES
OF ASTROPHYSICAL IMPORTANCE
Atmospheric Dynamics and Magnetic Fields on
Slrmll Spatial Scales
Several of the space experiments planned for the 1990s have
been designed to study dynamical processes and magnetic fields in
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117
the photosphere, chromosphere, and corona on scales as small as
100 km. The experiments are expected to provide data that will
help to unprove our understanding of fundamentally important
problems such as (~) the nature of granulation, (2) the interaction
between small magnetic elements and photospheric turbulence,
(3) the physics of spicules, and (4) the flow of matter and energy
between the chromosphere, corona, and solar wind. However, many
aspects of these problems will eventually require observations on
scales even smaller than 100 km.
For example, the present theory of granular convection envi-
sions a turbulent cascade involving a spectrum of sizes ranging
down to scales as small as 100 m. While the smallest scales of the
inertial range are probably not observable, it would be important
to observe at least one decade (esy, 100 km to 10 km) of the inertial
range to characterize the spectral distribution and hence to provide
a firmer observational basis for the theory of turbulent transport.
It ~ equally probable that such observations vnI} instead demand
a physical explanation outside the scope of present theory. The
superb whit - light observations of granulation obtained by the
SOUP instrument on Spacelab 2 have already shown, even with
0.4~arcsec (300 km) resolution, how oversunplified our conception
Of granular convection has been.
The upward extensions of fine-scale photospheric magnetic
fields are associated with the supergranular network, spicules, and
other structures that play a role in the transfer of mass and en-
ergy between the chromosphere, the corona, and the solar wind.
For example, theory medicates that the interaction between pho-
tospheric turbulence and magnetic fields on scales of the order of
10 to 100 km may induce electric currents capable of heating the
corona. Experunents planned for the 199Os promise to elucidate
aspects of these important processes, in particular by observing
dynamical phenomena in the cooler parts of the upper atmosphere
(T < 5 x lOsK) on scales as small as 100 km. The results of
these experiments will undoubtedly raise new questions regard-
ing the interactions between flows and magnetic fields on scales
significantly smaller than 100 km. For the hotter atmosphere ~ T
> 106K), current observation and theory imply that observations
of magnetic field structures and their interactions with flows of
matter on scales in the range of 100 to 1000 km are required in
order to study the processes involved in coronal heating and the
acceleration of the solar wind.
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118
Fme Structure and Dyn~m~ce of Flares
There ~ good observational evidence that the physical pro-
cesses that control solar flares manifest themselves on scales of
<50 km. For optically thin radiation, the observed emission mea-
sured Tom a single resolution element may be combined with an
independent estimate of the gas density and the known radiative
efficiency to yield the effective volume (V) of emitting material
and thereby a conservative estunate (Vi/3) of the dimensions of
the radiating structure. If the structure is filamentary (rather than
roughly spherical), its narrow dunension will be smaller than the
est~rnate.
This technique was applied to many extreme ultraviolet (EUV)
slit spectra from Skylab. For example, a flare-associated surge was
studied in which the Doppler shift of the surge material allowed it
to be isolated from other material in the field of view or along the
line of sight. Density-sensitive line ratios were used to estimate an
electron density greater than 10~3 cm~3 and a characteristic scale
of at most 60 km.
The scale lengths derived for this and other flares are consist
tent with optical spectra and with ultraviolet and x-ray emission
measure data from instruments on board the Solar Maximum Mis-
sion (SMM). In short, flare structures with characteristic scales
extending significantly below 100 km have been inferred at all
wavelengths (and temperatures) thus far observed.
The dynamics of flares is linked to their small-scale spatial
structure. SMM data have shown that lines such as Ca XIX ~ ~ ~
107 K) show significant nonthermal broadening during the impul-
sive phase of flares. The degree of broadening is nearly indepen-
dent of the location of the flare on the solar dek. The broadening
may be the result of spatial integration over many unresolved flare
loops, of time integration over highly transient, small-scale flows,
or of locally isotropic turbulence. It is essential to distinguish be-
tween these possibilities by achieving higher spatial (~10 km) and
temporal (~l s) resolution while maintaining spectral resolution.
One of the key advances that emerged from SMM was confir-
mation of the importance of high-energy electron beams in solar
flares. A substantial fraction and perhaps the majority—of the
flare energy may be channeled through beams. When high-energy
electrons interact with the ambient plasma, they produce hard
x-rays via bremsstrahiung and ultraviolet emission via collisional
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119
heating. However, it is unlikely that electron beams are the only
mechanism responsible for exciting flare emission at high densities.
High-spatial-resolution studies of flares, simultaneously in visible
and ultraviolet light, should clarify the role of electron beams and
elucidate the other mechanisms. For example, since an electron
beam moves along magnetic field lines, a measurement of the area
of the ultraviolet surface region in the chromosphere yields an up-
per limit to the cros+sectional area of the beam at that height
(an upper limit because scattering and absorption might produce
ultraviolet emission beyond the confines of the beam). Arguments
based on the observed relative intensities of ultraviolet and x-ray
bursts imply that the diameter of the beams is of order 10 km.
Plasma Heating and Microflares
The fundamental processes leading to the heating of extended
stellar atmospheres continue to puzzle us. Previous space exper-
iments have provided observational constraints on some plasma
heating mechanisms; for example, OS0-8 data showed that scour
tic waves alone cannot heat the transition region and corona.
Further progress has been impeded by the fact that the diagnose
tics necessary to differentiate between competing heating modem
cannot be applied using present instrumentation: there ~ simply
not enough spatial and temporal resolution.
An example of a basic, unanswered point is whether the heat-
ing process is steady or transient. Thus, one possible heating
process involves the relief of stresses built up In coronal magnetic
fields by the motion of the photospheric footpoint of magnetic field
lines, leading to steady flaring over a wide range of flare energies; at
the low-energy end of this flare spectrum, the flares are referred to
as "r~iicroflares." Observational evidence for the existence of such
m~croflares dates back to OS0-7 data of hard x-ray events, and
more recently to hard x-ray data obtained with balloon payloads.
These data show that the total heating rate by ~rflcroflares may
be comparable to the coronal luminosity if the energy spectrum of
electrons responsible for these transients extends down to ~5 keV
and the power law connecting the cumulative number of events
with 2~keV photon flux above a given threshold extends below
the present threshold of 10-2 photons/cm2/~/keV. Thus, the su-
perposition of such events could account for the steady coronal
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120
radiative output. Clear observational evidence that heating in so-
lar active regions ~ largely a transient process would exclude most
of the proposed coronal heating processes; the implications for
plasma heating in other astrophysical domains (such as accretion
disk coronae) would be similarly of major consequence.
An astrophysical situation in which the difference between
steady and transient heating ~ of great current interest ~ the
formation of coronae on very late-type stars (dMe stars) and very
young stars (T Tauri stars). For example, high-speed photometry
of T Tauri stars h" revealed short-term fluctuations sirn~lar to
solar flares, having power law cumulative spectra similar to those
mentioned above for solar hard x-ray transients. Such power law
spectra have also been detected In optical observations of ultravi-
olet Ceti flare stare. Moreover, it has now been shown that there
is a good correlation between the mean level of chromospheric and
x-ray emission and flare frequency for these low-mass stars. These
stellar data support the idea that the observed emission from the
hot outer layers of late-type stars is the result of a temporal super-
position of transient energy release events. Thus, a very important
question is whether quiescent EUV and x-ray emission from so-
lar active regions (and even the quiet corona) ~ due entirely to
superunposed transients.
~ order to answer this questions one is faced with the difficulty
that, as the energy of m~ividual events decreases, their visibility
above background at any given angular resolution decreases as
well. This problem ~ not acute at high photon energies (>20
keV) because the slowly varying (background) component at these
energies ~ ununportant even for full-disk (completely unresolved)
observations, at least at current sensitivity levels. However, at
lower photon energies (<5 keV) the slowly varying component
begins to affect our ability to detect individual events, so that
imaging becomes essential.
Since the observed duration of the rnicroflares at high photon
energies is a few seconds, it also becomes necessary to consider
tune-resolution constraints. For example, because of the thermal
inertia and the cooling rate of the transiently heated gas, it is
difficult to detect low-level microflaring at soft energies because
the coronal gas cooling time (103 s) far exceeds the time scale
of the transient heating event itself In contrast, ultraviolet and
EUV emission from gas at lower temperatures (~1 to 3 x 105K)
can vary at the transient time scale because the corresponding
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121
cooling tunes are sufficiently small. However, the smallest events
will require high spatial resolution in order to avoid averaging
within a resolution element.
An estunate of the size of the elemental ultraviolet-em~tt~ng
transient gas at the lower end of the energy spectrum is given by
the estimated extent of flare kerned, <60 km (see above). It Is
necessary to extend observations to even smaller scales in order to
resolve these events and to establish the cumulative contribution
of the power law m~croflare spectrum to the total ultraviolet/EUV
luminosity of active regions.
OBSERVATIONAL CONSIDERATIONS
The above discussion has shown that much would be gained by
observing physical conditions in the solar atmosphere on spatial
scales considerably finer than the best anticipated from current
program. Later in this appendix the technological considerations
that enter into observing small-ecale structure are discussed. How-
ever, intrinsic limitations that arise from the available photon flux
and the geometry of the source are first discussed.
The surface brightness of a feature limits the flux that can
be collected from a single resolution element in a time shorter
than the characteristic evolution time scale of the element. This
tune scale could be as short as the sound or the Alfven crossing
time often one second or less for subarcsecond structures.
At what rate will photons arrive at ~ AU from an element of
EUV-em~tting material? If the element ~ a cube of side it, the
number of photons detected per second at ~ AU will be
1.0(n2Ama,,/10~2)(d/10 km)3~/1001) (D/1 m)2 (~/10~2)photon s- ~
where Amiss is the cooling coefficient, nomad is the volumetric
radiative power, D ~ the aperture of the telescope, and ~ ~ the
overall efficiency of the instrument (photons counted divided by
photons incident). It ~ amumed that the pa~band try at wave-
length ~ includes all the radiation from the line or lines, and that
there is one element in the field of view. If the telescope resolves
distances smaller than it, the counts are assumed to be integrated
over ~ x d.
In the visual and near ultraviolet, the expected flux is encour-
aging for a meter-cIam telescope. For example, the C IV doublet
near 1548 ~ produces nc2Am'~= ~ 1.0 at a pressure ncT—10~6
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122
(roughly appropriate for an active region loop), leading to a pho-
ton flux ~~03 photon/s from a I~km element. However, for higher
temperature lines (T > 1055) in the EUV, typically ne2Ama': <
lo-2 at the same pressure, and, at ~ AU, only a few photons
would be detected per second. There could be little confidence
that the element would not evolve substantially or even disappear
during the time necessary to build up an image with adequate
signal-to-nome ratio.
These figures refer to structures of roughly average brightness.
In flares, pressures 10 or more tunes greater have been inferred.
The density enhancement in m~croflares ~ not known, but could
be comparable. Thus, there is good reason to expect that some
structures, come of the time, will have 100 or more times the
average brightness
Nevertheless, if we wish to observe typical EUV structures
of a size of <10 km on the quiet Sun, there is a flux problem.
This could be addressed either by larger collecting areas (of order
100 m2) or by meter-class telescopes in near-Sun orbit (say, at a
distance of 0.1 AU).
The example given above treats a single emitting volume 10
km on a side. Some lines of sight may include several structures.
Although the flux at Earth increases accordingly, the flux prom
lem is alleviated only at the expense of source confusion, since
the individual structures may have distinct physical conditions.
Source confusion could be addressed by stereoscopic observations,
in which two or more spacecraft simultaneously exanune the same
region of the solar surface from different vantage points.
A third problem ~ the reduction in contrast of very fine struc-
tures embedded in an optically thick medium such as the pho-
tosphere. In this case, image sharpness is limited by the photon
mean-free path (~100 km in the photosphere for visible light).
This problem is less important for observations of Doppler or Zee-
man line shifts to the extent that scattering causes diffusion in
position but not In wavelength. Moreover, the problem disap-
pears for optically thin media such as the chromosphere (for many
spectral lines), transition zone, and corona.
It may be that the twin problems of flux limitation and source
confusion will, more than purely technical considerations, limit
the remote sensing of solar features to scales of ~ km.
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130
contrast. All of these techniques should yield some improvement
even at very short wavelengths.
Aperture Synthesis (AS)
In AS, a mask with a set of transmitting holes is inserted
across the prunary mirror, or ~ a reduced image of the primary.
The resultant subapertures are positioned so that their pairwise
autocorrelations sample the (u,v) (spatial frequency) plane to the
greatest degree possible. Conventional images are recorded with
the array In place. Subsequent analysis of these coded images
allows estimation and correction of the phase errors across the
subaperture, and eventually, reconstruction of enhanced images.
The major advantage of this technique ~ its simplicity of
implementation, with full image reconstruction from one or two
exposures. Its drawback is that perhaps only 10 to 20 percent of
the total flux Is transrn~tted by the masks.
Speckle Nonaging
Speckle imaging techniques, which have been used to recover
diffraction-lim~ted images with large-aperture telescopes through
atmospheric turbulence, may be used to overcome the effects of
the uncorrectable errors in EUV optics. If pairs of small aper-
tures are scanned across an optical pupil plane, and images are
recorded for each position of the subapertures, then averaging
of the complex autocorrelations of the images would allow high-
resolution recovery. Each position of the pair of subapertures
provides a statistically independent realization analogous to ran-
dom atmospheric fluctuations between short exposure frames in
ground-based speckle. The major problem ~ that the tane required
for this serial procedure ~ significantly longer than a single-frame
technique such as AS, so its time resolution is Innited. Its advan-
tage is the potential to correct to much finer scales.
Deconvolution
A by-product of an actively controlled optical system is that an
accurate measure of the optical transfer function is obtained by the
system wavefront sensor. Because the sensor uses its own source,
good signal-to-nome and accuracies better than A/1000 (visible)
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131
can be achieved. An accurate deconvolution of each image with the
system point spread function may therefore be applied as a post-
processing step. Only the presence of zeroes in the optical transfer
function limits this technique, and these may be eliminated by
adequate compensation with active optics.
INTER1?EROMETRY
One way to obtain very high angular resolution is through
spatial interferometers, using either arrays of telescopes or arrays
of flat murrors feeding a central beam-combining telescope. The
goal of such an interferometer would be very high angular rem
olution unaging (m~Biarcseconds) from visible wavelengths (7000
1) to the EUV (~500 1~. Although the precise form of such an
interferometer remains to be defined, at least two options are
available. Both are so-called monolithic arrays, which means that
the interferometer system is mechanically coupled in such a way
that the pathlengthe between the Sun and the focal plane are
approxanately equal in all legs of the interferometer. Both are
also designed to have a wide field-of-,riew, which means that the
pupil configuration at the entrance and exit of the interferometer
· ~
is preserved.
In the first, a "thin monolithic interferometric "ray," HRSO
is used as a beam-comb~ner. As in the early Michelson and Pease
interferometer, pars of flat mirrors are used to feed the light into
the telescope from points outside the telescope. In one version,
five of these 4() cm-diameter interferometer beams are arranged in
a pentagon with a 12.~m diameter, feeding the lam diameter
optical telescope, giving l~iilliarcsec resolution in the visible or
3-milliarcsec resolution in the C IV lines near 15501. It could be
assembled in space to the required dimension. Because of the large
coherence lengths (~2/^ ~ l/~) that char~terme high-spectral-
resolution solar observations, the tolerances on the lengths of the
interferometer legs are rather loose for narrow bandpass observa-
tions. Such an interferometer could be implemented at an early
date at a relatively low cost. For unaging, it will be necessary
to determine the relative pathIengths of the interferometer legs.
This requires the development of the equivalent of phase closure
techniques used in radio image synthesis telescopes like the VLA
but for solar observations, and image synthesis by rotation of the
array and by changing the length of the arms. Because of the flux
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132
limitations in the ultraviolet, phase closure has to be done in the
visible wavelengths.
The second type of interferometer is a Thick monolithic arrays
in which the interferometer elements are coupled by an optical
metering system to achieve cophas~g. It gives complete (u,v)
plane coverage in one dimension. Rotation of the array will fill the
(u,v) plane. Phase closure ~ not needed in this case.
Both types of arrays are best used in conjunction with HRSO
on the Space Station or on co-orbit~ng platforms. Before the
implementation of a free-flying solar space interferometer, it is im-
portant to determine the visibility of different scales of solar fine
structures by means of two-element interferometers, using ground-
based interferometers for the visible and, e.g., a EURECA-ciass
experiment for the far ultraviolet, allowing milliarcsec resolution.
Phase closure techniques for solar observations can also be pio-
neered from the ground in the visible region of the solar spectrum.
POINTING SYSTEMS
.
The SOUP telescope on Spacelab 2 has demonstrated that
it is possible to point a white light solar telescope with internal
Image stabilization to a stability of 3 m~liarcsec using a 5~Hz
control bandwidth. Similar image control techniques have locked
two telescopes to an accuracy of a few milliarcseconds, which
essential for ultraviolet observations because it is often necessary
to use detectors that we insensitive to visible light or require that
visible light be excluded from the telescope.
The new generation of solar experiments should provide for
multiple-aspect Stereo studies of solar structures. Since the
structures to be studied may be ~ the I~m~liarcsec size range,
it would be desirable to be able to powt several widely separated
telescopes at the same feature on the Sun with an absolute accu-
racy of a few m~liarcseconds. Since the average solar diameter is
known to be stable at that level or better for periods longer than a
month, a pointing system based on metrology of the average solar
limb should yield the required accuracy.
If the solar diameter and the pointing position are measured
with a laser interferometer, which should yield )/10 accuracy, the
location on a Cm solar image can be measured to 1.6 parts in
lo6. Since the solar diameter ~8 about 2000 arcsec, this represents
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133
a positional accuracy of 1.3 m~liarceec. Co-powting of two inde-
pendent telescope systems also requires that their unage planes
be homogeneous to this level and that their roll orientation be
measured to 0.25 arcsec.
SITING
From the preceding discussion, it is clear that spatial res-
olutions substantially below 0.! arcsec, at wavelengths from 30
~ to visible light, can be achieved with techniques that will be
perfected in the twentieth century. The question of where to sit-
uate the resulting high-resolution solar instruments must now be
addressed.
Near-Sun Orbit
Close approach to the Sun offers a conceptually simple way
of achieving ultra-high angular resolution with instrumentation of
modest aperture, and it may be the only means of obtaining I~
km resolution of the solar surface with sufficient EUV flux. The
basic technology required for near-Sun orbits (e.g., heat shielding,
telemetry through the corona, injection into the correct orbit) will
be developed for the Starprobe progrmn. It would seem profitable
to use this technology in follow-on missions carrying solar unaging
experiments. Some of the additional studies for shielding the
instruments were done in the preliminary stages of the Starprobe
program and indicated that this concept ~ quite feasible.
Heliosynibronom Orbit
A spacecraft at ~30 solar radii approximately 0.! AU—will
have a Today orbit around the Sun, and in the ecliptic plane will
hover over a fixed point on the Sun. In addition to anportant long-
term studies of the evolution of solar structures, and stereoscopic
measurements of fine features when combined with another space-
craft in similar or near-Earth orbit, such a vantage point offers an
increase in spatial resolution by a factor of 10 and flux by a factor
of 100 compared to an orbit at ~ AU.
Lunar Basing
A solar observatory on the Moon would operate under almost
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ideal conditions, on a highly stable platform, free from atmospheric
turbulence and contamination, and able to view the Sun continu-
ously for Malay periods. Such an observatory could operate either
manned or unmanned, and should be considered In the context of
any future Moon landings. Two lunar observatories separated by
180° would give nearly 100 percent coverage.
Solar Orbit at 1 AU
Stereoscopic observations of the Sun can be achieved with
two observatories in orbit at ~ AU widely separated ahead of and
behind the Earth. Alternatively, one of the observatories could
be placed at a Lagrangian point. It Is felt that the simultaneous
pointing accuracies needed to achieve 0.01-arcsec resolution can
be achieved.
Earth Orbit Pree Flyer
The ideal near-Earth orbit for a solar observatory would be
a high-inclination, Sun-synchronous orbit, i.e., one perpendicular
to the Earth-Sun line. The first mission of the HRSO (High Rem
olution Solar Observatory) on the Space Shuttle is planned for a
high-inclination orbit, which will allow long periods of un~nter-
rupted observation.
Manned Vehicles Space Shuttle
It Is expected that the Space Shuttle in some version will still
be flying at the century's end. Spacelab has proven itself a valu-
able vehicle for astronomical observations. Advantages of Spacelab
include significant real-time scientific interaction with the exper-
iment as well as the ability to use either solid-state detectors or
recoverable fihn. While solar observations can make effective use
of week-Ion" flights, longer missions could significantly increase the
scientific return. Disadvantages include relatively short observing
periods for low-inclination orbits and the potential for contami-
nation of highly sensitive ultraviolet and EUV normal-incidence
optics. The principal contaminant is expected to be thin (10 to 20
R) layers of polymerized hydrocarbons, which are opaque to light
between 300 and 13001. Solar telescopes are particularly prone to
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135
organic contamination because the solar ultraviolet light polymer-
izes hydrocarbons rapidly. Any site in space where such telescopes
are deployed requires rigid material selection standards if the tele-
scope is to operate for an extended period. Skylab experience
demonstrates that this can be done.
Space Station
The Space Station shares many of the advantages and disad-
vantages of the Shuttle. An additional concern is the expected
broader spectrum of vibrations and disturbances, which make
pointing difficult; isolation platforms will probably '~e required.
Contamination Is also of great concern, and it is unperative to pay
careful attention to materials selection for the Space Station from
the outset. Of course, a co-orbiting platform circumvents many of
these difficulties and would provide a very attractive site for solar
observations.
PROSPECTS AND [IMITATIONS OF GROUN~BASE:D
OBSERVATIONS
Radio l~terferometr~r
The usefulness of high-angular-resolution observations for so-
lar physics has been demonstrated at radio wavelengths since the
early 1970s. The use of larg~synthesis radiotele~copes (such as
the Westerbork Array and the Very Large Array) at centuneter
and decimeter wavelengths made possible observations of a few
arcseconds resolution. Source sizes of 3 arcsec have commonly
been observed at 15 GHz during flares (somewhat larger sizes are
seen at lower frequencies, presumably from loops located higher
in the atmosphere). Radio observations with high spatial resolu-
tion have been used to pinpoint the site of initial energy release
in some flares, and such observations were the first to show the
expansion and thermalization of high-temperature flare plasmas
following the impulsive phase of the flare.
Further evidence for small-scale structure in the solar atmo-
sphere comes from observations with high temporal resolution.
Microwave spikes of millisecond duration are sometimes observed
during flares, implying source sizes of the order of a few hundred
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136
kilometers. Even smaller structures are suggested by decimeter-
wavelength observations of short-lived, narrow-band bursts. Di-
rect observations of such structures at radio wavelengths may
become possible during the next decade with the advent of the
Very Long Baseline Array. More limited information might be
obtained with existing very long baseline interferometers.
While high-angular and high-temporal resolution radio obser-
vations have yielded unportant new results, they are limited in
one sense. All radio emission mechanisms are continuum in na-
ture, and represent the signature of either thermal or energetic
electrons. Therefore, radio observations do not yield information
on the variety of ion species present in the solar atmosphere.
Eigh-Angndar-Resoh~tion Optical Nags Tom the Ground
There are a number of techniques that are currently yielding
high-angular-resolution information at visible wavelengths on sm
lar features. Adaptive optics, which use real-tune measurement of
atmospheric aberrations and phase correction of those aberrations,
has demonstrated a capability for substantive unage enhancement
of small solar features (pores) and the surrounding granulation.
Speckle interferometry and imaging have been applied to broad-
band sunspot and granulation data, yielding detail approaching
the diffraction limit of a l.~m-aperture (0.07 arcsec). Aperture
synthesis has shown promise of recovering enhanced imagery frown
a single short exposure, allowing very high time resolution.
As these and other techniques are improved and used on large
ground-based telescopes, the requirements for future space-based
systems will be greatly clarified. Of course, ground-based systems
have the severe limitation of the atmospheric transmission window.
[ong-Baselme Optical Interferometry
There has been recent, worldwide development of long-baseline
(up to 1 km) interferometers. All such instruments have been de-
signed for stellar astronomy. However, these or similar instruments
could be applied to high-angular-resolution solar measurements.
Special techniques that use field stops to isolate small features
would be necessary to actively control the interferometric base-
line, but these techniques are quite feasible. While no such solar
observations have yet been proposed, such a system would have the
potential of providing angular resolution approaching ~ milliarcsec
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at visible wavelengths. However, because of their sparse coverage
of the (u,v) plane, such observations are more difficult to interpret
than images proper, particularly for complex, low-contrast sources
like the Sun.
SUGARY AND CONCLUSIONS
There is now considerable evidence that all scales of structure
on the Sun, as well as other astrophysically interesting objects, are
strongly coupled to small-scale processes associated with interniit-
tent magnetic fields and turbulent stresses. Understanding the
physics of these dynamical structures and their interaction with
their surroundings ~ essential for a proper description of large-
scale structures (such as coronal active regions, flares, or the solar
wind) and their effects on interplanetary space and the near-Earth
environment.
The interplay between processes occurring on vastly different
spatial scales is ubiquitous in astrophysics. Whether in accretion
disks feeding black holes at the center of active galaxies or quasars,
in the magnetospheres of neutron stars, or in the x-ray coronae
now known to surround a wide range of stars, smaD-scale magne-
tohydrodynarn~c processes are thought to influence and sometunes
control the behavior of the object.
In these astrophysical situations, observations using even the
most advanced technology currently conceivable will not allow us
to directly observe the controlling smaD-scale processes. Using the
Sun, however, we can indeed imagine direct observations. The
Sun is therefore a unique too! for advancing our understanding
of a broad class of important astrophysical phenomena, if we can
penetrate to the domain of underlying processes that often operate
on spatial scales of 1 to 100 km.
An orderly progression of goals that could realize much of this
promise would include:
1. Implementation of the High Resolution Solar Observatory
on Spacelab, followed by the transfer of HRSO to the Advance
Solar Observatory on the Space Station, together with ultraviolet
and x-ray solar instruments capable of 0.1-arcsec angular resolu-
tion.
2. Development of interferometric experiments at visible and
ultraviolet wavelengths anned at preliminary reconnaissance of
solar features at angular scales much less than 0.1 arcsec.
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3. Development of meter-cIa" facilities exploiting the emerg-
ing multilayer coating technologies, designed to obtain resolution
in the 0.01-arcsec regime at extreme ultraviolet or soft x-ray wave-
lengths. Ideally, several spacecraft would be located in near-Sun
orbits to provide high flux, high-~near-resolution, and stereoscopic
· ~
Imaging.
4. Achievement of ultrahigh-resolution imaging ~ ultraviolet
and visible wavelengths, using baselines of order 10 m.
Two concepts related to these goals are presented in separate
recommendations of the task group: stereoscopic unaging using
meter-cIass telescopes at four locations around the ecliptic, and
an x-ray/ultraviolet/optical telescope in heliostationary orbit at
30 solar radii.
APPE:ND~: LIST OF PA1tTICIPANTS
S. Antiochos, Naval Research Laboratory
R. G. Athay, High Altitude Observatory, NCAR
J. Beckers, Advanced Development Program, NOAO
R. Bonnet, European Space Agency
G. Brueckner, Naval Research Laboratory
R. Catura, Lockheed Palo Alto Research Laboratory
L. Cram, Commonwealth Scientific and Industrial Research
Organization
I`. Dame, I,aboratoire de Physique StelIaire et Planetaire
J. -P. Delaboudiniere, I,aboratoire de Physique StelIaire et
Planetaire
G. Doschek, Naval Research Laboratory
G. Epstein, NASA, Goddard Space Flight Center
T. Gergely, NASA, Headquarters
M. Giampapa, National Solar Observatory, NOAO
L. Golub, Harvard-Sm~thsonian Center for Astrophysics
J. Harvey, National Solar Observatory, NOAO
J. Heyvaerts, Observatoire de Meudon
T. Holder, High Altitude Observatory, NCAR
R. Howard, National Solar Observatory, NOAO
R. Keski-Kuha, NASA, Goddard Space Flight Center and
University of Maryland
J. Leibacher, National Solar Observatory, NOAO
B. Lites, High Altitude Observatory, NCAR
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139
R. MacQueen, High Altitude Observatory, NCAR
P. Nmenson, Harvard-Sm~thsonian Center for Astrophysics
R. Noyes, Harvard-Sm~theonian Center for Astrophysics
D. Rabin, National Solar Observatory, NOAO
F. Roddier, Advanced Development Program, NOAO
R. Corner, Harvard-Sm~theonian Center for Astrophysics
L. Schmutz, Adaptive Optics, Inc.
E. Shoub, University of Colorado
D. Spicer, NASA, Goddard Space Flight Center
A. Title, Lockheed Palo Alto Research Laboratory
J. Toomre, University of Colorado
J. Underwood, Lawrence Berkeley Laboratory
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Representative terms from entire chapter:
glancing incidence
