Working Papers Astronomy and Astrophysics Panel Reports (1991) / Chapter Skim
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Solar Astronomy
Solar Astronomy
Pages 199-234
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From page 199... ...
1.2 Frontiers and Goals for the 1990s 1.2.1 The Solar Interior 1.2.2 The Solar Surface 1.2.3 The Outer Solar Atmosphere: Corona and Heliosphere 1.2.4 The Solar-Stellar Connection 2. Ground-Based Solar Physics .
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From page 200... ...
In addition, the solar imaging instruments on SoHO are designed to look in great detail at a totally different aspect of solar activity than is normally examined, namely the mass outflows. They will also be able to look at the "closed" corona, and here their principal virtue is their spectroscopic capabilities, combined with imaging, which also allows detailed density and temperature diagnostics to be performed.
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From page 201... ...
The formation of magnetic flux bundles we want to understand why solar magnetic fields are so highly spatially intermittent; (b) the evolution of surface magnetic fields—we want to understand how the distortions of surface magnetic fields by the surface convection can lead to plasma heating, including the creation of a multi-million degree corona enveloping the Sun; and how the process of magnetic field evolution proceeds (we know that solar magnetic fields must decay, but the observed decay rate is many orders of magnitude larger than what classical theory would predict.)
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From page 202... ...
The long-standing solar neutrino problem involves a remarkably broad range of physical issues, from stellar structure and opacity calculations to the physics of neutrinos themselves. More recent data have only deepened the mystery of these particles, as the possibility of correlations of the solar neutrino flux with the solar activity cycle has been raised.
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From page 203... ...
The activity at the surface of the Sun is a direct manifestation of this convective heat engine, which produces such diverse phenomena as sunspots, flares, coronal transients, the X-ray corona, and the solar wind, largely through the magnetic field as an intermediary. It seems not to be generally recognized in the astronomical community and elsewhere that the precise causes of solar activity are not yet reduced to hard science.
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From page 204... ...
What is more, the magnetohydrodynamic Reynolds number NM is 10~°, whereas the terrestrial laboratory can achieve no more than 102 or 103, so there is no general body of knowledge from which the subtleties of solar magnetic activity can be interpreted. The enormous heat flux in the convective zone producing the superadiabatic temperature gradient and driving the convective heat engine on all scales, and the extreme magnetohydrodynamic effects of solar activity combine to provide a dynamical scenario of exotic character that can be understood only after it is described and studied in detail - it is far too complex for a priory predictions.
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From page 205... ...
A large solar flare releases as much as 1032 ergs, and a significant fraction of this energy appears in the form of accelerated particles. It is believed that the flare energy comes from the dissipation of the non-potential components of strong magnetic fields in the solar atmosphere, possibly through magnetic reconnection.
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From page 206... ...
The next section, then, details some of the specific problems, measurements, observations, and theoretical studies that are necessary along the way to probe the mystery. 1.2 The Frontiers and Goals for the 1 990s By the year 2000 we hope to have a fairly detailed picture of the structure and dynamics of the solar interior, a better understanding of how the Sun generates magnetic fields, considerable measurements of how magnetic fields modulate the smooth outward flow of energy, a better description of the morphology of flaring plasmas, and some predictive capability for the flow of non-radiative energy through the heliosphere to Earth.
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From page 207... ...
While space offers the most certain route to improvement, the development of adaptive optics promises significant benefits using ground-based telescopes. A realistic goal for the '90s is to obtain a clear physical understanding of the interaction of magnetic fields and convective motions at and immediately beneath the surface, and specifically to understand the surprising shredding of the field into spatially intermittent "flux knots," which appears fundamental both to the evolution of surface magnetic fields and to their consequences for atmospheric heating.
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From page 208... ...
The same cannot be said for violent events such as coronal mass ejections that occur frequently during active times of the solar cycle better observations from the photosphere through the heliosphere, involving in particular coordinated in situ and remote sensing observations, are required for significant further progress. Appropriate goals for the decade of the 1990s include localizing coronal energy release sites, understanding in detail the mechanism of solar wind acceleration and fixing the height above the surface at which it occurs, and determining how the speed of the resultant wind depends on magnetic geometry.
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From page 209... ...
Within the general framework just discussed, we see the 1990s as the era in which ongoing key initiatives - discussed in Section 2.2 below - will be augmented by a major new initiative, which is needed to make really significant progress in our understanding of solar activity. This new "solar magnetohydrodynamics" initiative depends upon a concerted development program in high angular resolution optical observations and precision polarimetry, using existing ground-based telescopes; and aims for the establishment within this decade of a large-aperture ground-based optical facility - the Large Earth-based Solar Telescope - using adaptive optics techniques to image the solar surface in the subarcsecond range.
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From page 210... ...
2.3.3 The Major New Initiative: Solar Magnetohydrodynamics, and the LEST. A major scientific goal for ground-based solar research in the 1990s is to understand the physics of solar magnetic fields in the regions of the Sun that are observable from the ground.
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From page 211... ...
These advances in theory, coupled with the advances in observations, will allow a major improvement in our understanding of phenomena such as the turbulent diffusion of magnetic fields by confronting theory - such as the results of simulations - directly with high spatial resolution observations. In addition to excellent angular resolution, it is necessary to be able to make accurate polarimetric measurements of the Zeeman effect across profiles of spectral lines formed over a range of heights in the solar atmosphere.
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From page 212... ...
The LEST will build upon results from OSL characterizing the fine-scaled magnetic flux distribution and evolution at the solar surface, to investigate the full (non-potential) vector magnetic fields and their specific implications for energy transport and heating of the outer atmosphere.
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From page 213... ...
This advantage applies even with only moderate angular resolution. The McMath telescope of the National Solar Observatory is well suited to infrared solar research and, accordingly, several institutions are developing focal-plane instruments with modest resources to measure magnetic fields in the infrared.
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From page 214... ...
can provide important diagnostics of the flare plasma, particularly the electron energy distribution/temperature and magnetic field strength. In active regions, spectra provide unique measurements of the coronal magnetic field.
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From page 215... ...
The LARC, with its high spatial resolution, would neatly complement the high spectral resolution offered by the small reflecting coronagraph onboard SoHO. Tomography of the Convection Zone Early studies of the hydrodynamic structure of the solar interior through observations of pressure-driven (p)
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From page 216... ...
, to measure the time-dependence of the neutrino flux (as a function of solar cycle as well as on the much shorter time scales associated with solar flares) , and to determine the solar neutrino flux from the pep reactions which play the central role in the energetics of the solar core.
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From page 217... ...
Such a benefit has been realized for helioseismology and could be used to advance our understanding of solar activity in the same way. The Canadian Compact Cm/dm Array A study of a compact synthesis array for decimeter/centimeter waves is presently underway by Canadian radio astronomers; an international collaboration may be sought.
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From page 218... ...
As a starting point, we therefore strongly encourage the continued vigorous support of the GONG project (which will push the frontiers of exploring the solar interior from the ground) ; and we strongly recommend, first, the immediate development of adaptive optics necessary for high and uniform angular resolution optical observations, with the specific aim of building, together with international partners, the LEST facility to attack the frontier of high angular resolution solar surface observations; second, the vigorous development of infrared imaging and spectroscopy instrumentation, together with development of a detailed plan for a large-aperture infrared facility; third, a concerted effort to implement the moderate-scale initiatives listed in Section 2.3.4, and fourth, support for the interdisciplinary initiative listed in Section 2.3.5.
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From page 219... ...
A primary goal for OSL research is the nature of solar magnetic fields from the deepest observable layers of the photosphere upward to high temperature regions of the solar corona. A particularly important location is where the solar plasma changes from domination by radiative and convective processes to magnetic control; processes in this region are fundamental in creating solar activity.
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From page 220... ...
. · ~ information encoded in gamma-ray lines, such as abundances in both the ambient gas and the accelerated particles, beaming of the accelerated particles, temperatures and states of ionization of the ambient gas, and the structure of the magnetic fields.
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From page 221... ...
To carry out its mission properly, BESP should be accompanied with diagnostic observations of parameters of solar plasma in the 104 - 107 K temperature range, and with vector magnetic field measurements in the photosphere. Such observations could be provided by the instruments on OSL if OSl is operating during the next peak of solar activity.
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From page 222... ...
· Solar-Stellar Activity Mission: The stars teach us about solar activity by allowing the study of analogs to the solar mechanisms; they also give us rich fields for new discovery. Stellar magnetic activity can be sensitively studied with wide-field (one degree)
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From page 223... ...
We have addressed this prerequisite with the "Solar Watch" program described above, which will provide qualitative and quantitative improvements in the data base for solar activity forecasting. Deep-space observations of the invisible hemisphere of the Sun alone will appreciably improve activity forecasting via the early warning capability of active-region growth.
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From page 224... ...
S.6 Conclusions and Summary To meet the challenge of the next decade in space, we recommend the continued development of OSL and the other approved missions. Beyond these, our top priority for a new space project is a small-t~moderate mission capable of studying solar activity through the maximum of the forthcoming solar cycle (ca.
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From page 225... ...
Improvements in infrared narrow-band filters, polarizers, and photodetector arrays will contribute to continuing progress in measurement of magnetic and electric fields in the solar photosphere and chromosphere, as well as improved thermodynamic diagnostic capability in those same parts of the solar atmosphere. Optical photodetector arrays with shorter readout times than are presently available will produce an immediate benefit in any measurements such as the above, including development of adaptive optics.
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5.2. Facilitating Solar Research Two steps can be taken by NASA and NSF to facilitate solar research through minor restructuring of existing funding programs.
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3. Networks: Efforts to provide high speed digital networking capability to large numbers of solar researchers should be enhanced.
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Thus the committee recommends that the internal structure for funding of solar research within NSF be changed so that support for both grants and centers is administered by a single entity within NSF whose primary responsibility is solar physics. Such a reorganization will permit the development of appropriate advocacy within NSF, the definition of an overall coherent approach to the subject, a unified vision of the field's national facilities and university grants program its scope and its development and the implementation of new efforts.
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From page 229... ...
and the accelerated particles (thought to be accelerated in the corona)
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From page 230... ...
Normal-incidence optics represents a great advance over previous methods of solar X-ray imaging and will be the basis for high-resolution X-ray imaging on the Orbiting Solar Laboratory. Image courtesy of IBM Research and the Smithsonian Astrophysical Observatory.
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From page 231... ...
. Spicules may be very important to our understanding of the energy balance of the solar atmosphere.
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From page 232... ...
The dramatic change holds important clues to the nature of the magnetic cycle. It is important that new high-resolution observations, both from OSL and LEST, follow the changing magnetic patterns, and their consequences in the overlying atmosphere, throughout a large fraction of a solar cycle.
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From page 233... ...
. These observations, made in a single day, do not have sufficient frequency resolution for accurate characterization of solar internal structure or rotation; needed are continuous observations for extended periods, such as are planned with the GONG experiment as well as helioseismology instruments on the SOHO mission.
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From page 234... ...
COCHRAN, University of Texas, Austin DALE P CRUIKSHANK, NASA Ames Research Center IMKE DE PATER, University of California, Berkeley JAMES L
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