Solar and Space Physics Space Science in the Twenty-First Century -- Imperatives for the Decades 1995 to 2015 (1988) / Chapter Skim
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Appendix D: Probing Fundamental Astrophysical Scales with High-Resolution Observations of the Sun: Prospects for the Twenty-first Century
Appendix D: Probing Fundamental Astrophysical Scales with High-Resolution Observations of the Sun: Prospects for the Twenty-first Century
Pages 112-140
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From page 112... ...
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.
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From page 113... ...
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 anticipated from solar space missions planned for the coming decade (~100 km on the Sun, or ~0.!
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From page 114... ...
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)
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From page 115... ...
Coronal loops, a fundamental building block of the solar atmosphere, 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 properties of a loop (e.g., temperature, pressure, magnetic field strength)
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From page 116... ...
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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From page 117... ...
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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From page 118... ...
resolution while maintaining spectral resolution. One of the key advances that emerged from SMM was confirmation of the importance of high-energy electron beams in solar flares.
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From page 119... ...
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.
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From page 120... ...
This problem ~ not acute at high photon energies (>20 keV) because the slowly varying (background)
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From page 121... ...
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.
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From page 122... ...
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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From page 123... ...
2. At wavelengths below 1000 A, nearly diffraction limited resolution probably requires normal incidence optics, and this in turn entails the development of multilayer, high-reflectance coatings.
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From page 124... ...
Even higher resolution, of order 0.2 ~m, can be achieved with a semitransparent photocathode followed by rennaging of the photoelectrons onto a suitable detector, with acceleration of the electrons and expansion of the plate scale achieved simultaneously. Some increase in plate scale can be accomplished by suitable design of the optical system, such as using relay optics in a grazing incidence configuration or using a Camegra~n design with normal incidence optics and x-ray multilayers.
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From page 125... ...
In addition, such coatings can be applied to glancing incidence mirrors in order to enhance their short-wavelength performance. The principal advantage of normal incidence optics in comparison with glancing incidence is the ability to achieve substantially higher image quality with a given level of effort.
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From page 126... ...
is roughly equal to the number of layer pairs and varies from ~10 at 200 AL to ~103 at 201. Imaging in a narrow bandpass that reflects a single line, or lines, of a single ionization state, formed over a narrow temperature range, may provide the best opportunity to study fine-sc~e solar features at EUV wavelengths.
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From page 127... ...
Resolution of A/~> ~ 3 x 104 can be achieved, thereby permitting separation of nearby spectral lines, measurement of line broadenings and asymmetries, and deterrn~nation of radial velocities to a few kilometers per second. Glancing Incidence Optics Although the multilayer technology just discussed opens up the exciting prospect of applying the fine-scale imaging capabilities of normal incidence optical systems to solar observations at soft x-ray and EUV wavelengths (30 to 400 Hi, there will still be a need for glancing incidence optical systems at shorter wavelengths.
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From page 128... ...
This improvement in surface smoothness by an order of magnitude Anti reflect itself in an unprovement in attainable resolution by an order of magnitude, the limiting factors then being the ability to achieve and maintain relative alignment of the primary and secondary mirrors and the diffraction effects caused by the large central obscurations of glancing incidence telescopes. Primary Figure Confection and Control The prunary resolution of the telescope is affected by the alignment accuracy of the optical train and the consequences of variations ~ element position having temporal periods of hours (thermal)
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From page 129... ...
Furthermore, wave front sensors can easily resolve 10,000 or more pupil elements, which can be used to drive tower resolution correctors. The residual error ~ a dynamic measure of the instrumental optical transfer function, which can be used for post facto resolution enhancement (see following subsection)
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From page 130... ...
If pairs of small apertures 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 highresolution recovery. Each position of the pair of subapertures provides a statistically independent realization analogous to random atmospheric fluctuations between short exposure frames in ground-based speckle.
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From page 131... ...
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.
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From page 132... ...
Before the implementation of a free-flying solar space interferometer, it is important to determine the visibility of different scales of solar fine structures by means of two-element interferometers, using groundbased 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 pioneered from the ground in the visible region of the solar spectrum.
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From page 133... ...
SITING From the preceding discussion, it is clear that spatial resolutions substantially below 0.! arcsec, at wavelengths from 30 ~ to visible light, can be achieved with techniques that will be perfected in the twentieth century.
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From page 134... ...
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 contamination of highly sensitive ultraviolet and EUV normal-incidence optics.
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From page 135... ...
. Radio observations with high spatial resolution 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.
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From page 136... ...
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.
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From page 137... ...
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 magnetohydrodynarn~c processes are thought to influence and sometunes control the behavior of the object.
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From page 138... ...
Howard, National Solar Observatory, NOAO R Keski-Kuha, NASA, Goddard Space Flight Center and University of Maryland J
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From page 139... ...
Spicer, NASA, Goddard Space Flight Center A Title, Lockheed Palo Alto Research Laboratory J
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