Appendix I

Infrared Solar Physics

NOTE: The material in this appendix is reprinted from material made available to the Task Group on Ground-based Solar Research by the scientific staff of the National Solar Observatory.



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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Appendix I Infrared Solar Physics NOTE: The material in this appendix is reprinted from material made available to the Task Group on Ground-based Solar Research by the scientific staff of the National Solar Observatory.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Infrared Solar Physics Beyond 2.5 µm: Why and How (Including a Program for the McMath-Pierce Telescope) Scientific Staff National Solar Observatory National Optical Astronomy Observatories July 1997 1 Summary Infrared observations beyond 2.5 µm are expected to contribute uniquely to solving two key problems of solar physics: the role of weak surface magnetic fields in the solar cycle and the origin of the chromosphere. More generally, infrared measurements of magnetic field strength as well as temperature, density, and chemical composition are the most direct and sensitive of any known techniques. Infrared solar observations from space with useful angular resolution are impractical, and only one ground-based telescope in the world currently accesses the full infrared spectrum. This unique capability, exploited using modern detectors, has given us a new window on solar physics. A larger aperture is needed to further develop the powerful diagnostic possibilities available in the infrared. 2 Two Key Scientific Goals 2.1 Weak Magnetic Fields: the Dark Matter of Solar Physics? Since the invention of the solar magnetograph, our understanding of photospheric magnetic fields has undergone one revolution and may well be in the process of another. The first revolution established that most of the magnetic flux in plages and the magnetic network is in the form of kilogauss-strength, sub arc-sec field concentrations rather than the much apparent weaker field that corresponds to the measured flux. The second revolution concerns the importance of the “magnetic carpet” that covers the rest of the Sun outside active regions. Several indirect but independent estimates (Stenflo 1994) indicate that the flux in this component is comparable to the strong-field flux, even at solar maximum. The weak-field component continually renews itself on a time scale of a few days at most. How do strong fields and weak fields interact? Does the weak-field component have large-scale structure? How is it generated? How does it disappear? Stenflo (1994) emphasized the importance of understanding the role of weak fields in the solar cycle: “A tiny, non-random component would however suffice for the IN [internetwork] emergence to be the dominating source of the large-scale pattern, since the IN flux emergence rate is 104 larger than that of AR

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE [active regions].” Our ambitious but attainable goal should be to measure the magnetic flux at every point on the solar surface and to measure the field strength at as many points as possible. The “12-µm” emission lines (e.g., Mg I 12.32 µm) will play a key role in reaching this goal because they alone permit a model-independent Zeeman measurement of magnetic field strength down to ~100 gauss (Solanki 1994). The well-known pair of Fe I lines near 1.56 µm is sensitive down to ~250 gauss and will complement the 12-µm lines because the Fe I lines are formed near the base of the photosphere while the 12-µm lines are formed about two pressure scale heights above. Kilogauss flux concentrations with low filling factor will spread and weaken significantly between these two heights, whereas space-filling weak fields may change change only slightly. Another advantage of the 12-µm lines, connected with their height of formation, concerns the extrapolation of surface magnetic field measurements into the outer atmosphere, an application that has become increasingly important with the advent of high-quality coronal images from Yohkoh and SOHO. The extrapolation techniques usually assume that the field is force-free, but this is a poor assumption for visible magnetograph lines such as Fe I 630.2 nm and even poorer for the deep 1.56-µm lines. For the 12-µm lines, however, the plasma β (gas pressure divided by magnetic pressure) is typically of order 10-1. Thus, the 12-µm lines provide a consistent “platform” for inferring the structure of the magnetic field at higher levels. 2.2 The Origin of Chromospheres The mystery of outer stellar atmospheres begins, not in the corona, but at the top of the photosphere in the so-called temperature minimum region. In the absence of non-radiative heating, there would be no temperature minimum: the temperature would decline steadily with height until it reached equilibrium with the interstellar medium. Classical chromospheric models are one-dimensional, semi-empirical, hydrostatic atmospheres that assume whatever distribution of non-radiative heating is necessary to produce the “desired” run of temperature from photosphere to corona. Nearly every textbook presents a similar figure. Various sources for non-radiative heating have been proposed; none has been proven. Infrared observations of the carbon monoxide molecule provide some of the strongest evidence to date that the textbook picture – of the Sun, and late-type stars in general – is wrong. Models (Ayres and Rabin 1996 and references therein) founded on spectra of the CO vibration-rotation band system near 4.8 µm show that, below a height of about 1000 km, over most of the solar surface, most of the time, there is no chromosphere. At any one time, only about 20% of the solar surface produces the radiation from what we have traditionally called the chromospheric “layer.” Most of the volume below the magnetic canopy is occupied by a “COmosphere” that is cooler than the temperature minimum of standard models. In this respect, one-dimensional models have little physical meaning because the mean properties represented by the model may be encountered almost nowhere in the actual atmosphere. Dynamical simulations by Carlsson and Stein (1995) demonstrate that this apparently extreme characterization has a physical basis. Carlsson and Stein (1997) reinforce from a modeling perspective the startling conclusion implied by CO observations: “Despite long held beliefs, the Sun may not have a classical chromosphere in magnetic field free internetwork regions at heights below 1 Mm.” The unique advantages of CO in elucidating this new picture are that

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE CO dissociates strongly above 4000 K; the vibration-rotation lines are formed closely in LTE; they have narrow contribution functions that dissect the mean vertical structure of the temperature minimum region. CO is not the only molecule that can probe the transition from the photosphere to the outer atmosphere. Deming et al. (1984) used LTE rotational lines of the OH radical near 11 µm to show that the thermal inhomogeneity seen strongly in CO is present even lower in the atmosphere, at (500 nm) optical depths ~10-2. The molecular observations to date have made progress in directly imaging the “unchromosphere,” in establishing the range of its physical properties, and in demonstrating substantial time variability. However, the measurements are acquired too slowly (due to inadequate flux and under-instrumentation at the focal plane) to resolve the time variations (including pmode oscillations) over a useful field of view (several supergranules). In other words, we have gone some way toward answering “what” and “where” but have barely scratched the surface of “when” and “why.” 3 Other Applications of Infrared Measurements In highlighting two important research areas in which thermal infrared observations play a central role, we short change a broad range of applications. The scope and rapid growth of infrared solar physics can be judged from the two recent volumes, “Infrared Solar Physics ” (Rabin et al. 1994) and “Infrared Tools for Solar Astrophysics: What's Next” (Kuhn and Penn 1995). We confine ourselves here to the briefest reminder that the infrared spectrum offers proven advantages for measuring the “bread-and-butter” quantities of stellar physics: temperature, density, and chemical composition, as well as magnetic field strength. 3.1 Temperature At photospheric temperatures, the intensity of the thermal infrared continuum closely approximates a Rayleigh-Jeans distribution. It is easier to measure temperature in the infrared (including the effects of spatially unresolved fluctuations) because the intensity is linear in temperature. Moreover, a true continuum is often accessible (unlike the heavily-blanketed visible spectrum), and the opacity is dominated by a single, well-understood source (H- free-free absorption). The infrared continuum (1-20 µm) at unit optical depth spans 250 km in height, compared to 40 km in the visible. Kopp and Rabin (1992) used infrared intensities to demonstrate a relationship between temperature and magnetic field strength in sunspots (the lower contrast and stray-light contamination of sunspots in the IR was an additional advantage). Moran et al. (1992) verified that faculae are dark at disk center at wavelengths beyond 1.6 µ m, a property that may also extend to visible wavelengths (as micropores) at high angular resolution (Topka et al. 1992). As discussed above, the CO lines are a sensitive thermometer for cool gas.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE 3.2 Density Like the Zeeman effect, the linear Stark effect is quadratic in wavelength. For hydrogen lines in the mid-infrared region (8-20 µm), the Stark broadening in prominences is so strong that the Stark and Doppler components of the line profile can be separated. Chang and Deming (1997) have used this property to derive order-of-magnitude more accurate temperatures and densities in both active and quiet prominences. Casini and Foukal (1996) have shown that the H I 15-9 transition is sensitive to directed electric fields as small as 0.5 V cm-1 an order of magnitude better than the upper limits from visible-light measurements. This gain in sensitivity is particularly important because it opens the door to electric fields generated by processes that are thought to occur continually – such as MHD wave heating – as opposed to the larger fields that occur only transiently in very small regions (Foukal and Hinata 1991). 3.3 Chemical Composition Accurate values of the solar abundances of C, N, and 0 are fundamental anchor points for the calculation of stellar opacities. Grevesse et al. (1994) stress that the most secure indicators, both atomic and molecular, occur in the infrared spectrum beyond 2 µm; this is even more true for isotopic ratios. In deriving abundances, it is vital to use spatially resolved observations so that each spectrum can be assigned a unique temperature and density. The combination of reasonable (arcsec) angular resolution with the very high signal-to-noise ratios (~103) needed for accurate analysis demands a substantial aperture. 4 The Frontier Science is a balance between understanding what we have seen and opening windows onto the unknown. This document has emphasized the known and tangible applications of solar infrared measurements. Still, it is important to remember how incompletely explored the infrared spectrum still is compared to the visible. For example, almost nothing is known about the infrared spectrum of a solar flare, or about the spectrum of the corona beyond 2.5 µm. The first mid-infrared (8-21 µm) atlas of a sunspot was produced at the McMathPierce only three years ago (Wallace et al. 1994) and quickly led to the identification of the water molecule on the Sun (Wallace et al. 1995). Follow-up analysis published this month in Science (Polyansky et al. 1997; Oka 1997) required a new approach to the solution of the vibration-rotation Schrödinger equation to reproduce the solar spectrum; the authors expect that the solar results will spur similar improvements in the chemical analysis of other hot polyatomic molecules. Still, the diagnostic potential of molecules such as H20, CN, and OH has barely been scratched. Why? Primarily because imaging technology – so crucial to understanding the pervasively inhomogeneous solar atmosphere – has only become available in the infrared during this decade.

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE 5 Technical Requirements The decade of wavelength between 1 and 22 µm is comparable in size and richness to the entire visible spectrum, so it is not surprising that it cannot be reduced to a single objective or figure of merit – comparable, say, to resolving magnetic flux tubes in the visible. Indeed, it will not be practical to achieve very high (0.1 arcsec) angular resolution in the thermal infrared through direct imaging. However, some general statements can be made. First and foremost, the thermal infrared is useless by definition if it cannot be observed. Currently, only one telescope in the world can observe the infrared beyond 2.5 µm with angular resolution sufficient (~1 arcsec at 5 µm) to relate directly to visible-light measurements. A larger aperture would greatly improve the quality and value of wide spectral range observations; but to preclude future access to the thermal infrared altogether would be akin to completely eliminating infrared or radio wavelengths as tools of nighttime astronomy – which is not even a remote possibility. The forefront of angular resolution is ~0.1 arcsec. A three-meter class telescope has a diffraction limit at this scale in the near infrared. The infrared offers the advantage of a larger isoplanatic patch size as one moves toward longer wavelengths. This means that reduced angular resolution is offset by an increasing high-quality field of view. For those investigations that do not need the highest possible angular resolution, but benefit from a big field, the infrared is very attractive. The value of infrared measurements will be maximized when they can be compared directly with visible measurements. The following apertures are required to reach various diffraction limits at three infrared wavelengths (a highly Zeeman-sensitive Fe I line is near 1.56 µm; the fundamental CO vibration-rotation bands are centered around 4.8 µm; the extremely Zeeman-sensitive Mg I lines are near 12.2 µm). Telescope Aperture (m) λ Resolution (arcsec) (µm) 1.0 0.5 0.3 0.1 1.6 0.4 0.8 1.3 4.0 4.8 1.2 2.4 4.0   12.2 3.1 6.1     An aperture of about 3 meters or larger will access the full range of 1-5 µm at 0.5 arcsec or better; moreover, the diffraction limit should be routinely attainable with low-order adaptive optics (even just tip-tilt at the longer wavelengths). Angular resolution much below 1 arcsec is not practical at 12 µm through direct imaging. However, the real power of the 12-µm lines lies in their ability to measure weak fields that are not expected to be concentrated like kilogauss flux tubes in the low photosphere. For studying the role of weak fields in the solar cycle, a 12-µm magnetograph with 1-2 arcsec resolution would ideally complement the next generation of synoptic vector magnetographs represented by SOLIS (which will have 1 arcsec pixels).

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Polarimetry of the 1.6-µm lines from a meter-class telescope in space would be scientifically valuable in combination with visible measurements from the same spacecraft. At longer wavelengths, solar infrared measurements with useful angular resolution are logically done from the ground. The photon flux in the infrared is reasonably well matched to the attainable apertures. For example, the cadence of current CO measurements is too slow by a factor of ~5 to study the dynamical properties of the COmosphere over a reasonable area (~2 arc min). However, currently about 80% of the time is occupied by inefficient image scanning and data transfer. The warm spectrograph is inefficient (~1%) and contributes high thermal background. A 3-m telescope with precise pointing and scanning, coupled to a high (~8%) efficiency cryogenic spectrograph, would provide more than enough flux to increase the cadence by an order of magnitude using 0.25 arcsec pixels. In terms of thermal background, the emissivity of the telescope is not very important for non-coronal observations, but a well-baffled cryogenic spectrograph or high-resolution filtergraph is essential. The relative importance of focal-plane instrumentation in the infrared is even greater than it is in the visible because the technology is not as mature and is improving rapidly. This is one reason why existing telescopes, despite their inadequacies, have by no means exhausted their research potential in the IR. The next generation of 1-5 µm IR arrays (1024 × 1024) framing at 10 Hz) should be in place within the next three years; comparable arrays for the 8-20 µm range may be 3-5 years farther off. 6 Near-Term Scientific Objectives at the McMath-Pierce The McMath-Pierce Telescope can make progress toward all the scientific objectives discussed above, as well as others that were not highlighted. Specifically, we will: investigate the thermodynamics of the photosphere and chromosphere using CO, OH, and He I 1083-nm spectroscopy in conjunction with visible measurements such Ca K imaging; measure the distribution of magnetic field strength down to 100 G outside active regions using simultaneous 12-µm and 1.6-µm imaging magnetometry; measure temperature and magnetic field strength in sunspots using Ti I Zeeman lines, molecules (including H20), and continuum imaging; make photometric measurements of facular contrast in the range 1-5 µm (Moran et al. 1992); obtain infrared (1-22 µm) spectra of flares during the coming rise in solar activity; measure temperature and densities in prominences using the Stark broadening of high-n hydrogen lines; attempt to measure directed electric fields at the level of 1 V cm-1 or below; survey the IR spectrum of the corona above an active region; carry out spectroscopic imaging in lines such as Brackett α (4.05 µm) as a diagnostic of the active chromosphere and flares (Falchi et al. 1994).

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Our next-generation 1-5 µm camera will be based a 1024 × 1024 Aladdin array currently under development by NOAO and Hughes/SBRC in partnership with the USNO. As a proof of concept, high-resolution IR spectroscopy was successfully carried out earlier this year at the McMath-Pierce using the NOAO Phoenix cryogenic spectrometer, which is based on an Aladdin array. Funds have been identified and committed for an NSO array controller to be delivered early in 1999. Long-wavelength work will be carried out with the 1-m FTS and with the Athena 12-µm imaging magnetograph built by NASA/GSFC. Athena will be commissioned in 1998 and permanently stationed at the McMath-Pierce. Like all NSO facilities, the McMath-Pierce is a national resource allocated through peer-reviewed proposals. We expect that scientists will propose to carry out infrared measurements that are not mentioned above because no one has thought of them yet! In addition, the facility carries out externally-supported programs in stellar and solar-system astronomy, upper atmospheric physics and chemical physics, many of which rely on the unique IR capabilities of the McMath-Pierce. References Ayres, T. A., and Rabin, D. 1996, ApJ 460, 1042 Carlsson, M., and Stein, R. F. 1995, ApJ 440, L29 Carlsson, M., and Stein, R. F. 1997, in Solar Magnetic Fields, ed. V. H. Hansteen ( Oslo:U. Norway), 59 Casini, R., and Foukal, P. 1996, Solar Phys. 163,65 Chang, E. S., and Deming, D. 1997, BAAS 29, 906 Deming, D., Hillman, J. J., Kostiuk, T., and Mumma, M. 1984, Solar Phys. 94,57 Falchi, A. Falciani, R., and Mauas, P. 1994, in Infrared Solar Physics (IAU Symp. 154), eds. D. Rabin et al. ( Dordrecht: Kluwer), 113 Foukal, P., and Hinata, S. 1991, Solar Phys. 132,307 Grevesse, N., Sauval, A. J., and Blomme, R. 1994, in Infrared Solar Physics (IAU Symp. 154), eds. D. Rabin et al., ( Dordrecht: Kluwer), 539 Kopp, G., and Rabin, D. 1992, Solar Phys. 141,253 Kuhn, J. R., and Penn, M. J. 1995, Infrared Tools for Solar Astrophysics ( New Jersey: World Scientific) Moran, T., Foukal, P., and Rabin, D. 1992, Solar Phys.142,35 Oka, T. 1997, Science 277, 328 Polyansky, 0. L., Zobov, N. F., Viti, S., Tennyson, J., Bernath, P., and Wallace, L. 1997, Science 277,346 Rabin, D., Jefferies, J. and Lindsey, C., eds. 1994, Infrared Solar Physics (IAU Symp. 154) (Kluwer: Dordrecht) Solanki, S. 1994, in Infrared Solar Physics (IAU Symp. 154), eds. D. Rabin et al. ( Dordrecht: Kluwer), 393 Stenflo, J. 0. 1994, Solar Magnetic Fields ( Dordrecht: Kluwer), 20 Topka, K. P., Tarbell, T. D., and Title, A. M. 1992, ApJ 396, 351 Wallace, L., Bernath, P., Livingston, W., Hinkle, K., Busler, J., Guo, B., and Zhang, K. 1995, Science 268,1155

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GROUND-BASED SOLAR RESEARCH: AN ASSESSMENT AND STRATEGY FOR THE FUTURE Wallace, L., Livingston, W., and Bernath, P. 1994, An Atlas of the Sunspot Spectrum from 470 to 1233 cm-1(8.1 to 21 µm) and the Photospheric Spectrum from 460 to 630 cm-1 (16 to 22 µm) ( Tucson: NSO)

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