A brief overview of the field of infrared solar physics is given with particular reference to the topics to be considered during the Symposium.

The early 1990's heralded the deployment of vastly improved space instruments in the ultraviolet (HST) and X-ray (ROSAT) bands, where thermal inhomogeneities in high-excitation chromospheres and coronae are seen in their most favorable light. The infrared spectrum provides a key complementary view of inhomogeneities, but only recently has begun to be seriously exploited for studies of the solar chromosphere and the outer atmospheres of other late-type stars.

Imaging of active regions in continuum around 1.6 μm shows that many facular regions are less bright than the photosphere when observed nearer to disk center than μ = cos θ ~ 0.75. The contrast of these dark faculae increases with magnetic flux above a threshold of approximately 2 × 1018 Mx. This explains why not all faculae are dark at 1.6 μm, since the magnetic flux density in many regions of bright Ca K plage emission falls below this threshold. After correction for blurring, the typical contrast value is about 4-5%, so the brightness temperature deficit is about 130 K. Faculae are brighter than the photosphere at 1.63 μm nearer to the limb than μ ~ 0.5. The negative contrast of dark faculae may arise from cooling of the surrounding photosphere, or from increased visibility of cool layers of the facular flux tube itself. Quantitative comparison of these IR data with MHD models awaits calculation of flux tube contrasts at realistic angular resolution.

An investigation of a solar plage based on observations of UV, visible, and IR spectral lines is presented. Using model calculations, we have established distributions of the main physical parameters (temperature, density, turbulent velocity) in both plage and quiet-Sun atmospheres. Details of the models are presented and the role of magnetic pressure is discussed.

One-dimensional hydrostatic-equilibrium models are shown here for faint, average, and bright components of the quiet Sun, and for a plage region, describing in each case how the atmosphere is stratified through the photosphere, chromosphere, and transition region up to a temperature of 105 K. The observed coronal line radiation is assumed to be the inward incident radiation at the 105 K boundary. This coronal radiation penetrates into the upper chromosphere causing sufficient helium ionization to populate the lower level of the He I 10830 Å line, producing optically-thin absorption of the photospheric continuum at 10830 Å. The amount of absorption, which is proportional to the optical thickness of the upper chromosphere in the 10830 line, depends on 1) the strength of the coronal lines at wavelengths in the He I 504 Å ionizing continuum, and 2) the density and geometrical thickness of the upper chromosphere. The computed 10830 Å line is shown for the four atmospheric models and for three values of the coronal illumination. The calculated off-limb 10830 intensity distribution shows a minimum in the low chromosphere and a maximum at roughly 2000 km above the photosphere, in general agreement with observations, indicating that this is the predominant height of the transition region over most of the solar surface.

Spectra-spectroheliograms obtained in the 10830 Å line with the NASA/NSO Spectromagnetograph are analyzed to produce images in equivalent width, line depth, velocity, and continuum intensity. These and other images imply that if large-scale deficits in coronal irradiation induce weakening of the 10830 line, the ionizing radiation must be produced in close enough spatial proximity to the region of formation for 10830 to allow sharply defined spatial boundaries. The complex morphology of the images shows the importance of underlying chromospheric structure in providing a highly variable “substrate” which is modulated by varying illumination. Clear evidence is seen of He I 10830 Å formation in magnetic loops where the velocity field, particularly at footpoints, directly affects the excitation of the line. Steady flows as seen by Lites et al. (1985) are also observed, particularly near the limb. The observations and radiative transfer calculations are consistent with a simple, optically thin “cloud” model of line formation, and observed line profiles in weak network elements compare favorably with predictions from mean models.

The He I 10830 Å triplet gives a unique view of the solar chromosphere. Digital spectroheliograms have been made regularly since early 1974 using this line and the NSO Vacuum Telescope on Kitt Peak. For many purposes (detection of coronal holes, giant two-ribbon flares, and dark point events) these images are sufficient. A Sun-as-a-star signal is also produced by averaging all the pixels in each daily image. To calibrate this ‘irradiance’ signal in terms of line equivalent width, a comparison is made with integrated sunlight spectrophotometric measurements obtained less frequently. After correction for the effects of water vapor blends, we find a linear relation between the two measurements. The daily averages have been assembled into a time series covering nearly two solar cycles. This time series shows cycle modulation of about ±30% and rotational modulation of about ±10%. The general variation is similar to that of other activity indices but with some interesting small differences. Since images are available, it has been possible to decompose the full disk index into components due to plages, filaments, coronal holes and background. At all times during the cycle, most of the signal comes from the background but most of the variability from plages.

In continuation of a series of studies devoted to the dynamics of the solar photosphere and chromosphere we have attempted to further extend the range of heights in the atmosphere towards the transition region by including observations of the He I 10830 line. We have recorded simultaneous time series of He I 10830 and Mg I 8807 spectra in the quiet solar atmosphere using the echelle spectrograph at the German Vacuum Tower Telescope in Izaña, Tenerife. The velocity signal derived from the Doppler shifts of He 10830 clearly reveals oscillatory motions. The intensity of He 10830, on the other hand, is hardly affected by the oscillations. In the cell interior the 3-min oscillations prevail. Longer periods are found ain the cell boundaries of the chromospheric network where He absorption is enhanced. The V–V phase difference spectrum between the oscillations of He 10830 and those of Mg 8807 confirms previous observations of a non-propagating component that dominates the acoustic wave spectrum in the chromosphere.

The characteristics of large-scale two-ribbon flares and dark points observed in He I λ10830 are summarized and compared with observations of these phenomena at other wavelengths and with the underlying magnetic field.

This paper gives a description of the technique and equipment used to make solar observations in the near IR region with a coronagraph. Examination of active-region spectra in the neighborhood of the He 110830 Å line confirm that the the line is enhanced in sunspot umbrae. The line profiles of umbrae and faculae differ qualitatively. Flares show a decrease in the line depth in comparison to faculae, and in flare kernels the line undergoes an emission reversal. Simultaneous observations of spectra of chromospheric spicules at two heights above the solar surface reveal that the half-width of the 10830 line increases with height.

We examine the potential for observations of the solar corona using forbidden lines in the near infrared.

Continuum observations in the far IR have given us a broad spectrum of new and powerful diagnostic utilities for the solar atmosphere. The infrared continuum is formed in LTE with thermal free electrons by free-free interactions. This gives us a flexible and accurate atmospheric thermometer that has made infrared measurements fundamental to modeling of the quiet solar medium for more than two decades. The submillimeter and millimeter continua are particularly useful with respect to thermal diagnostics of the low chromospheric temperature minimum, where non-radiative heating of the solar medium becomes clearly manifest. Modern submillimeter telescopes and instrumentation on Mauna Kea, in Hawaii, are now revolutionizing solar observations in the submillimeter spectrum, giving us the first observations of detail on the scale of the chromospheric supergranular network, sunspots and prominences. These observations are showing us a remarkable and unexpected view of thermal structure that emerges as one probes to successively higher levels above the chromospheric temperature minimum.

Solar flare observations in the sub-mm spectral bands are essentially non-existent. There is evidence that some solar bursts exhibit a spectral component rising in intensity towards wavelengths shorter than 3 mm, displaying fast sub-second pulses at different repetition rates. On the other hand, the spectral features of white light flares are also unknown in the infra-red range of frequencies. In both wavelength ranges the physics of the emission processes may involve particles accelerated to high energies. The diagnostics of solar flare continuum emission in the IR and sub-mm spectral regions will provide crucial tests on various flare models and bring some clues on the initial primary energy release mechanisms. We propose the construction and operation of a ground-based telescope, operating at two sub-millimeter wavelengths (at about 210 GHz and 405 GHz), with high time resolution (one millisecond), capable of determining the spatial position of burst emission centroids with high definition (a few arcseconds) using the multiple beam technique. Final installation and operation at a high-altitude site in the Argentinian Andes mountains are planned in a joint cooperation with Argentina's Instituto de Astronomia y Fisica del Espacio, IAFE (M. Rovira and associates) and Complejo Astronomico El Leoncito, CASLEO, San Juan (H. Levato and associates); and Switzerland's University of Bern, Institute of Applied Physics, IAP, Bern (A. Magun and associates).

The mechanisms for emission in the submillimeter and far-infrared (1011 and 1013 Hz) regions by solar flares and expected fluxes at these frequencies are described and evaluated. These inferences are based on observations of flare emission at other frequencies and on models for these emissions. In the impulsive phase, non-thermal synchrotron emission by electrons responsible for > 10 MeV gamma-ray emission can give rise to significant radiation in the 1011 to 1013 Hz region from large flares. Free-free or thermal gyrosynchrotron from the hot plasma responsible for the gradual soft X-ray emission can produce significant radiation in the 1011 to 1013 Hz range. However, only radiation in the lower end of this range would have a brightness temperature exceeding the quiet sun brightness.

We give a critical review of the observations of solar activity in the IR and sub-mm range, which are quite scarce, except for the Fe I triplet at 1.56 μm and the Mg I emission lines at 12.32 μm. These, however, are mainly intended for solar magnetic field studies rather than physical diagnostics on activity phenomena. We compute the emission in some continuuum windows and in some detectable Paschen and Brackett lines in two extreme flare models, viz. a “chromospheric” and a white-light flare model. The utility of the Paschen and Brackett lines as diagnostics of the atmospheric state is questionable since more information can be obtained more easily by observing the higher Balmer lines. On the other hand, observations in various continuum windows can be of high scientific value and efficiency. We also discuss possible coordination with simultaneous visible observations, in order to increase the diagnostic efficiency of a prospective observing run.