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 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.

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.

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.

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.

Coronal images were taken in the light of the He I 10830 Å line, the 10000 Å continuum, and the Fe XIV 5303 Å line, with the aim of studying the thermal structure of the corona. In addition, spectroscopic observations were made in the violet wavelength region (3760-4060 Å) and near-infrared (10745-10835 Å), to obtain details of physical conditions of the corona, especially of its cool part. The data obtained do not show any distinct cool structures other than ordinary prominences. Some preliminary results concerning the corona and prominence structures are given.