A photometric solar seismograph, as part of an international network, was installed at Yunnan Observatory, the Chinese Academy of Sciences, and put into operation in the spring of 1991. This instrument is used to detect solar oscillations by measuring the continuum radiation intensity on the solar disk with a spatial resolution of 50 arcsec, in heliocentric coordinates, for research on p-modes and g-modes when l < 50, where l is the angular degree of the eigenfunction. The solar oscillations can be observed simultaneously at two wavelengths of the detector with a sensitivity of 10 of the average intensity. This paper reports on the optical system of the instrument. Also introduced in this paper is the compensation system for the noise signals produced by the changes in the transparency of the Earth's atmosphere. This is based on solar photometry at wavelengths of 0.55 mm and 1.6 mm.

The High Resolution Telescope and Spectrograph (HRTS) instrument has obtained broadband spectroheliograph images at 1600 Å of the solar temperature minimum region. I discuss HRTS observations of quiet areas and their relation with magnetic fields, five minute oscillations, and heating. The brightness temperature of solar fine structure elements composing the supergranular network is found to be linearly proportional to the local absolute value of magnetic field strength. There is evidence for a 250-s period oscillation occurring in 10-arcsec scale patches, which however is energetically unimportant to the local heating budget. A general nonmagnetic background heating and five minute oscillations occur globally, while the network bright points occur in magnetic regions, heated perhaps from partial dissipation of Alfvén waves (whose energy flux is linearly proportional to B) in individual elemental 1500-G (at the photosphere) flux tubes which expand to form the temperature minimum fine structure bright points.

The 12 μm emission lines were unexpectedly detected about a decade ago. Great progress has been made in understanding the atomic physics underlying these high-l Rydberg transitions in Mg I and other atoms. In a magnetic field, their Landé g factor is shown to be unity. At disk center, the shift of the absorption trough relative to the emission peak is demonstrated to be due to the quadratic Stark Effect, permitting measurement of the photospheric electric field strengths. Other related lines of Mg I require accurate atomic fine structure data to interpret properly their complex line profiles. Related lines are found in the ATMOS spectra for C I, Na I, Al I, Si I, Ca I, and Fe I, in addition to H I.

We review the formation of infrared solar spectral lines from highly excited levels in neutral atoms. The lines of Mg I and H I are the most interesting ones. We explain the NLTE processes by which they are affected and we study the sensitivity of the Mg I 12 μm lines to granulation and to flux tubes.

The emission lines of Mg I at 7.4, 12.2, and 12.3 μm are now known to be formed in the upper photosphere; the line emission is due to collisional coupling of higher levels with the continuum together with radiative depopulation of lower levels. These combined effects cause the line source functions of high-lying transitions to exceed the corresponding Planck functions. However, there are uncertainties in a) the relevant atomic data, particularly the collisional rates and ultraviolet photoionization rates, and b) the sensitivity of the calculated results to changes in atmospheric temperature and density. These uncertainties are examined by comparing twelve calculated Mg I line profiles in the range 2.1-12.3 μm with ATMOS satellite observations. We show results based on different rates, and using different atmospheric models representing a range of dark and bright spatial features. The calculated Mg profiles are found to be relatively insensitive to atmospheric model changes, and to depend critically on the choice of collisional and photoionization rates. We find better agreement with the observations using collision rates from van Regemorter (1962) rather than from Seaton (1962). We also compare twelve calculated hydrogen profiles in the range 2.2-12.4 μm with ATMOS observations. The available rates and cross sections for hydrogen seem adequate to account for the observed profiles, while the calculated lines are highly sensitive to atmospheric model changes. These lines are perhaps the best available diagnostics of the temperature and density structure of the photosphere and low chromosphere. Further calculations based on these infrared hydrogen lines should lead to greatly improved models of the solar atmosphere.

We compare infrared hydrogen lines observed with ATMOS with computations for two models of the solar atmosphere, one without and one with a chromosphere. The weaker H I lines are formed in the photosphere. Proper evaluation of Stark broadening is required to reproduce their profiles; the heavy ion contribution is most important. The cores of the stronger lines are sensitive to the structure of the chromosphere, but detailed NLTE modeling is needed for diagnostic applications.

Theoretical oscillator strengths are reported for transitions between excited terms of the singlet and triplet S, P, D, F and G manifolds of atomic magnesium. The principal quantum numbers of the jumping electron range from n = 4 to 100. The calculations are based on the Coulomb approximation. Low-n results, for which other data are available previously, agree reasonably well with the sophisticated calculations of Moccia and Spizzo (1988). Data for 1,3 F–1,3 G transitions may deviate from hydrogenic values by as much as 50%.

We have investigated the impact broadening of the 12 μm lines. Ion broadening is found to follow the adiabatic theory, whereas electron broadening needs a non-adiabatic treatment. This relaxes the fixed width/shift ratio, as found by Chang and Schoenfeld (1991), using a pure adiabatic analysis.

We have identified the Fe I 4f–5g lines at 4 μm in the ATMOS solar spectra. Using the polarization model as previously applied to silicon, we predict and identify the 5g–6h lines at 7 μm. Additional absorption features at 2.5 and 12 μm are also shown to be due to high-l Rydberg transitions in Fe I.

This paper reports the observation of high-n lines in emission from n = 12-11, 13-12, 14-13 and 16-15 Rydberg transitions in H, Mg and Si in solar far IR spectra taken from balloon altitudes, in which the H I line intensities are found to exceed those from the heavier elements. Tentative identification is also made of the n = 8-7 hydrogen line in emission on 20 μm spectra taken from Mauna Kea. The characteristics of the hydrogen lines are compared with lower-n transitions seen in the Space Shuttle ATMOS spectra, in which Brackett, Pfund and n = 6 lines with Δn = 1, 2, 3 and 4 are seen as broad absorption features, while the n = 7-6 line shows a small emission peak within a broader absorption line and the n = 9-7, and possibly the 11-8, transitions appear as weak emission lines. These results indicate that the transformation from absorption to emission occurs at longer wavelengths for hydrogen lines than for those of heavier elements.

A two-beam Martin-Puplett polarizing interferometer has been used in the rapid-scan mode on the 15 meter JCMT in conjunction with the facility detector, UKT14, to survey the solar sub-millimeter and millimeter spectrum in the four wavebands at 7-11, 11-15, 21-24 and 27-30 cm–1 to a spectral resolution of 0.01 cm–1 and at spatial resolutions of 19″, 16″, 7″ and 6″, respectively. Overall atmospheric transmission through these windows has been evaluated by comparison with synthetic spectra generated with FASCOD/HITRAN. A search has been made for contributions to these spectra from high-n transitions of H and heavier elements by several methods, including the comparison of solar with lunar and limb with disk center spectra.

Recent polarimetric observations of the 12.32-μm emission line have provided the observational basis for deriving vector magnetic fields in the upper photosphere with great sensitivity. We use a line source function from the non-LTE model of Carlsson, Rutten and Shchukina, and calculate the radiative transfer of the Stokes I, Q, U, and V profiles. The results show that the profiles are not significantly affected by magneto-optical effects or by saturation, and reliable vector fields can be extracted by simply fitting the Seares relations to the Stokes profiles. Vector field observations for sunspots have shown that the field extends well beyond the photometric boundary of the sunspot, but that the field strength at the penumbral/photospheric boundary is less than half of the sunspot-center value. Within a mature sunspot, the 12-μm line profiles contain essentially no unpolarized radiation, indicating that the field is not intermittent in the sense of containing discrete flux tubes separated by field-free regions. We describe the design of a 12-μm Stokes polarimeter incorporating a high-resolution Fabry-Perot etalon and a 128 × 128 infrared array detector.

An overview is given of the structure and the physics of magnetic features in solar plages, as derived from observations of near-infrared lines. First, the diagnostic potential of near-infrared lines is compared with that of lines in the visible and at 12 μm. Then, the results on the magnetic and velocity structure of magnetic features obtained from 1.5 μm lines are described, discussed and compared with results of observations in the visible and with theoretical predictions. Finally, the past and present achievements of near-infrared investigations of Zeeman-split lines are summarized.

Two types of model calculations for small scale magnetic flux tubes in the solar atmosphere are reviewed. In the first kind, one follows the temporal evolution governed by the complete set of the MHD and radiative transfer equations to a (quasi) stationary solution. From such a solution the continuum contrasts of a photospheric flux tube in the visible and in the infrared continuum at 1.6 μm have been computed and are briefly discussed. The second, more empirical type of method assumes the flux tubes to be in magnetohydrostatic equilibrium. It is computationally faster and more flexible and allows us to explore a wide range of parameters. Models and insights obtained from such parameter studies are discussed in some detail. These include an explanation for the peculiar variation of the area asymmetry of Stokes V profiles across the solar disk in terms of mass motions in the surroundings of magnetic flux tubes.

Furthermore, a two-dimensional model of the lower chromosphere that has been developed is presented. Emphasis is laid on the effect of thermal bifurcation of the lower chromosphere on the structure of the chromospheric magnetic field. If the cool carbon monoxide clouds, observed in the infrared, occupy the non-magnetic regions, the flux tubes expand very strongly and form a magnetic canopy with an almost horizontal base. This has consequences for the spatial distribution of the Ca II K spectral line emission.

Finally, some consideration is given to the formation and destruction of intense magnetic flux tubes in the solar photosphere. The formation is described as a consequence of the flux expulsion process that leads to a convective instability. A possible observational signature of this mechanism is proposed.

The continuum intensity observations of sunspot umbrae and penumbrae in the visible and infrared are reviewed. The intensity in the darkest part of the umbra and the average penumbral intensity are known with relatively high accuracy in large sunspots. The importance of including infrared observations in the construction of semi-empirical sunspot models is emphasized.

Magnetic field measurements are discussed. Special attention is given to recent high-spatial-resolution observations that show large fluctuations in magnetic field inclination, suggesting that the sunspot magnetic field changes its inclination – but not its magnitude – between bright and dark penumbral features.

I review the advantages, techniques, and results of measurement of magnetic fields on cool stars in the infrared (IR). These measurements have generated several important results, including the following: the first data on the magnetic parameters of dMe and RS CVn variables; evidence for field strength confinement by photospheric gas pressure; support for the correlation between magnetic flux and rotation, with possible saturation at high rotation rates; indications of horizontal and/or vertical magnetic field structure; and evidence of spatial variations in B over a stellar surface. I discuss these results in detail, and suggest future directions for IR magnetic field research.

Infrared array detectors are a new and promising tool for investigating the properties of magnetic field concentrations in the solar photosphere. Array measurements provide large statistical samples of polarized line profiles and display the spatial organization of the magnetic field. The wavelength region near 1.6 μm has important advantages for magnetometry: spectral lines with magnetic sensitivites ranging from low to very high; low continuum opacity; high continuum flux; and the possibility of sub-arcsecond angular resolution with existing telescopes. Initial results have extended earlier work on the distribution of field strength and flux in plages and revealed new properties specifically connected with spatial structure. The quality and flexibility of near infrared magnetographs can be expected to improve rapidly.

The interchange instability of solar magnetic flux tubes and possible stabilization mechanisms are reviewed. Special attention is paid to the influence of magnetic tension forces and the internal atmosphere, both of which were neglected in earlier studies of this instability. It is found that whirl flows with velocities of only 2.2 km s–1 are strong enough to stabilize the flux tubes. However, their absence or the excitation of other instabilities might lead to a shredding of the tubes. The observability of such a scenario in the infrared is briefly discussed.

Subsurface filamentation of sunspot magnetic fields has been postulated as a source of the visible small-scale structure of sunspot umbrae. We examine this possibility by investigating the magnetohydrodynamic structure of thin, vertical magnetized gas columns embedded in sunspot umbrae. These umbral fluxtubes are assumed to have weaker field strengths than the surrounding umbral atmosphere. The steady-state magnetohydrodynamic equations are solved numerically in the slender fluxtube approximation, thus allowing for stationary internal mass flows. We include the radiative exchange of heat between the umbral fluxtube and the ambient medium using a simple relaxation time approach.

The geometric shape of the steady flow solution is a gas column converging with height. We discuss the relationship of our results to observed properties of umbral brightenings (umbral dots). We show that, even if there is a large difference in magnetic field strength between the dot and the ambient medium in deeper layers, the field strengths are nearly equal in the observable layers, a result required by the observations. We also show that either high temperatures at the lower boundary of the dots or strong upflows are needed in order to produce bright continuum structures.

The magnetic structure of a simple, relatively symmetric sunspot is determined using the extremely Zeeman sensitive Landé g = 3 line of Fe I at 1.5648 μm. From the measured strength and inclination of the magnetic field we estimate the fraction of the total magnetic flux of the sunspot passing through the solar surface in the penumbra. It is found that on average approximately 1/2–2/3 of the total magnetic flux of the spot emerges in the penumbra. Sunspot penumbrae are therefore deep, i.e., the τ = 1 level does not correspond to the lower magnetic boundary of the spot in its penumbra.