This review summarizes the main properties of the available theoretical models of the solar photosphere and the semiempirical models based primarily on observed spectra in different wavelength regions. LTE and non-LTE semiempirical models are compared with LTE and non-LTE theoretical (radiative equilibrium) models calculated with and without convective energy transport. Atomic and molecular lines throughout the spectrum affect the model calculations in essential ways. The temperature-minimum region, considered as the upper boundary of the photosphere, is discussed in detail, in view of the substantial non-radiative cooling in this region. Results are shown to illustrate the relationship between average one-dimensional models and Nordlund's three-dimensional convection models.

The degree of thermal heterogeneity at the base of the solar chromosphere is substantially beyond that simulated in the best-available multi-component models; casting serious doubts on inferences drawn from them.

We use NLTE modelling of the alkali resonance lines and of C I and 0 I high-excitation multiplets to test their quality as temperature diagnostics of the upper photosphere, in the context of Avrett's NLTE recovery of the LTE Holweger-Müller model below the temperature minimum and Ayres' bifurcation into hot and cool components above the temperature minimum.

We briefly review the Kiev program for determining oscillator strengths of Fraunhofer lines from the optical solar spectrum, which has recently resulted in a compilation of solar gf-values for 1958 lines from 40 chemical elements (Gurtovenko and Kostik 1989). These gf-values were determined empirically by fitting solar lines using standard plane-parallel LTE modelling. Errors in this modelling propagate into the gf-values; reversely, the deviations in the latter may serve as diagnostics of the modelling and thus of spectral line formation in the solar photosphere. For a small subset, comparison with reliable laboratory data can be made; for other lines there is information in the differences between fits of the line area and of the line depth.

A series of 17 Fourier transform spectra taken at the McMath telescope near disk center in regions of different magnetic field strengths were analyzed. Applying a multi-variate regression analysis magnetic filling factors 0 < α ≥ 0.11 were determined. With α increasing from 0 to 0.11, line bisectors averaged over groups of lines of similar depth are found to show a blue shift decreasing from 0.35 km s–1 to nearly 0.1 km s–1, when referred to the MgI line λ5172.7å. The bisectors of FeII lines exhibit smaller blue shifts than FeI lines. The increase of bisector red shift near the continuum with increasing α, found earlier by Brandt and Solanki (1987), was confirmed and is tentatively interpreted as a manifestation of downdrafts in the vicinity of flux tubes (Deinzer et al., 1984).

A significant increase of line width (typically between 3 and 8%, depending on line strength) and a decrease of line depth is found with increasing filling factor. For strong lines the equivalent width W shows no variation or a slight increase, while for the weaker lines a reduction of W between a few % and > 10% is found.

High resolution observations, theoretical models, and simulations are discovering many new and exciting phenomena in the solar atmosphere. In recent years, there have been a number of very high quality observations of the solar surface and lower photosphere made on the ground at Sacramento Peak Observatory, Pic du Midi, and at the Swedish Solar Observatory, La Palma. In space the Solar Optical Universal Polarimeter (SOUP) has made diffraction limited (30 cm aperture) time sequences completely free from atmospheric disturbances. The recognition that significant progress is possible in non-linear dynamics has encouraged a number of theoretical groups to attack the problem of convection in the solar atmosphere. Two, two and a half, and three dimensional simulations yield the geometry of the flow below the surface and a prediction of the response of the atmosphere above the surface. Models of magnetic flux tubes are now very sophisticated, and modern high resolution observations should be able to test these theories. The development of the technique of Local Correlation Tracking (LCT) has allowed the direct measurement of horizontal velocities in the atmosphere near disk center. The combination of Doppler and LCT measurements allows a direct measurement of the photospheric vector flow field. Measurements from SOUP, Sacramento Peak, Pic du Midi, and La Palma have shown that mesoscale flows cover the surface and that there exist still larger scale flows associated with emerging pores and active regions. Much of the recent experimental and theoretical progress in processing and understanding high resolution data has resulted from the availability of powerful scientific workstations for user interaction, large amounts of memory for image storage, and supercomputers for the massive fluid dynamics calculations. We are now in the very early stages of learning how to use these new computer tools to identify and follow processes in the solar atmosphere.

Until recently the theory of the photospheric structure has had a modest role. This situation has qualitatively changed with the appearance of the numerical simulations of Nordlund [1], Uus [2], Gadun [3], and others.

An observing program in the spectral region of the opacity minimum region near 1650 nm was started at NSO/SP–VTT many years ago and few results were presented. Thanks to an image processing unit connected to a rather conventional TV camera, IR images taken at 1650 nm can be displayed on the TV monitor and images stored on a VCR for subsequent comparison with corresponding images in the visible spectrum. This rather qualitative analysis is substantiated thanks to results coming from one–dimensional time series obtained with an IR pin–hole spectrophotometer. Power spectra of the o.m.r. granulation as well as its temporal behaviour are compared with results obtained using the same method in the optical region of the solar spectrum. Noticeable differences are emerging, including the reduced effect of 5-min oscillations.

Properties of the photospheric bright points associated with magnetic flux tubes are reviewed both in faculae (facular points) and in the photospheric network (network bright points - NBPs) out of active regions. A special attention is given to their size distribution, to their location relative to the granular, mesogranular and supergranular patterns, and to their relation with the small scale magnetic features, both in active and quiet regions. In particular a new granulation movie reveals that NBPs form in large intergranular spaces, compressed by the surrounding granules.

At the center of the solar disk, bright points are much brighter than the mean photosphere; their contrast increases toward the limb up to μ = 0.3 − 0.2 and then decreases to the limb, as it is now widely accepted. But, all the published contrasts are of little significance because of center-to-limb selection effects. New center-to-limb contrast variations of individual network bright points are presented, which take into account the selection effects.

There are two kinds of bright threads in spectrograms of the Ca II II and K lines: the ones in the outer line wing are due to enhancements of the continuum inside granules, while the others in the inner wing are due to bright features in the intergranular regions.

The empirically derived properties of magnetic flux tubes at both ends of the size spectrum, i.e. magnetic elements and sunspots, are reviewed. Emphasis is placed on quantitative results. The following parameters are discussed in greater detail: The strength and structure of the magnetic field, the temperature stratification and the structure of the velocity field.

We present results of an inversion procedure that derives the turbulent velocity, the magnetic field strength, and the temperature stratification of the photospheric layers of solar magnetic fluxtubes from 10 FeI and FeII Stokes V line profiles around 5250 Å and from the continuum contrast. The free parameters of two-dimensional magnetohydrostatic fluxtube models are determined by minimizing the difference between observed and calculated Stokes V parameters in an iterative manner. Results of this inversion procedure applied to observations of a plage and a network region at disk center indicate a temperature deficit (at equal geometrical height) of the fluxtubes at the level of continuum formation and a temperature excess at the highest levels of line formation in general agreement with the latest theoretical fluxtube models.

Data from magnetograph observations, obtained in six Fe I and Mg I lines with the Crimean magnetograph with a spatial resolution of 1 × 1 and 1 × 2 arcsec have been interpreted using the line-ratio method and a two-component model with spatially unresolved fluxtubes and a background field. Magnetograph data obtained by other authors have also been taken into account. The magnetic field strength, its variation with distance from the fluxtube axis, the characteristics of the spectral line profiles, as well as other properties of the fluxtubes have been determined.

Small-scale solar features identifiable on the quiet sun in magnetograms of the line-of-sight component consist of network, intranetwork, ephemeral region magnetic fields, and the elementary bipoles of ephemeral active regions. Network fields are frequently observed to split into smaller fragments and equally often, small fragments are observed to merge or coalesce into larger clumps; this splitting and merging is generally confined to the borders and vertices of the convection cells known as supergranules. Intranetwork magnetic fields originate near the centers of the supergranule convection cells and appear to increase in magnetic flux as they flow in approximate radial patterns towards the boundaries of the cells.

On 29 September 1988, filtergrams of the solar photosphere with excellent resolution (0.3 to 0.5 arcsecond) were obtained at the Swedish Solar Observatory on La Palma, Canary Islands. An outstanding 2.5 hour run of digital filtergram observations was obtained, looking at a small area within an active region near disk center. On 6 August 1987, an 80 minute run of similar observations was obtained at the Vacuum Tower Telescope of the National Solar Observatory at Sacramento Peak. Digital and video movies have been made of Dopplergrams, magnetograms, line center, continuum, and white light images. Several examples of magnetic field emergence and formation of flux tubes can be studied in detail in the movies. The relationship between photospheric bright points, “filigree”, the line center brightness, and the magnetic field has been established for individual images in analysis to date.

Using a time sequence of filtergrams in the magnetically sensitive λ6103A CaI line (with circular polarization measurements), obtained with the SPO Vacuum Tower Telescope and the universal filter we mapped the line of sight velocity and the longitudinal magnetic field in three quiet solar regions. After elimination of the effect of the 5-minute photospheric oscillations we found in two regions of concentrated magnetic field no association with the line of sight velocity, while the third region was associated with small (<300 m/s) downflows.

Active sunspot group n° 4567 was observed with high angular magnification on Sept. 2, 1984, using the instrument FPSS at Meudon Observatory. Within the sunspots umbrae, the longitudinal magnetic field shows a collection of small elements down to the limit of resolution of around 600 km. Between the trailing and leading spots, there were very sharp gradients, both in magnetization (450 gauss over 1000 km) and in Doppler velocity (350 m s-1 over 1 000 km). The line defined by these gradients was the location of a flare. Evershed driving forces may be responsible for building up these gradients and flare.

The state of theoretical description of small-scale concentrated magnetic fields in the solar photosphere (excluding oscillations and wave propagation) is reviewed with emphasis on work done since 1982. The processes which probably lead to the formation of strong fields (flux expulsion, convective collapse) are discussed in some detail and the present understanding of the subsequent (quasi-)equilibrium state is summarized. We consider in particular the magnetic and thermal structure of the basic magnetic flux concentrations (magnetic elements) and stress the importance of radiative transfer effects, e.g. the horizontal heat exchange with the surroundings and the effect of radiation from the hot bottom and walls on the upper layers. Velocity fields within and around magnetic flux concentrations are discussed with emphasis on shift and asymmetry of the observed Stokes V-profiles which have recently been understood in terms of a downflow in the immediate vicinity outside magnetic structures. Reconnection and instabilities are considered as possible destruction processes for magnetic elements.

Multi-dimensional radiative energy transport is coupled self-consistently to magnetohydrostatic solutions for fluxtubes with rotational symmetry. It is shown that the photospheric layers of plage and network fluxtubes are heated by radiation by as much as 300 K at equal geometrical height. The amount of heating depends on the density reduction within the tube. The results are compared with observational data and the most recent semi-empirical model.