Solar-cycle temperature variations in sunspots

Abstract

The observed umbra/photosphere intensity ratio varies from the beginning to the end of each solar cycle by ∼ 30% at 1.6 μm and by factors > 2 at visible wavelengths. We present the intensity ratios measured in 10 wavelength bands extending from 0.387 to 2.35 μ m for 22 large sunspots observed during the period 1968-82, thus covering most of solar cycles 20 and 21. These results together with new observations of umbral limb darkening, and available data on photospheric absolute intensities, are used to estimate the dependence of the relative umbral intensity, and the absolute umbral intensity, on wavelength, heliocentric angle, and phase of the solar cycle. These umbral intensities are used to determine preliminary sunspot models which show the temperature as a function of depth in early, mid-, and late phases of the solar cycle. In the model calculations we use an extensive new compilation of atomic and molecular line data, allowing us to carry out the analysis by means of a detailed synthesis of the observed spectral bands.