Spatial Resolution of Solar Total Irradiance Variability: The YOHKOH White-Light Observations

Abstract

The observations from the ACRIM instrument on board the Solar Maximum Mission, and other recent experiments, have allowed us to study the time variations of solar irradiance. These observations identified several mechanisms, ranging from sunspots to global oscillations produced by waves trapped in the solar interior (the p-modes) that cause variations in the total irradiance. In all cases the variations have small amplitudes in integrated light from the visible photosphere. The SXT instrument on board Yohkoh has provided a new data set that can resolve some of these variations spatially. Most of these data are in the form of whole-Sun images: rate about 5 per day; effective pixel size of 4.92 arc sec; passband 28Å centered at 4310Å; data interval beginning in September, 1991 and ending in October, 1992. These observations have several advantages over ground-based observations for the characterization of solar global variability, namely the lack of atmospheric effects and the long-term stability of the instrumentation. We present an assessment of these data in the context of the existing total irradiance data, and also discuss their application to determination of the figure of the Sun, particularly measurements of the variations of the solar radius.