The Sun has been the object of close scrutiny for many centuries. An occasional glance at its surface shows many unusual features. The more persistent observer will notice rapid changes in these features, as well as many recurrent events. The Sun is usually viewed in two distinct ways. One is the global view in which the Sun has evolved stably over billions of years and can be compared to a number of other nearby stars of a similar character. The other view is very localized and focuses on specific aspects of the Sun, such as its rate of rotation (both on the surface and in the interior) and the presence of sunspots. The study of solar variability requires a bridge between these two views. Using the detailed physics of many isolated features, can a picture be developed of how the Sun varies on time scales of decades to centuries and beyond?
One principal record of solar variability is called the solar irradiance: the total amount of energy per second per unit angle, normalized to 1 AU (Astronomical Unit, the average Sun-Earth distance). This quantity has been directly available with the required sensitivity only since 1979, when the satellite Nimbus-7 began a series of measurements that continued until recently. These measurements were augmented in 1980 with the activation of the ACRIM (Active Cavity Radiometer Irradiance Monitor) experiment on the Solar Maximum Mission, then with the Earth Radiation Budget Experiment (ERBE), and currently with ACRIM II on the UARS (Upper Atmospheric Research Satellite). The most striking result of these measurements is that the solar irradiance varies by about 0.1% on time scales of at least a decade.
To examine what solar variations actually exist we will first take a look at the Sun, discuss what we see, and then try to piece together an understanding of what we can infer about future solar change.
This image of the Sun, taken on 1 January 1958 at Mt. Wilson Solar
Observatory, through a filter that shows the regions
of emission of ionized calcium, clearly shows magnetic active regions,
sunspots, the magnetic network, and faculae on the solar surface. These regions are responsible for
many of the variations in the outer atmosphere of the Sun and the
disturbances that eventually reach the Earth.
Last revised: Wed May 17 11:28:21 MDT 2000