Solar Physics Information

Another major scientific advance came in 1817 with Fraunhofer's discovery of 'dark lines' in the spectrum of light from the Sun. Coupled with the understanding of atomic energy levels, studies of the solar spectrum gave us a scientific method to learn the atomic composition of the sun and the temperatures in its outer layers. Solar spectroscopy advanced rapidly late in the 19th century to the development of specialized instruments showing the existence of magnetic fields on the Sun at Mt. Wilson Observatory in 1908.

Solar astronomers continued fruitful studies of the Sun from the ground until the space age dawned in 1946 with rocket observations and then accelerated in 1957 with the launch of Sputnik. Rocket instruments and orbiting satellites allowed observation of the ultraviolet and x-ray regions of the solar spectrum for the first time (since these wavelengths are normally absorbed by the earth's atmosphere and do not reach the ground) and thus opened the way for high temperature astrophysics to be applied to the solar atmosphere where temperature can exceed 1 million degrees. Although our understanding of solar-type stars required ultraviolet radiation from the Sun, such short wavelength emission cannot be observed from the ground because of absorption by the Earth's atmosphere.

Above the photosphere are two additional layers, the chromosphere and corona, which were first identified at eclipses of the Sun by the Moon. The chromosphere is an inhomogeneous layer extending 10,000 km above the photosphere. It is best thought of as the transition from the photosphere to the corona. The very outer extent of the Sun proper is the tenuous corona which can extend several million kilometers into the interplanetary medium. Such extensions of the solar atmosphere produce the striking images seen at the times of solar eclipses.

It is the variation of the visible solar atmosphere that affects the Earth's atmosphere directly as a source of heat as well as a modifier of the its chemistry . If we are to anticipate the effects of solar change on short times of minutes to decades, we must have a clear understanding of how the Sun's atmosphere produces radiation at all wavelength from x-rays to the infrared. Since the solar surface is mottled by darks spots and bright vein-like features called faculae, their combination determines the net variability in the radiation reaching the earth.

The outer solar atmosphere also expels solar gas and magnetism into the interplanetary medium as a steady solar wind as well as transients called coronal mass ejections. This flow of mass from the sun results in variation in the amount of material trapped inside the dipolar magnetic field of the earth. Therefore, today we study the total solar output of radiation and material with the view to understanding its sources inside the Sun as well as their variation and visibility in the outer solar atmosphere. The parallel research to understand the detailed effects of this variability in changing the Earth's atmosphere gives the research strategy behind the larger study of coupling of the outer atmosphere of the Sun with that of the Earth.