In the early 1960s, radial oscillations of the photosphere were detected that have since been explained as the resonant trapping of acoustic waves between certain layers of the convection zone of the Sun. Such studies are enabling scientists to probe the density, temperature, and velocity patterns of the Sun's layers hidden below the photosphere.
A sophisticated satellite, the Solar and Heliospheric Observatory ( SOHO), was launched on Dec. 2, 1995, to study the Sun during a relatively quiet period in the cycle of solar activity. As it circles the first stable point in Earth's orbital path around the Sun, its instruments have obtained the first images of the extended outer corona, along with views of gigantic "tornadoes" near the solar poles. The research program is being conducted jointly by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA); Japan is also taking part in this investigation into the physics of the Sun-Earth relationship.