The SOHO satellite is a solar observatory to study: the structure, chemical composition, and dynamics of the solar interior, the structure (density, temperature and velocity fields), dynamics and composition of the outer solar atmosphere, and the solar wind and its relation to the solar atmosphere. To accomplish this, SOHO will carry a set of telescopes to study phenomena initiated by processes commencing below the photosphere, and propagating through the photosphere, chromosphere, and the transition region into the corona. The SOHO instruments are designed to investigate problems such as how the corona is heated and transformed into the solar wind that blows past the Earth at 400 km/s. To do so they will have spectrometers to study the emission and absorption lines produced by the ions present in the different regions of the solar atmosphere. From this information it will be possible to determine densities, temperatures and velocities in the changing structures. These measurements are complemented by the ``in situ'' study of the composition and energy distribution of the solar wind ions and energetic particles that emanate from the coronal structures observed by the telescopes. SOHO will thus greatly enhance our knowledge of the solar wind and its source region.
While the solar interior is the region that generates the kinetic and magnetic energy driving outer atmospheric processes, almost no direct information can be obtained about any region below the photosphere. The neutrinos generated by the nuclear reactions, taking place in the core, are the only direct radiation that reaches us from anything that is below the photosphere. A relatively new technique, helioseismology, has developed in the last two decades that allows us to study the stratification and the dynamical aspects of the solar interior. It uses the study of the acoustic and gravity waves that propagate through the interior of the Sun and can be observed as oscillatory motions of the photosphere. An analysis of these oscillations allows us to determine the characteristics of the resonant cavities in which they resonate, much in the same way as the Earth's seismic waves are used to determine the structure of the Earth interior.
To study the solar interior, SOHO will carry a complement of instruments whose aim is to study the oscillations at the solar surface by measuring the velocity (via the Doppler effect) and intensity changes produced by pressure and gravity waves. The study of such oscillations requires both high resolution imaging and long uninterrupted time series of observations. In addition, because it is paramount to understand the structure of the Sun in relation to the oscillation measurements, the total solar irradiance, or solar constant, and its variations will be measured.