SOHO is a joint venture of ESA and NASA, and it is an element of both the ESA Solar Terrestrial Science Programme (STSP) and the ISTP Science Inititiative. ESA will procure the spacecraft in Europe, where it will undergo its final integration and environmental testing, and NASA will provide the launcher, launch services, and the ground segment system to support all pre-launch activities and in-flight operations.

The scientific instruments on-board are provided by the Principal Investigators participating in the project.

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) and dynamics 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. These 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 energies of the solar wind and energetic particles that result from the coronal structures observed by the telescopes. This is done with the help of particle detectors carried by SOHO that sample the solar wind as it passes through it. 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 the outer atmosphere, 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's 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.

Instrumentation

Helioseismology
The helioseismology investigations primarily aim at the study of those parts of the solar oscillations spectrum that cannot be obtained from the ground. The required sensitivity for observing the very low modes (l<6) and the very high modes (l>200) is difficult to achieve from the ground because of noise effects introduced by the Earth's diurnal rotation for the low modes, and the transparency and seeing fluctuations of the Earth's atmosphere for the high modes.

Solar atmosphere remote sensing

The solar atmosphere remote sensing investigations are carried out with a set of telescopes and spectrometers that will produce the data necessary to study the dynamic phenomena that take place in the solar atmosphere at and above the chromosphere. The plasma will be studied by spectroscopic measurements and high resolution images at different levels of the solar atmosphere. Plasma diagnostics obtained with these instruments will provide temperature, density, and velocity measurements of the material in the outer solar atmosphere.

"In situ" measurements

The instruments to measure "in situ" the composition of the solar wind and energetic particles will determine the elemental and isotopic abundances, the ionic charge states and velocity distributions of ions originating in the solar atmosphere. The energy ranges covered will allow the study of the processes of ion acceleration and fractionation under the various conditions that cause their acceleration from the "slow" solar wind through solar flares.

Spacecraft, Orbit, Attitude

The SOHO spacecraft is three-axis stabilized and points to the Sun within an accuracy of 10 arcsec and has a pointing stability of 1 arcsec per 15 minutes interval. It consists of a payload module which accommodates the instruments and a service module carrying the spacecraft subsystems and the solar arrays. The total mass is about 1350kg and 750W power is provided by the solar panels. The payload weighs about 650kg and consumes 350W in orbit.

SOHO will be injected in a halo orbit around the L1 Sun-Earth Lagrangian point, about 1.5 x 10^6km sunward from the Earth. The halo orbit will have a period of 180 days and has been chosen because, 1) it provides a smooth Sun-spacecraft velocity change throughout the orbit, appropriate for helioseismology, 2) is permanently outside of the magnetosphere, appropriate for the "in situ" sampling of the solar wind and particles, and 3) allows permanent observation of the Sun, appropriate for all the investigations. The Sun-spacecraft velocity will be measured with an accuracy better than 0.5 cm/s.

SOHO is designed for a lifetime of two years, but will be equipped with sufficient on-board consumables for an extra fours years.