The 610kg SOHO spacecraft orbits the L1 Lagrange point, the point between the Earth and the Sun where the Earth's gravity exactly counterbalances the Sun's, which is about 1.5 million kilometres from the Earth. Although sometimes described as being at L1, the SOHO satellite is not exactly at L1 as this would make communication difficult and is not a stable orbit. Rather it orbits L1 once every six months, while L1 itself orbits the sun every 12 months as a direct consequence of the motion of the Earth. This keeps SOHO at a good position for communication with Earth at all times.
In normal operation the spacecraft transmits a continuous 200Kb/s data stream of photographs and other measurements via the NASA deep space network of receiving stations. SOHO's data about solar activity are used to predict solar flares, so electrical grids and satellites can be protected from their damaging effects.
SOHO contains twelve main instruments, each capable of independently observing the sun or parts of the sun. These are:
- Global Oscillations at Low Frequencies (GOLF) which measures velocity and magnetic field variations of the whole solar disk to explore the core of the sun.
- Variability of Solar Irradiance (VIRGO) which measures oscillations and solar constant both of the whole solar disk and at low resolution, again exploring the core of the sun.
- Michelson Doppler Imager (MDI) which measures velocity oscillations at high resolution, gaining information about the convection zone which forms the outer layer of the interior of the sun.
- Solar UV Measurement of Emitted radiation (SUMER) which measures plasma flows, temperature and density in the corona.
- Coronial Diagnostic Spectrometer (CDS) which measures density, temperature and flows in the corona.
- Extreme UV Imaging Telescope (EIT) which studies the low coronial structure and activity.
- UV Coronagraph and Spectrometer (UVCS) which measures density and temperature in the corona.
- Large Angle Spectrometer Coronagraph (LASCO) which studies the structure and evolution of the corona
- Solar Wind Anisotropies (SWAN) which uses telescopes sensitive to a characteristic wavelength of hydrogen to measure the solar wind mass flux, map the density of the heliosphere, and observe the large-scale structure of the solar wind streams.
- Charge, Element, Isotope Analysis (CELIAS) which studies the ion composition of the solar wind.
- Suprathermal & Energetic Particle Analyser (COSTEP) which studies the ion and electron composition of the solar wind.
- Energetic Particle Analyser (ERNE) which studies the ion and electron composition of the solar wind.
Some of these observations can be formatted as images, many of which are also readily available on the internet, see the official website. Others such as spectra and measurements of particles in the solar wind do not lend themselves so readily to this. These images range in wavelength or frequency from optical (H) to extreme ultraviolet (UV). Images taken partly or exclusively with non-visible wavelengths are shown on the SOHO page and elsewhere in false color.
See also solar astronomy