The Solar and Heliospheric Observatory (SOHO) is a spacecraft launched in 1995 to study the sun. It is a joint project of the European Space Agency (ESA) and NASA.

SOHO Observatory

Organization	ESA, NASA
Wavelength regime	optical through UV, also magnetic information
Orbit height	1.5×106 km (heliocentric, sunwards at L1)
Orbit period	1 year
Launch date	2 December 1995
Deorbit date	(on going)
Mass	1850 kg (610 kg payload)
Webpage	http://sohowww.nascom.nasa.gov/
Instruments
GOLF	core oscillations
VIRGO	core oscillations
MDI	convection zone oscillations
SUMER	corona characteristics
CDS	corona characteristics
EIT	UV, low corona
UVCS	UV inner corona spectrograph
LASCO	outer corona spectrograph
SWAN	solar wind density
CELIAS	solar wind ions
COSTEP	solar wind ions
ERNE	solar wind ions

The 610 kg SOHO spacecraft orbits the L1 Lagrange point, the point between the Earth and the Sun where the balance of the (larger) Sun's gravity and the (smaller) Earth's gravity is equal to the centripetal force needed for an object to have the same orbital period in its orbit around the Sun as the Earth, with the result that the object will stay in that relative position. It is about 1.5 million kilometres from the Earth. Gravity from the Sun is 2% (118 µm/s²) more than at the Earth (5.9 mm/s²), while the reduction of required centripetal force is half of this (59 µm/s²). The sum of both effects is balanced by the gravity of the Earth, which is here also 177 µm/s².

Although sometimes described as being at L1, the SOHO satellite is not exactly at L1 as this would make communication difficult (an antenna on Earth focused on SOHO would be "looking" right into the Sun) and is not a stable orbit. Rather it lies in the (constantly moving) plane which passes through L1 and is perpendicular to the line connecting the sun and the Earth. It stays in this plane, describing on the plane an elliptical orbit centered about L1. 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.

Instruments

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.

Large solar flare recorded by SOHO EIT304 instrument in the ultraviolet. (Animation (980kB MPEG)).

Observations from some of the instruments can be formatted as images, many of which are also readily available on the internet for either public or research use (see the official website (http://sohowww.nascom.nasa.gov/data/realtime-images.html)). 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 (Hα) to extreme ultraviolet (UV). Images taken partly or exclusively with non-visible wavelengths are shown on the SOHO page and elsewhere in false color. Unlike many space-based and ground telescopes, there is no time allocated for proposals: due to the precise nature of this space observatory, there is no need for such a process.

See also: solar astronomy, space observatory

External links

• A Description of the SOHO Mission (http://sohowww.nascom.nasa.gov/mission)
• Latest SOHO Images (http://sohowww.nascom.nasa.gov/data/latestimages.html), free to use for educational and non-commercial purposes[1] (http://sohowww.nascom.nasa.gov/data/summary/copyright.html)
• Space Weather Now (http://www.sec.noaa.gov/SWN/)

de:Soho fr:SOHO