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SOHO mission graphic

Solar and Heliospheric Observatory

Phase: Operating

Launch Date: December 02, 1995

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Program(s):Heliophysics Research

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SOHO, the Solar and Heliospheric Observatory, is a project of international cooperation between ESA and NASA to study the Sun, from its deep core to the outer corona, and the solar wind. Together with two other ESA missions, Cluster and Ulysses, SOHO is studying the Sun-Earth interaction from different perspectives. SOHO’s easily accessible, spectacular data and basic science results have captured the imagination of the space science community and the general public alike.

SOHO - Prominence on the Sun
SOHO’s Extreme Ultraviolet Imaging Telescope (EIT) captured this image of a huge, handle-shaped prominence taken on Sept. 14,1999 taken in the 304 angstrom wavelength. Prominences are huge clouds of relatively cool dense plasma suspended in the Sun's hot, thin corona. At times, they can erupt, escaping the Sun's atmosphere.
Emission in this spectral line shows the upper chromosphere at a temperature of about 60,000 degrees K. Every feature in the image traces magnetic field structure. The hottest areas appear almost white, while the darker red areas indicate cooler temperatures.

SOHO was designed to answer the following three fundamental scientific questions about the Sun:

  • What is the structure and dynamics of the solar interior?
  • Why does the solar corona exist and how is it heated to such an extremely high temperature
  • Where is the solar wind produced and how is it accelerated?

Clues on the solar interior come from studying seismic waves that are produced in the turbulent outer shell of the Sun and which appear as ripples on its surface. This technique, called helioseismology, will be extended to other stars by ESA’s forthcoming Eddington mission.

SOHO has provided an unprecedented breadth and depth of information about the Sun, from its interior, through the hot and dynamic atmosphere, to the solar wind and its interaction with the interstellar medium. These findings have been documented in an impressive, still growing body of scientific and popular literature.

Some of the key results include:

  • Revealing the first images ever of a star’s convection zone (its turbulent outer shell) and of the structure of sunspots below the surface.
  • Providing the most detailed and precise measurements of the temperature structure, the interior rotation, and gas flows in the solar interior.
  • Measuring the acceleration of the slow and fast solar wind.
  • Identifying the source regions and acceleration mechanism of the fast solar wind in the magnetically "open" regions at the Sun's poles.
  • Discovering new dynamic solar phenomena such as coronal waves and solar tornadoes.
  • Revolutionizing our ability to forecast space weather, by giving up to three days notice of Earth-directed disturbances, and playing a lead role in the early warning system for space weather.
  • Monitoring the total solar irradiance (the ‘solar constant’) as well as variations in the extreme ultra violet flux, both of which are important to understand the impact of solar variability on Earth’s climate.

Besides watching the Sun, SOHO has become the most prolific discoverer of comets in astronomical history: as of May 2003, more than 620 comets had been found by SOHO.