The Solar Heliospheric Observatory

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The Solar and Heliospheric Observatory (SOHO) is an exciting international collaboration between ESA ( European Space Agency) and NASA (National Aeronautics and Space Administration).  The SOHO website is very colorful, with easily accessible data, spectacular results, and has many basic science explanatory functions.  This project, a satellite launched in 1995, is for the purpose of studying the Sun from its deep core to its outer corona and solar winds.  SOHO is studying the Sun-Earth interactions from different perspectives, along with ESA’s Cluster mission.  Initially proposed for a 2-year mission, the project has been extended five times, currently approved through 2012.  This is a direct result of its spectacular data.

The SOHO spacecraft was built in Europe by prime contractor Matra Marconi Space, now known as EADS Astrium, and an industry team with management by ESA.  It was launched from Cape Canaveral, Florida, USA.

The twelve complementary instruments on board were designed and built by a consortium involving 29 institutes from 15 countries:

Coronal Diagnostic Spectrometer (CDS)
Charge, Element, and Isotope Analysis System (CELIAS)
Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP)
Extreme ultraviolet Imaging Telescope (EIT)
Energetic and Relativistic Nuclei and Electron experiment (ERNE)
Global Oscillations at Low Frequencies (GOLF)
Large Angle and Spectrometric Coronograph (LASCO)
Michelson Doppler Imager/Solar Oscillations Investigation (MDI/SOI)
Solar Ultraviolet Measurements of Emitted Radiation (SUMER)
Solar Wind Anisotropies (SWAN)
UltraViolet Coronograph Spectrometer (UVCS)
Variability of Solar Irradiance and Gravity Oscillations (VIRGO)

SOHO is a three-axis stabilized spacecraft that constantly faces the Sun.  Its design is based on a modular concept with two main elements: the payload module, housing the 12 instrument packages, and the service module, providing essentials such as thrusters, power and communications.  After some difficulties with gyroscope malfunctions in 1998, the craft was retro-fitted  and now the SOHO is the first three-axis stabilized spacecraft operated without gyroscopes, breaking new ground for future spacecraft designs.

Originally designed to answer three fundamental questions about the Sun, it has also become a prolific source for the discovery of comets.  The basic questions the project is geared to address are about what are the structure and dynamics of the solar interior, why does the solar corona exist and how it is heated to the extreme of 1,000,000 degrees C, and where solar wind is produced and how it is accelerated.  The clues about the interior are read by the seismic waves produced in the turbulent solar outer shell and observed as ripples on the surface of the Sun.

Key Results:

• 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.

SOHO moves around the Sun in step with the Earth, by slowly orbiting around the First Lagrangian Point (L1), where the combined gravity of the Earth and Sun keep SOHO in an orbit locked to the Earth-Sun line. The L1 point is approximately four times the distance of the Moon away from the Earth, in the direction of the Sun. There, SOHO enjoys an uninterrupted view of our daylight star.  Its unprecedented compilation of new material has captured the imagination of both the science community and the general public. 

More about this author: Andrea Theisson

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