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The SOHO Satellite

Diagram showing the position of the SOHO satellite

The Solar and Heliospheric Observatory (SOHO) is a joint venture between the European Space Agency (ESA) and NASA. It was launched on December 2nd 1995 (near the minimum of the 11 year sunspot cycle) and was originally going to be de- commissioned in 1998, but has been so successful that both the ESA and NASA decided to extend its life until 2003. The spacecraft was designed to study the Sun's outer atmospheric layers, its internal structure (helioseismology) and the origin and nature of the solar wind, using a variety of 12 different instruments. It is hoped that the information and data SOHO will take during its lifetime will help to solve some of the remaining mysteries surrounding the Sun and its activity, as well as helping us understand the effect it has on our environment. The aim of SOHO was to give Astronomers and Physicists alike the opportunity to study the Sun with an un-interrupted view. This was done by putting the spacecraft in an orbit some 1.5 million kilometres from Earth (towards the Sun). This point is referred to as the L1 Lagrangian Point (see above picture), and is where the gravitational field of the Sun is in balance with the gravitational field of Earth, keeping the spacecraft in an orbit continuously in view of the Sun. As said above, SOHO consists of 12 instruments, each designed to study one of three specific areas: the solar corona, helioseismology and energy particles. I am now going to say a bit about each instrument and what it has discovered so far. CDS (Coronal Diagnostics Spectrometer): Measures intensities of the corona over a wide range of sharply defined wavelengths. It can find a lot about densities and temperatures of gases.

EIT (Extreme UV Imaging Telescope) : Observes the corona at four different ultraviolet wavelengths to highlight atmospheric gases at different temperatures, ranging from 80,000K to 2 million K. LASCO (Large Angle and Spectrometric Coronagraph) : Images the corona using a set of three coronagraphs which take images from 1.1 to 32 solar radii (from 140,000km out to 20 million km). Its main aims are to find out why the corona has such a high temperature and how the solar wind is accelerated to high speeds. SUMER (Solar Ultraviolet Measurement of Emitted Radiation) : Designed to investigate magnetic activity in the chromosphere, corona and the transition region, plasma flow, densities and temperatures. It will take images of the upper chromosphere and corona in extreme ultraviolet. UVCS (UltraViolet Coronagraph Spectrometer) : Studies the solar corona and generation of the solar wind. Unlike LASCO, it will make measurements and images between 2 and 10 solar radii with high spectral and spatial resolution. GOLF (Global Oscillations at Low Frequencies) : Studies internal structure of the Sun by measuring oscillations in frequency range 10-7 � 10-2 Hz. MDI/SOI (Michelson Doppler Imager/Solar Oscillations Investigation): Studies the internal structure and dynamics of the Sun. This instrument is responsible for detecting jets streams of plasma inside the Sun as well as observing sun quakes as a result of a solar flare producing seismic waves. VIRGO (Variability of Solar Irradiance and Gravity Oscillations): Studies variations in brightness of the Sun due to the oscillations taking place. It also measures the diameters of the poles and the equator. CELIAS (Charge ELement and Isotope Analysis System) : Studies the composition of the solar wind and solar and interplanetary energetic particles. COSTEP (Comprehensive SupraThermal and Energetic Particle Analyser) :Studies steady state processes in the solar atmosphere, energy release and particle acceleration in the atmosphere and samples the atmospheric materials. ERNE (Energetic and Relativistic Nuceli and Electron) : Studies energy eruptions leading to fast streams of particles streaming through the solar atmosphere. SWAN (Solar Wind Anisotropies) : Observes interplanetary Lyman alpha radiation in all directions of the sky, away from the Sun.

One of the main objectives for SOHO was to find out about coronal heating (how the corona is heated). As the corona is above the photosphere and the core, one might, logically, think that it will be cooler than the photosphere. In fact it is quite the opposite. As you move up through the layers of the solar atmosphere, you find the temperature varies slightly, until you reach the transition region, where the temperature rises at a phenomenal rate (from 10,000K to 1,000,000K) in a very short distance. With the help of SOHO's many instruments, scientists have collected data showing that the magnetic field of the Sun produces and destroys sufficient magnetic flux in the quiet regions, to heat the solar corona ten times over. This is a significant development in the search for the reason behind coronal heating.

As well as making observations of the sun, SOHO's instrumentation has, without initial intention, become quite efficient at discovering comets even though that wasn't in its original design specifications. Just recently, its comet count reached 500. The main instruments which have been discovering these comets are LASCO, SWAN and UVCS.

The success of the SOHO spacecraft is highlighted by the fact that its original mission was extended until 2003. Although it has solved some long-standing problems associated with solar physics, some other interesting science resulted from its 6 years of orbiting and observing the sun. For instance, the discovery of a tornado within the solar atmosphere.

This section was written with the help of the SOHO websites of the European Space Agency (ESA) and NASA.