Solar and Heliospheric Observatory

The Solar and Heliospheric Observatory (SOHO) is a spacecraft that was launched on 2 December 1995 to study the sun, and began normal operations in May 1996. It is a joint project of international cooperation between the European Space Agency (ESA) and NASA. Originally planned as a two-year mission, SOHO currently continues to operate after over ten years in space. In addition to its scientific mission, it is currently the main source of near-real time solar data for space weather prediction. Along with the Advanced Composition Explorer (ACE), SOHO is one of two spacecraft currently in the vicinity of the Earth-Sun L1 point, a point of gravitational balance located approximately 0.99 AU from the Sun and 0.01 AU from the Earth.

**The Solar and Heliospheric Observatory**
Instruments
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/
GOLF	core oscillations (Doppler-sensitive photometer)
VIRGO	core oscillations (photometric imager)
MDI	oscillations and magnetic fields (Doppler imager)
SUMER	coronal physics (UV spectrograph)
CDS	corona and chromosphere physics (UV spectrograph)
EIT	low corona (UV telescope)
UVCS	solar wind acceleration (UV spectrograph)
LASCO	mid to outer corona (visible light camera)
SWAN	solar wind density (UV camera)
CELIAS	solar wind ions (material sampler)
COSTEP	solar wind ions (material sampler)
ERNE	solar wind ions (material sampler)

The 610 kg SOHO spacecraft is in a halo orbit around 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 due to radio interference generated by the Sun, and because this would not be 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 200 kbit/s data stream of photographs and other measurements via the NASA Deep Space Network of ground 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.

In 2003 ESA reported the failure of the antenna Y-axis stepper motor, necessary for pointing the high gain antenna and allowing the downlink of high rate data. At the time, it was thought that the antenna anomaly might cause two to three week data-blackouts every three months. However, ESA and NASA engineers managed to use SOHO's low gain antennas together with the larger 34 and 70 meter DSN ground stations and judicious use of SOHO's Solid State Recorder (SSR) to prevent total data loss, with only a slightly reduced data flow every three months.

Scientific Objectives

The three main scientific objectives of SOHO are:

Investigation of the outer layer of the Sun, which consists of the chromosphere, transition region, and the corona. CDS, EIT, LASCO, SUMER, SWAN, and UVCS are used for this solar atmosphere remote sensing.
Making observations of solar wind and associated phenomena in the vicinty of L1. CELIAS and CEPAC are used for "in situ" solar wind observations.
Probing the interior structure of the Sun. GOLF, MDI, and VIRGO are used for helioseismology.

Instruments

The SOHO Payload Module (PLM) consists of twelve instruments, each capable of independent or coordinated observation of the Sun or parts of the Sun, and some spacecraft components. The instruments are:

Coronial Diagnostics Spectrometer (CDS) which measures density, temperature and flows in the corona.
Charge ELement and Isotope Analysis System (CELIAS) which studies the ion composition of the solar wind.
COmprehensive SupraThermal and Energetic Particle analyser collaboration (COSTEP) which studies the ion and electron composition of the solar wind. (COSTEP and ENRE are sometimes referred to together as the COSTEP-ENRE Particle Analyser Collaboration (CEPAC).
Extreme ultraviolet Imaging Telescope (EIT) which studies the low coronial structure and activity.
Energetic and Relative Nuclei and Electron experiment (ERNE) which studies the ion and electron composition of the solar wind. (See note above in COSTEP entry.)
Global Oscillations at Low Frequencies (GOLF) which measures velocity variations of the whole solar disk to explore the core of the sun.
Large Angle and Spectrometric COronagraph experiment (LASCO) which studies the structure and evolution of the corona
Michelson Doppler Imager (MDI) which measures velocity and magnetic fields in the photosphere to learn about the convection zone which forms the outer layer of the interior of the sun and about the magnetic fields which control the structure of the corona. The MDI is the biggest producer of data by far on SOHO. In fact, two of SOHO's virtual channels are named after MDI, VC2 (MDI-M) carries MDI magnetogram data, and VC3 (MDI-H) carries MDI Helioseismology data.
Solar Ultraviolet Measurement of Emitted Radiation (SUMER) which measures plasma flows, temperature and density in 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.
UltraViolet Coronagraph Spectrometer (UVCS) which measures density and temperature in the corona.
Variability of solar IRradiance and Gravity Oscillations (VIRGO) which measures oscillations and solar constant both of the whole solar disk and at low resolution, again exploring the core of the sun.

Observations from some of the instruments can be formatted as images, most of which are also readily available on the internet for either public or research use (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 (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.

As a consequence of its observing the Sun, SOHO (specifically the LASCO instrument) has inadverdently discovered comets by blocking out the Sun's glare. Approximately one-half of all known comets have been discovered by SOHO. Recently, it discovered its 1000th comet.

References

External links

ESA SOHO webpage
, free to use for educational and non-commercial purposes.
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Scientific Objectives

Instruments

References

External links