Objective:

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Solar and Heliospheric Observatory (SOHO)

• Objective
• 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 the extremely high temperature of about
  1 000 000C?
• Where is the solar wind produced and how is it
  accelerated?
• Science highlights include
• Revealing the first images ever of a stars
  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.
• Revolutionising 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 Earths climate.
• Also 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.

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Spacecraft and Launch SOHO is a three-axis
stabilised spacecraft pointing at Sun. The
spacecraft was built for ESA by European
industry. Dimensions 4.3 2.7 3.7 metres
(9.5 metres with solar arrays deployed). Mass
1850 kilograms at launch. Launch Launched by
NASA using an Atlas rocket. Orbit SOHO orbits
the Sun in step with the Earth, by slowly
orbiting the First Lagrangian Point (L1) 1.5
million Km from Earth, where the combined gravity
of Earth and Sun keep SOHO in an orbit locked to
the Earth-Sun line. There, SOHO enjoys an
uninterrupted view of the Sun. Mission lifetime
Designed for a nominal mission lifetime of two
years. the mission has been extended, through
March 2007. This will allow SOHO to cover a
complete 11-year solar cycle. Loss Recovery
Control of the spacecraft was lost in June 1998,
and restored three months later through superb
efforts of the SOHO recovery team. All 12
instruments were still us-able, most with no ill
effects. Two of the three on-board gyroscopes
failed immediately and a third in December 1998.
After that, new on-board software that no longer
relies on gyroscopes was installed in February
1999. It allowed the spacecraft to return to full
scientific operations, while providing an even
greater margin of safety for spacecraft
operations. This made SOHO the first three-axis
stabilised spacecraft operated without
gyroscopes, breaking new ground for future
spacecraft designs.
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Instruments The scientific payload consists of
12 instruments, developed and furnished by 12
international consortia involving 29 institutes
from 15 countries. More than 1500 scientists in
countries all around the world are either
directly involved in SOHO's instruments or have
used SOHO data in their research programs.
OPTICAL Coronal Diagnostic Spectrometer (CDS)
CDS measures emission lines in the solar corona
and transition region, providing diagnostic
information on the solar atmosphere, especially
of the plasma in the temperature range from
10,000 to more than 1,000,000K. Extreme
ultraviolet Imaging Telescope (EIT) EIT provides
full disc solar images at four selected EUV
wavelengths, mapping the plasma in the low corona
and transition region at temperatures between
80,000 and 2,500,000K. Global Oscillations at
Low Frequencies (GOLF) GOLF studies the internal
structure of the Sun by measuring velocity
oscillations over the entire solar disc. Large
Angle and Spectrometric Coronograph (LASCO)
LASCO observes the outer solar atmosphere
(corona) from near the solar limb to a distance
of 35 Rsun (1/7th AU). LASCO used an occulter,
creating an artificial solar eclipse, 24 hours a
day, 7 days a week. LASCO has also become SOHOs
principal comet finder.
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Michelson Doppler Imager/Solar Oscillations
Investigation (MDI/SOI) MDI records the vertical
motion (tides) of the Sun's surface at a
million different points every minute.
Measurements of the acoustic waves inside the
Sun as they perturb the photosphere, enables
study of the structure and dynamics of the Suns
interior. MDI also measures the longitudinal
component of the Suns magnetic field. Solar
Ultraviolet Measurements of Emitted Radiation
(SUMER) SUMER acquires detailed spectroscopic
plasma diagnostics (flows, temperature, density,
and dynamics) of the solar atmosphere, from the
chromosphere through the transition region to the
inner corona, over a temperature range from
10,000 to 2,000,000K and above. Solar Wind
Anisotropies (SWAN) SWAN does not look at the
Sun. It watches the rest of the sky, measuring
hydrogen that is blowing into the Solar System
from interstellar space. By studying the
interaction between the solar wind and this
hydrogen gas, SWAN determines how the solar wind
is distributed. UltraViolet Coronograph
Spectrometer (UVCS) UVCS makes UV measurements
of the solar corona (between about 1.3 and 12
solar radii from the center) by creating an
artificial solar eclipse. UVCS provides valuable
information about the microscopic and macroscopic
behaviour of the highly ionised coronal plasma.
Variability of Solar Irradiance and Gravity
Oscillations (VIRGO) VIRGO characterises solar
intensity oscillations and measures the total
solar irradiance (known as the solar constant)
to quantify its variability over periods of days
to the duration of the mission.
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IN SITU MEASUREMENTS Charge, Element, and
Isotope Analysis System (CELIAS) CELIAS samples
the solar wind and energetic ions of solar,
interplanetary and interstellar origin, as they
sweep past SOHO. It analyses the density and
composition of particles present in this solar
wind. It warns of incoming solar storms that
could damage satellites in Earth orbit.
Comprehensive Suprathermal and Energetic
Particle Analyzer (COSTEP) COSTEP detects and
classifies very energetic particle populations of
solar, interplanetary, and galactic origin. It is
a complementary instrument to ERNE (see below).
Energetic and Relativistic Nuclei and Electron
experiment (ERNE) ERNE measures high-energy
particles originating from the Sun and the Milky
Way. It is a complementary instrument to COSTEP.
Ground Control and Science Operations SOHO
is operated from NASAs Goddard Space Flight
Center (GSFC) by an integrated team of scientists
and engineers from ESA, NASA, partner industries,
research laboratories and universities. Ground
control is provided via NASAs Deep Space Network
antennae, located at Goldstone (California),
Canberra (Australia), and Madrid (Spain).
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White Light Image from MDI near solar maximum
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SOHO Peers Beneath a Sunspot
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Quakes on the Sun Observed by MDI instrument
on SOHO Seismic waves triggered by solar
flare Wave speed increased as waves moved out
from 10 km/s to 115 km/s
930
220,000 km
936
940
946
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MDI/GONG Helioseismology Images of Changing
Interior
Near Surface
Interior Cut-away
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TOP IMAGES Rotation rates near the bottom of
the convection zone (white line), the level of
the suspected dynamo, change markedly over 6
months at solar minimum. (Left 1996 January
right 1996 July) Faster/slower rates are
shown in red/blue. Near the surface (seen on
the left of each cutaway) bands of faster (red)
and slower (green) rotation move towards the
equator. LOWER IMAGE Shows how bands of
faster/slower rotating material below solar
surface move toward equator from solar minimum
(1996) to near maximum (1999) SOHO is the
NASA/ESA Solar Heliospheric Observatory. GONG is
an NSF ground-based helioseismology network. The
helioseismology instrument on SOHO provides high
resolution data not obtainable from the ground,
GONG provides long term measurements.
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Imaging Solar Farside via Helioseismology
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He II l304 Image from SOHO August 27, 1997
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Spectacular CME Observed January 4, 2002 by SOHO
Spectacular Coronal Mass Ejection (CME) observed
in the early hours of January 4, starting off as
a filament eruption seen by the Extreme
ultraviolet Imaging Telescope (EIT) in the 195 Å
images. The complexity and structure of the CME
as it passed through the Large Angle and
Spectrometric Coronagraph (LASCO) C2 and C3
fields of view amazed even experienced solar
physicists at the SOHO operations center.
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CME Images
July 1, 2002
Comet Images
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Discovery that Coronal Mass Ejections are a
Global Phenomenon
Discovery that Coronal Mass Ejections are a
global phenomena with a CME at one location
apparently triggering CMEs at other
locations. Discovery of slow solar wind outflow
in streamers due to episodic small coronal mass
ejections with constant acceleration out to 30
Rsun. Discovery of coronal temperatures for
ions much higher than for electrons in polar
coronal holes 106K for electrons, 3 times
higher for protons, 30 times higher for oxygen
ions. Results consistent with heating by MHD
waves via ion cyclotron resonance process.
White Light Corona and Background Star Field
Small inner circle is size of Sun.
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Halo CME Ejected Toward Earth