Flamsteed Astronomy Society

Bursting the Solar Bubble

Prof Richard Harrison,  February 7, 2005

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In October 1998 Richard Harrison heard that his baby had come back from the dead.

Understandably, he was moved to tears.  Richard’s baby, though, is SOHO, the Solar and Heliospheric Observatory.  SOHO orbits the Lagrangian point L1 about 1.5 million kms toward the Sun, on the line joining Sun and Earth.   It is the most sophisticated solar observatory ever built—a Rolls Royce of a satellite.  An ESA mission, in conjunction with NASA,  SOHO was launched in 1995 with an intended life of 2 years.  10 years later it’s still going strong with a remaining fuel reserve of 20 years.  But in June 1998 SOHO was lost in space.  Out of radio contact and known to be spinning, its solar panels were deprived of power.  Persistence pays however.  The controllers refused to give up and by sending repeated commands they managed to switch on heaters to thaw-out the frozen fuel tanks (a bit like lighting a fire under your car, but any port in a storm!).  By October SOHO was properly re-aligned to the Sun once more and all the instruments were back up.

SOHO was built ‘to take the Sun apart’.  It weighs almost 2 tonnes and carries 12 instrument packages to watch solar activity at a variety of wavelengths, analyse its spectrum, and ‘listen’ to its vibrations (sun-quakes in the interior, via helio-seismology techniques which also allow activity on the back of the Sun to be modelled).  Richard’s team at Rutherford Appleton Labs (RAL) led development of the Coronal Diagnostic Spectrometer CDS package.   The LASCO Large Angle and Spectrometric Chronograph was partly built by the University of Birmingham.   SOHO is our sentinel giving us early warning about the Sun misbehaving.  It sits 4 times further away than the Moon and has a constant unobstructed view of the Sun, uncontaminated by atmosphere.


Courtesy SOHO ESA & NASA

Why do we care?  Events on the Sun have major consequences for us on Earth.  The Sun is our sole (almost) source of energy.  Its medium and long-term behaviour is literally a matter of life & death.  It is really close to Earth — the only star we can study in detail.  It is a physics laboratory in space.  Helium was discovered on the Sun before it was found on Earth and we can study all kinds of effects happening on the Sun due to its very high temperatures and intense magnetic fields that we can hardly create on Earth.   In the short run, ‘space weather’ is of significant interest.  Solar events like flares and CMEs (Coronal Mass Ejections) trigger Aurorae, cause power failures, black-out communication and navigation systems, and destroy satellites.   Future manned space missions must pay careful attention to solar activity or the crew will not survive.  The Apollo astronauts were really lucky to avoid major flares or CMEs.


SOHO/CDS high res image scanning in the Helium I 584 Angstrom line

The Sun is a huge nuclear reactor sitting in space, pulled into a globe by the gravitational power of its enormous mass.  In the interior, nuclear fusion, ‘burning’ of hydrogen into helium, drives temperatures into the millions of degrees.  The hydrogen and helium gas at such pressures behave as a fluid.  Further from the core, convection processes kick-in to carry energy toward the surface.  The Sun’s outer layers seethe and boil producing the granulated surface visible in H-alpha and Helium filters.  The temperatures strip electrons from the gases creating electrically-charged plasma.  Electrically-charged particles in motion build intense magnetic fields and, seen in UV and X-Ray pictures, the Sun writhes in a spaghetti of twisting super-heated gas plasma trapped in the loops of magnetic field.