INFO 24-1995: SOHO hunts elusive solar prey
31 Oct 1995The Sun, our nearest star, will be studied in unprecedented detail when the European Space Agency's SOHO spacecraft is launched by NASA later this year. The name SOHO is an acronym for the SOlar and Heliospheric Observatory. SOHO was also a medieval Anglo-French hunting cry; but this time the hunt is for answers to basic questions about the sun.
SOHO will carry twelve sophisticated telescopes and other instruments, developed in record time by twelve international consortia involving scientific institutes in 15 countries. Roger M. Bonnet, the Director of ESA’s Scientific Programme said: "Each one of these instruments by itself would be enough to make major breakthroughs in our understanding of the Sun. But what makes SOHO such an exciting mission is that we will operate all the instruments together and find possible links between various phenomena at different levels in the volume of the Sun and in the interplanetary medium."
Four years of intense efforts by space engineering teams in ESA and across Europe, under the leadership of the prime contractor Matra Marconi Space of Toulouse, France, have fulflled the dream of scientists who wished to build a superb space observatory for examining the Sun. SOHO, together with the four-spacecraft Cluster mission – which will explore near-Earth space, forms the Solar-Terrestrial Science Programme, the first cornerstone in ESA’s long-term programme Horizon 2000.
No night time for SOHO
Instead of being placed in orbit around the Earth, SOHO will be lofted to a position where the gravitational pulls of the Earth and the Sun cancel each other out exactly, at 1.5 million kilometres sunward from the Earth. This is known in astronomy as the inner Lagrangian point after the French mathematician, Joseph Louis Lagrange, who first calculated its position near the end of the eighteenth century.
SOHO will fly in an elliptical (or halo) orbit around the Lagrangian point, with an orbit radius of about 600 000 kilometres, allowing the spacecraft to experience perpetual day. It will have a continuous, uninterrupted view of the Sun for twenty four hours of the day, all three hundred and sixty five days of the year, producing an extraordinary amount of data.
All previous solar observatories have either been on the Earth or in orbit around our planet. On the Earth, telescopes are limited by inclement weather conditions and atmospheric distortion of the Sun’s signal, and of course they cannot observe the Sun at night. Although the weather problem has been removed in orbit around the Earth, observations are still periodically interrupted when an Earth-orbiting spacecraft enters our planet’s shadow. In contrast, SOHO will provide the first long, clean uninterrupted views of the Sun.
SOHO will look beyond the visible soar disk, observing through new windows from the centre of the Sun to the Earth. It will examine three regions – the hidden interior of the Sun, the hot transparent solar atmosphere, and the eternal solar wind of charged particles and magnetic fields that continuously flow outward from the Sun. The twelve instruments on board SOHO are designed to study one or two of these regions in a different, yet complimentary way. Their combined data will link events in the Sun’s atmosphere and solar wind changes taking place deep within the Sun.
The SOHO mission has three principle scientific objectives:
"Never before have solar physicists had the opportunity to work with such a comprehensive observatory giving them access literally to the whole Sun," said Martin C. E. Huber, the Head of ESA's Space Science Department.
Taking the pulse of the Sun
SOHO will illuminate the unseen depths of the Sun by recording widespread throbbing motions of the Sun's visible surface, or photosphere. These oscillations are caused by sounds that are trapped inside the Sun. On striking the surface and rebounding back down, the sound waves cause the gases there to move up and down.
Sound waves that penetrate deep within the Sun produce global surface oscillations with longer periods of up to a few hours; smaller, shorter oscillations refer to shallower layers. By considering a sequence of oscillations with longer and longer periods, describing sound waves that penetrate deeper and deeper, SOHO will 'peel away' progressively distant layers of the Sun and establish physical properties inside the Sun's deep interior. Since the technique is similar in scientific principle to using earthquakes, or seismic waves, to decipher the Earth's internal structure, it has become known as helioseismology.
SOHO's helioseismology data may shed light on solar neutrinos; they are insubstantial, subatomic particles created in prodigious quantities inside the Sun's energy-generating core. Neutrinos move at the velocity of light and travel almost unimpeded through the Sun, the Earth and nearly any amount of matter. The difficulty is that underground detectors always observe fewer neutrinos than theory says they should detect, a discrepancy known as the solar neutrino problem. Either the Sun does not shine the way we think it ought to, or our basic understanding of neutrinos is in error.
SOHO's record of surface oscillations may establish the temperature at the centre of the Sun, and tell us if there is something wrong with our knowledge of the way stars shine. If the centre of the Sun is about a million degrees cooler than is presently thought, nuclear reactions would produce fewer neutrinos and resolve the solar neutrino problem. But if the internal temperature has the expected value, then the neutrinos may have an identity crisis, undergoing metamorphosis before reaching terrestrial detectors that therefore cannot see them.
Future SOHO helioseismology observations will also improve our understanding of the solar dynamo responsible for the Sun's magnetic field. The dynamo is located somewhere in the solar interior where the hot, rotating material generates electrical currents and converts the energy of motion into magnetic energy. Magnetic fields, spawned by the dynamo inside the Sun, thread their way out into the solar atmosphere where they mould the electrified gas into an ever-changing shape. The entire atmosphere is continuously transformed by the Sun's varying magnetism, producing activity on a scale unknown on Earth.
Looking inside the Sun
There are three helioseismology experts on board SOHO that will acquire long uninterrupted observations of solar oscillations. Two of them emphasise global, long-period oscillations and sound waves that can penetrate the deep solar interior. They are known as GOLF, for Global Oscillations at Low Frequency, and VIRGO, an acronym for Variability of solar IRradiance and Gravity Oscillations. The third SOHO helioseismology instrument will obtain data for oscillations on smaller spatial scales with unprecedented precision; it is called the Solar Oscillations Investigation/Michelson Doppler Imager, or SOI/MDI for short.
GOLF and MDI employ the familiar Doppler technique for measuring motions of the solar photosphere. When part of the visible surface heaves up towards us, the wavelength of a spectral line formed in that region is shortened; if the region moves away from us, back toward the solar interior, the wavelength is lengthened. (A spectral line absorbed or emitted by an atom or an ion at a specific wavelength that identifies the element; it looks like a line in a spectral display of radiation intensity as a function of wavelength).
Sound waves can also be used to determine the internal rotation of the Sun. Waves propagating in the direction of rotation will appear, to a fixed observer, to move faster and their measure speeds will be shorter. Waves propagating against the rotation will be slowed down with longer periods. Accurate measurements of this oscillation period splitting will determine rotation within the solar interior.
GOLF aims to measure velocities as low as 1 millimetre per second for global surface oscillations with periods from 3 minutes to 100 days. SOI/MDI will obtain precise oscillation data with high spatial resolution, investigating surface oscillations of relatively small spatial scales and short periods. Both instruments will determine the radial distribution of density, pressure and temperature, establish the depth and latitude variation of rotation, and determine interior conditions that lead to the development of solar magnetic activity.
VIRGO will measure variations in the Sun's irradiance, or its total luminous output, with extremely accurate, precise and stable radiometers. As the Sun fades and brightens, VIRGO will obtain a sensitive record of global, long-period oscillations, refining our knowledge of the physical and dynamic properties of the deep solar interior.
The precise, long-duration measurements from GOLF and VIRGO may also lead to the unambiguous detection of solar waves for the first time. They are largely confined to the Sun's energy-generating core, and the force of gravity determines how quickly they rise and fall, much like waves in the ocean. Gravity waves that manage to reach the visible solar surface are expected to have long periods of an hour or more and to reveal conditions at the very centre of the Sun. They will shed new light on the solar neutrino problem and determine if the Sun's rotation speed increase near its centre.
The Solar Atmosphere
The entire Sun is just a huge, gaseous sphere that is compressed at its centre and becomes tenuous further out. So, the sharp visible edge of the Sun is an illusion. It is enveloped by gases that are so rarefied that we can see right through them, just as we see through the Earth's transparent air. This tenuous outer part of the Sun is therefore called the solar atmosphere.
The lowest, densest level of the Sun's atmosphere is the photosphere, which simply means the sphere from which visible light comes - from the Greek photos for light. Just above the photosphere lies a thin layer called the chromosphere, from chromos, the Greek work for colour. Still higher, above the chromosphere, is the corona, or crown; this outermost layer of the solar atmosphere extends to the planets and beyond.
The solar corona is extremely hot, with a temperature of a few million degrees. Its very existence is one of the most fundamental, unresolved paradoxes of modern solar physics. The photosphere is closer to the Sun's centre than the corona, but it is several hundred times cooler. Heat should not flow outward from a cooler to a hotter region; it violates common sense and the second law of thermodynamics. Despite more than a half century of investigation, the exact mechanism for heating corona still remains a mystery, and it is one of the main scientific objectives of the SOHO mission.
Sunlight passes right through the corona without depositing substantial quantities of energy in it. So, radiation cannot resolve the heating problem. Possible mechanisms involve the kinetic energy of moving material and/or magnetic energy. Unlike radiation, both of these forms of energy can flow from cold to hot regions.
In and out motions within the solar interior within the solar interior generate sound waves that could accelerate into supersonic shocks; they apparently dissipate energy and generate heat in the lower chromosphere. However, observations suggest that sound waves cannot significantly heat the corona since they cannot reach that far. SOHO will test this conclusion, looking for the varying spectral signatures of sound waves in the chromosphere and corona.
Magnetic energy should play a role in coronal heating. Magnetic fields shape the highly structured corona, and the brightest coronal structures are located where the magnetic field is the strongest. SOHO's spectral instruments will therefore also look for the oscillating intensity and velocity signatures of magnetic waves that are produced by changing magnetism. Magnetic energy can also be converted into heat by numerous small, localised explosive events that have already been observed with space-borne telescopes at ultraviolet wavelengths. SOHO will provide new insight to the frequency, locations and power of such explosions.
SOHO tunes in the Sun's atmosphere
The solar atmosphere will be studied by five SOHO instruments. Three of them will study the chromosphere and the transition region in the low corona. They are known as SUMER for Solar Ultraviolet Measurement of Emitted Radiation, CDS, an acronym for Coronal Diagnostic Spectrometer, and EIT, which is short for Extreme-ultraviolet Imaging Telescope. Two SOHO instruments will examine the middle corona between 1.1 and 10 to 30 solar radii from Sun-centre. They are known as UVCS for UltraViolet Coronagraph Spectrometer and LASCO, an acronym for Large Angle and Spectrometric COronagraph.
Four of these instruments detect invisible radiation at ultraviolet (UV) or extreme ultraviolet (EUV) wavelengths. UV light has wavelengths somewhat less that those of visible light, and waves of EUV are a little shorter than the UV ones. Since this radiation is partially or totally absorbed in our air, it must be observe using telescopes that have been lofted above the Earth's obscuring atmosphere in satellites such as SOHO.
We can tune into different parts of the solar atmosphere by isolating UV or EUV radiation at just one wavelength and forming an image there. Certain UV and EUV lines act like thermometers, specifying the temperature when they are formed, while others are sensitive to the local density. Velocities of moving material can also be inferred from wavelength shifts or broadening of the lines. Temperature, density, and velocity measurements from all four experiments will be used to specify heating, flows and wave motions in different magnetic structures and at various levels in the solar atmosphere. When combined, they will uniquely describe an unseen world of violent change, extreme temperatures and powerful explosion, quite unlike the bland white-light face of the Sun.
SUMER, CDS and EIT will observe lines over a temperature range of 10 000 to a few million degrees, and determine velocities down to 1 kilometre per second. SUMER and CSA will obtain images of the chromosphere and corona with high spatial and temporal resolution (down to 1 second of arc and as brief as 1 second) with a field view of about 4 minutes of arc; EIT will provide full disk images with coarser resolution. UVCS is an occulted telescope equipped to measure UV line intensities and profiles, determining physical parameters of the solar corona from 1.2 to 10 solar radii from Sun-centre with an angular resolution down to 12 seconds of arc.
The remaining SOHO atmosphere instrument, LASCO, uses an occulting disk to mask the Sun's photosphere and view the dim visible sunlight scattered by free coronal electrons. (At a million degrees, several electrons are set free from each atom, leaving an ion behind.) Since the sky's light confuses such images, the finest detail is obtained from space where the daytime sky is truly and starkly black. The LASCO instrument contains three such coronagraphs with nested and overlapping annular fields of view from 1.1 to 30 solar radii from Sun-centre, looking closer to, and further from, the Sun than all previous space-borne coronagraphs.
The coronagraph images will provide electron densities, or the number of electrons per unit volume, specifying their global distribution and radial variation. The inner coronagraph will also permit high-resolution imaging spectroscopy from 1.1 to 3 solar radii. It will measure the intensities and wavelength (Doppler) shifts of visible lines emitted by coronal ions, determining temperature, density and velocity information that will also be used to understand the currently-unknown mechanism for heating the Sun's corona.
The Solar Wind
The hot solar atmosphere, or corona, is expanding into interplanetary space, filling the solar system with a perpetual flow of electrified matter called the solar wind. Unlike any wind on Earth, the solar wind is a rarefied mixture of protons, electrons and magnetic fields, streaming radially outward from the Sun. Thus, the space between the planets is not completely empty, it is filled with charged pieces of the Sun.
At increasing distances from the Sun, where the solar gravity weakens, the hot coronal material creates an outward pressure that overcomes the Sun's gravity, creating a wind that accelerates away to supersonic speeds, like water overflowing a dam. And as the corona disperses, it must be replaced by gases welling up from below to feed the eternal solar wind.
Spacecraft have made in situ measurements of the solar wind near the Earth, showing that it manifests itself in two ways, either as wind moving at a relatively slow speed of 300 to 400 kilometres per second, or as high-speed streams of 600 to 800 kilometres per second. What forces propel the solar wind to these supersonic velocities with such tremendous energy, and where do the components of the solar wind come from? The acceleration and origin of the solar wind are not completely known, and are included as principal scientific objectives of the SOHO mission.
Given the high observed temperatures, the ordinary slow-speed wind is a consequence of its expected outward flow. As this component breaks away from the Sun, it will gain speed with distance, reaching supersonic speeds of hundreds of kilometres per second at a few solar radii from Sun-centre. So, the basic mystery for the acceleration of the slow-speed component is the unknown heat source of the corona. No one really knows how the high-speed stream is accelerated.
And where does the solar wind come from? The high-speed component of the solar wind apparently squirts out of extended regions of low density and temperature in the solar corona. These regions, called coronal holes, appear as large dark areas in EUV or X-ray images, seemingly devoid of radiation. The magnetism in the coronal holes stretches radially outward, providing a fast lane for the high-speed wind. However, the source of the low-speed wind remains a mystery to be solved by future SOHO observations.
SOHO: from the Sun to the Earth
Coronal remote sensing and in-situ experiments on board SOHO will provide a comprehensive data set to study the solar wind from its source at the Sun to the Earth. We have already discussed two of them, UVCS and LASCO, that will determine temperature, density and velocity information in regions near the Sun where the solar wind is accelerated and has its origin (see previous section; SOHO Tunes in the Sun’s Atmosphere). Three SOHO instruments, CELIAS, COSTEP and ERNE, will analyse in situ that charged particles in the solar wind.
The Charge, ELement and Isotope Analysis System, or CELIAS, will measure the mass, ionic charge and energy of the low-speed and high-speed solar wind, as well as energetic particles emitted during explosions on the Sun. COSTEP (COmprehensive SupraThermal and Energetic Particle analyser) and ERNE (Energetic and Relativistic Nuclei and Electron experiment) , together form a collaboration to study the energy release and particle acceleration processes in the solar atmosphere, as well as particle propagation in the interplanetary medium. COSTEP will measure energy spectra of electrons (up to 5 MeV), protons and helium nuclei (up to 52 MeV/nucleus). ERNE will measure energy spectra of heavier ions (up to 540 MeV/nucleus), abundance rations of isotopes and the anisotropy of the particle flux. The Study of Solar Wind ANisotropies, or SWAN, will make complete sky maps of the hydrogen density in the solar wind, determining the distribution of the solar wind mass flux from equator to pole as well as the variation of this distribution.
Command and control of SOHO
ESA has overall responsibility for the SOHO mission, but NASA will provide the launch, tracking and control. The spacecraft will be launched from Cape Canaveral Air Station in Florida by Atlas IIAS, the most powerful of the Atlas-Centaur rockets. The satellite will maintain contact with the ground through NASA’s Deep Space Network (DSN). The DSN is a network of three radio antennas spread around the world. One is in Goldstone, USA, a second near Madrid, Spain, and a third is placed in Canberra, Australia. Together, these antennas provide continuous links to spacecraft wherever thy happen to be in relation to Earth.
After the DSN has collected the SOHO data, it will be routed to NASA’s Goddard Space Flight Centre in Greenbelt, Maryland, USA, from where SOHO will be commanded. A special facility, known as the SOHO Experiment Operations Facility, has been set up at Goddard.. This will serve as the fulcrum for all SOHO operation. Scientists will meet there in order to use the spacecraft and to plan the scientific investigations it will be carrying out. The data from all observations will be stored there in an archive and researchers from all over the world will be able to access the information electronically, via computers.
Energising Space Near Earth
Fortunately for life on Earth, the terrestrial magnetic fields shield us from the full blast of the solar wind, deflecting it away from hr Earth and hollowing out a cavity in it. Yet, this magnetic cocoon, called the magnetosphere, is constantly being buffeted, distorted and reshaped by the variable solar wind, and some of it manages to penetrate the Earth's magnetic defence at its weak points. The Sun thereby feeds a vast and shifting web of energetic particles, electric currents and magnetic fields that encircle the Earth in space.
The Sun's gusty solar wind can therefore affect our environment significantly. It can disturb the Earth's magnetic field, producing geomagnetic storms, create the northern and southern lights (the aurora), disrupt navigation and communication systems, destroy electronics, endanger astronauts and create electrical power blackouts on Earth. SOHO's investigations of the acceleration, evolution and origin of the solar wind therefore have a direct impact on human activity. Indeed, all of these effects are of such vital importance that national centres employ space weather forecasters and continuously monitor the Sun from the ground and space to warn of threatening solar activity.
The Solar-Terrestrial Science Programme - comprising of SOHO and its sister-mission Cluster - are aimed at obtaining a fuller understanding of the vital link between the Sun and the Earth. SOHO will look back at the ultimate source of it all, the Sun, in order to identify and analyse the ultimate source of the phenomena that cause terrestrial effects. Cluster will investigate in detail and - being a flotilla of four magnetospheric spacecraft - in three dimensions, the physical mature of the processes that are induced in the near-Earth environment.
"All those who have worked tirelessly on the SOHO payload, spacecraft and ground segment are to be congratulated in their excellent work and for having developed the most remarkable tool to help us understand the Sun and its environment, the heliosphere," said Roger Bonnet.