Pages 581-594

SOHO, ITS DAY IN THE SUN 

V. Domingo 

European Space Agency (ESA), at NASA GSFC Mail Code 682 Greenbelt MD 20771 USA, E-mail:vdomingo@esa.nasacom.nasa.gov

 

 ABSTRACT 

SOHO, the Solar and Heliospheric Observatory, is a project of international cooperation between ESA and NASA to study the Sun, from its deep core to the outer corona, and the solar wind. Three helioseismology instruments are providing unique data for the study of the structure and dynamics of the solar interior, from the very deep core to the outermost layers of the convection zone. A set of five complementary remote sensing instruments, consisting of EUV, UV and visible light imagers, spectrographs and coronagraphs, give us our first comprehensive view of the outer solar atmosphere and corona, leading to a better understanding of the enigmatic coronal heating and solar wind acceleration processes. Finally, three experiments complement the remote sensing observations by making in-situ measurements of the composition and energy of the solar wind and charged energetic particles, and another instrument maps the neutral hydrogen in the heliosphere, and its dynamic change of the Solar Wind. This paper highlights some of the first results from SOHO.  

 INTRODUCTION 

SOHO, the Solar and Heliospheric Observatory, is a project of international cooperation between ESA and NASA to study the Sun, from its deep core to the outer corona, and the solar wind. It carries a complement of twelve sophisticated, state-of-the-art instruments (see Table 1), developed and furnished by twelve international PI consortia involving 39 institutes from fifteen countries (Belgium, Denmark, Finland, France, Germany, Ireland, Italy, Japan, Netherlands, Norway, Russia, Spain, Switzerland, United Kingdom, and the United States). (Domingo, et al. 1995.) 

SOHO was launched by an Atlas II-AS from Cape Canaveral Air Station on 2 December 1995, and was inserted into its Halo Orbit around the L1 Lagrangian point on February 14, 6 weeks ahead of schedule. The launch was so accurate and the orbital manoeuvres were so efficient that there remains sufficient fuel on board to maintain the Halo Orbit for more than a decade, at least twice the time originally foreseen. 

Already in the commissioning phase the SOHO experiments have returned a wealth of data, impressive through their quality and diversity. Some of the images can be viewed on the SOHO pages (http://sohowww.nascom.nasa.gov) on the World Wide Web, and on the individual experiment pages, all with links from the SOHO page. 

In the following sections we show examples of the kind of results that are being obtained by the SOHO instruments. It is worth noting that the main results from SOHO will mainly come from joint analysis of coordinated observations.

  Table 1: The SOHO Scientific Instruments.

 

Investigation

 

Principal Investigator

 

GOLF

 

Global Oscillations at Low Frequencies

 

A. Gabriel, IAS, Orsay, France

VIRGO  

Variability of Solar Irradiance and Gravity Oscillations

 

C. Fröhlich, PMOD Davos, Switzerland

 

MDI/SOI

 

Michelson Doppler Imager / Solar Oscillations Investigation

 

P. Scherrer, Stanford University, USA

 

SUMER

 

Solar Ultraviolet Measurements of Emitted Radiation

 

K. Wilhelm, MPAe Lindau, Germany

 

CDS

 

Coronal Diagnostic Spectrometer

 

R. Harrison, RAL, Chilton, England

 

EIT

 

Extreme-Ultraviolet Imaging Telescope

 

J.-P. Delaboudinière, IAS, Orsay, France

 

UVCS

 

UltraViolet Coronagraph Spectrometer

 

J. Kohl, SAO, Cambridge, USA

 

LASCO`

 

Large Angle Spectroscopic Coronagraph

 

G. Brueckner, NRL, Washington, USA

 

SWAN

 

Solar Wind Anisotropies

 

J.-L. Bertaux, SA, Verrières, France

 

CELIAS

 

Charge, Element and Isotope Analysis System

 

P. Bochsler, Univ. Bern, Switzerland

 

COSTEP

 

Comprehensive SupraThermal and Energetic Particle Analyzer

 

H. Kunow, Univ. Kiel, Germany

 

ERNE

 

Energetic and Relativistic Nuclei and Electron Experiment

 

J. Torsti, Univ. Turku, Finland

 THE SOLAR INTERIOR 

Just as seismology reveals the Earth's interior by studying earthquake waves, solar physicists are probing inside the Sun using helioseismology. Oscillations detectable at the visible surface are due to sound waves reverberating through the Sun's interior. The three helioseismology instruments (GOLF, VIRGO, MDI) are harvesting the benefit of the stability of observation and continuity of the measurement that SOHO provides. Using seismology techniques and wave measurements from SOHO's Michelson Doppler Imager (MDI) which measures the vertical motion of the Sun's surface at a million different points once a minute (Scherrer et al., 1995) scientists were able to generate the first maps of horizontal and vertical flow velocities as well as sound speed variations in the convection zone just below the visible surface (Figure 1). The convection zone lies directly beneath the photosphere, which forms the Sun's visible surface and effectively hides what is below. As a result, very little is known about the convection zone's internal structure, despite the fact that it is the source of sunspots, solar flares and most other forms of solar activity that affect Earth.

 MDI data were used to calculate the time it takes sounds to travel between many different points on the solar surface. Because the paths of these sound waves loop down into the interior, one can use this information to map the temperature and flow patterns beneath the surface. Figure 1 provides a tantalizing first view of how the convection zone is organized internally. For example, the map provides the first direct evidence for the depth of the features called supergranules that average about 25000 km across. Theoretical calculations predicted that supergranule thickness should be between 25 and 30 percent of their width. But the mapping effort suggests that they are shaped more like pancakes, with a thickness only one-tenth of their width. More significantly, the new map shows no evidence of giant convection cells that had been predicted by a popular theory called the global circulation model. It does, however, show evidence of narrow plumes of cooler gases streaming downward toward the boundary with the radiative layer - a view consistent with the result of some numerical simulations of the region. Surprisingly, however, the plumes appear to originate from the middle of the supergranules, rather than at their edges as had been proposed. Additional observations at other times and locations are needed to determine whether the features that the map reveals are characteristic. Future observations will also allow the researchers to make a "movie" of this part of the convection zone so that they can observe how its structure changes over time.

  Fig.1: A vertical cut through the outer 1% of the Sun showing flows and temperature variations inferred by helioseismic tomography using measurements from the Michelson Doppler Imager (MDI) onboard SOHO. The arrows indicate directions and relative speeds of the vertical motions within the Sun. Colour shadings indicate temperature changes. (Duvall et al., 1997)

Random motions on the Sun, due to convection and other dynamic activity, lead to a continuum of frequencies when the measured surface velocities are Fourier analysed. It is possible to model this velocity spectrum, by making assumptions about the physical properties of the various sizes of convection cell and their distribution. Harvey (1985) proposed a tentative model based upon the combined effects of granulation, mesogranulation, supergranulation and active regions. This he integrated over the whole disk, giving data in the same format as that which is observed from GOLF, or from the ground-based networks IRIS and BISON.

Earlier comparisons of the model with ground-based data showed agreement to within a factor of 2 over the frequency range in which g-modes would be expected. This had been taken as confirmation of the model, as well as an indication that the Earth's atmospheric disturbances do not make a significant contribution to the observed data. This presumed solar background level imposes the eventual limit for the detection of weak g-mode oscillations.

     Fig. 2: Comparison of Harvey's background solar noise model with measured velocity power spectra from GOLF on board SOHO and the ground-based Mark-1 instrument at the Teide Observatory (IAC, Tenerife). Note the significantly reduced noise in the GOLF spectrum at low frequencies. (P.Pallé and A.Gabriel, private communication.)

  Now, for the first time the frequency spectrum of surface velocity has been measured from space. Observations from SOHO have completely changed this picture. Fig.2 shows plotted the Harvey model together with preliminary results from GOLF (Gabriel et al., 1995) and from Mark-1, a ground-based instrument at IAC Tenerife, regarded as one of the best of the ground-based observing stations. The peak at 3 MHZ is the effect of the 5-minute p-mode oscillations. At lower frequencies, the signal level from GOLF is an order of magnitude lower than Mk I or the model. We conclude that atmospheric disturbances account for the major part of the noise received on the ground at these frequencies and that the model greatly over- estimates the solar background level. The consequences for SOHO are very positive. The strategy of making such observations from space is fully vindicated and the limit for detection of low-level oscillations or g-modes is lower than anticipated, by a factor of ten or more.

 VIRGO started with measurements of the sun photometers (SPM) for the spectral irradiance at 402, 500 and 862 nm and with the radiometers for total irradiance in mid January 1996. The measurements with the Luminosity Oscillation Imager (LOI) only started the end of March after the successful opening of its cover, which before bounced back to the closed position every time it was actuated. The power spectra calculated from the time series of the total and spectral observations have a signal-to-noise never seen before. The range between 50 and 1000 µHz (0.5-5h periods) could never be observed before (Fröhlich et al., 1997).

 For the study of solar irradiance variations, the passage by the solar disk of small activity features during this time of quiet Sun (minimum of activity) has proven the extreme sensitivity of the VIRGO detectors. An example is a very small active region which passed the central meridian mid May. It could be followed by the LOI passing through the 4 pixels north of the equator showing an increase of intensity close to the limb, a decrease near the central meridian and again an increase before vanishing at west limb. The simultaneous observation with the SPM and radiometers allow now to disentangle time and spatial variations which will much improve our understanding of solar irradiance variability. An overall effect of less than 0.02% in the total solar irradiance can be decomposed into sunspot and facular components, thanks to the sensitivity of the instrument.

 THE SOLAR CORONA

 The combined operation of EIT, SUMER and CDS, is a continuous source of discoveries. The Extreme-Ultraviolet Imaging Telescope (EIT) produces images at four wavelengths centered around spectral lines produced 80 000 K, 1.3x10 6 K, 1.6x10 6 K and 2.0x10 6 K (Delaboudinière et al., 1995). "Plumes" of outward flowing hot gas in the Sun's atmosphere may be one source of the solar wind of charged particles. Figure 3 shows (top) magnetic fields on the Sun's surface near the south solar pole; (middle) an ultraviolet image of the 1 million K plumes from the same region; and (bottom) an ultraviolet image of the "quiet" solar atmosphere closer to the surface. The top image was taken by the Michelson-Doppler Imager/Solar Oscillations Investigation instrument (MDI/SOI) on board SOHO. The center and bottom images were taken by EIT. These images represent the first opportunity scientists have had to see the detailed development over time of the plume structures in which the solar wind is accelerated, at least at the solar poles.

 SUMER - Solar Ultraviolet Measurements of Emitted Radiation - observed its first light on the

24th of January 1996, and obtained a detailed spectrum in the wavelength range from 500 to 1490 Å of a solar region near the north pole. Using the second detector of the instrument, this range was later extended to 1610 Å. Many more features and areas of the Sun have been observed since, including coronal holes, polar plumes and active regions. Because of the technological advances employed in SUMER, we have been able to detect lines that are much fainter than previously observed. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made. (Wilhelm et al., 1997; Lemaire et al., 1997.)

The Coronal Diagnostic Spectrometer (CDS) on board SOHO is a twin extreme ultraviolet spectrometer looking at the Sun in the wavelength range 150-780 Å (Harrison et al., 1995). CDS has recorded spectral atlases in normal and grazing incidence from a variety of features: active regions, quiet sun and coronal holes (Harrison et al., 1997). These spectra represent a great improvement over earlier spectral mapping of the solar emission, particularly in the short end of the CDS spectral range. The reason for this is the combination of good angular and spectral resolutions, with a CDS spatial element size of 2 arc seconds and the spectral element in the range 0.07 - 0.2 Å, depending on the spectrometer type and spectral range. Thus, the short wavelength region below 220 Å with many strong lines from different ionisation stages of iron, from Fe IX to Fe XIV, have provided good temperature information in a typical coronal plasma of 1-2 million K. A comparison of these spectra taken in different targets shows strong emission from Fe XIV lines in an active region. These lines become weaker in a quiet sun area while the lower ionisation stages (Fe IX), repre-sentative of cooler plasma, completely dominate in the coronal hole. The evolution and structure of coronal holes, and their role in supplying the fast component of the solar wind, are currently being studied.

   Fig. 3: The south pole of the Sun: Magnetic field (top), the 1 million K corona as seen in the Fe IX emission line at 171 Å (middle), and the upper chromosphere as seen in the He II emission line at 304 Å (bottom) (P. Delaboudinière, P.Scherrer, private communication).

 Analysis of spectral line profiles recorded with the Normal Incident Spectrometer (NIS) of CDS has allowed us to find significant flows of plasma taking place in active region loops (Brekke et al., 1997). Up-flowing plasma reaching velocities of 100 km/s is seen in hot coronal lines like Fe XVI (335 Å). Other observations of an active region show down-flowing material with a velocity of 50 km/s. Similar flows are observed in lines emitted at transition region temperatures, demonstrating that the transition region and corona are very dynamic in nature. From the present preliminary measurements it seems clearly possible to measure relative line shift corresponding to velocity differences as low as 20 km/s.

 

  Fig. 4: Images of an active region recorded by CDS in two ionisation states of Magnesium, Mg IX and Mg X. The field-of-view is 4x4 arcmin (R.Harrison, private communication).

 CDS has also observed strong high velocity events (see upper panel of Figure 4 for example). The event in Figure 4 was located in the leg of an active region loop and is characterised by extremely wide emission lines corresponding to a velocity dispersion of approximately 300-450 km/s. The spatial extent is small, less than 4 arc seconds. The event occurred in all lines from He I to Fe XVI i.e. over a temperature range from 20,000 K to 2.5 million K. This is a new result which has not been reported before. The fact that such events extend simultaneously over a wide temperature range is a challenge to theoretical models and may cause a re-examination of the contribution of explosive events to coronal heating.

 Two coronagraphs (LASCO and UVCS) produce observations and measurements over the extended corona, and the wind at its inception. For the first time the Solar corona=s expansion into Solar wind can be observed up to 30 Solar radii (LASCO) and plasma diagnostics spectroscopy obtained up to 10 Solar radii (UVCS).

 LASCO uses three coronagraphs to observe the outer solar atmosphere from the solar limb to a distance of 30 solar radii (Brueckner et al., 1995). Figure 5 shows a frame of a large coronal mass ejection (CME) as observed by the LASCO C-2 and C-3 coronagraphs on February 3, 1996. This event because it was seen in the field of view of two coronagraphs, could be followed from 1.1 million km above the solar surface out to 15 million km. Bright clouds are seen travelling outward in the equatorial plane with speeds ranging from 90 km /s to 540 km/s over both the east and the west limb of the Sun. The acceleration takes place over a distance of 15 million km. The sectors above the solar limb in which the bright clouds are seen seem to extend over an angle of 120 degrees in the equatorial plane. Although the instruments cannot see material moving toward or away from the Earth-Sun line, it is safe to assume that the CME extended all the way around the Sun. A faint event can be seen in this picture above the south pole of the Sun.

 The Ultraviolet Coronagraph Spectrometer (UVCS) on board SOHO uses ultraviolet spectroscopy to obtain an empirical description of regions in the Sun's extended atmosphere or corona where the primary solar wind acceleration takes place (Kohl et al., 1995). This information is used to address a broad range of scientific questions regarding the nature of the extended solar corona and the acceleration of the solar wind. UVCS has observed helmet streamers which are believed to be a source of normal speed solar wind, and it has observed coronal holes, which are the known source of high speed solar wind streams. An understanding of the physical processes that control solar wind acceleration would also contribute to the understanding of mass loss in other stars.

 Since the start of UVCS observations in late January of this year, it has made the first ultraviolet images of the extended solar corona above two solar radii from the center of the Sun. It has sensed the presence of a broad range of chemical elements in the extended corona and it has actually measured the speeds of coronal material as it accelerates from the Sun. UVCS has confirmed that protons and the more massive oxygen particles are hotter than the electrons in the out-flowing coronal gas. This temperature difference may be the key to identifying the physical processes responsible for solar wind acceleration and for controlling the com-position and temperatures of solar wind particles near the Earth. UVCS has made the first measurements of the speed of highly charged oxygen as it flows out of the tips of streamers and has made the first measure-ments of supersonic outflow of highly charged oxygen flowing out of coronal holes. This information is being used to test theoretical explanations of how the solar wind is accelerated.

SOLAR WIND

Three instruments measure the composition and energy spectrum of particles in the Solar Wind. The Charge, Element, Isotope Analysis System (CELIAS) investigation on SOHO is a multi- sensor experiment consisting of three sensors that measure the composition and energy spectra of plasma (solar wind) and energetic ions of solar, interplanetary, and interstellar origin, and a fourth sensor for monitoring the absolute EUV (extreme ultraviolet) flux from the Sun (Hovestadt et al., 1995). The CELIAS solar wind mass spectrometer (MTOF, Mass Time-of-Flight Sensor) has unprecedented mass resolution for solar wind composition studies, and has already measured rare elements and isotopes that were previously not resolvable from more abundant neighbouring species, or were not previously observable at all. For example, as seen in Figure 6, the elements of sulphur, argon, and calcium are now easily distinguished from the neighbouring species of silicon and iron, as is nitrogen from carbon and oxygen. The rare elements phosphorus, chlorine, potassium, titanium, chromium, and nickel are being measured in the solar wind for the first time. The determination of the elemental abundances of these rarer species allow us to fill in the "blanks" of the solar wind versus photospheric abundance tables. This is important in obtaining a much better analysis of the solar wind feeding and acceleration processes in the chromosphere and inner corona. The solar wind and coronal abundances indicate an ordering of relative abundance enhancement (or depletion) to photospheric values partially correlated with the first ionization potential (FIP) of the element (the so-called "FIP effect"). Since these newly observed elements have different properties (such as first  ionization potentials and times, charge state equilibrium times, atomic mass, etc.), knowledge of their relative abundances serve as diagnostic tools for determining conditions in the chromosphere/ transition region, where ions which eventually become the solar wind are separated from neutrals.  

   Fig. 5: Coronal disturbances as seen from the LASCO C2 and C3 coronagraph: Instead of a single coronal mass ejection erupting in one direction, movies made with LASCO confirm that material is simultaneously ejected both to the East (left) and the West (right) from the Sun. The combined field of view of the images spans 25 solar radii. The inner edge is at 1.6 solar radii. The dark circle marks the boundary between the C2 and the C3 field of view (G.Brueckner, private communication).

 The COSTEP instruments measure electrons from 45 KeV to 10 MeV and Hydrogen and Helium nuclei from 45 KeV to 53 MeV (Müller-Mellin et al., 1995). Due to the phase in the solar activity cycle close to solar minimum the sun was extremely quiet during the first phase of the mission. Of the solar wind particle detectors on SOHO, ERNE covers the highest energies, roughly from 1 MeV/n to 500 MeV/n (Torsti et al., 1995). At these energies, the present solar activity minimum enables ERNE to collect particles originating outside our solar system, the galactic cosmic radiation. For the next five years, solar activity can be expected to increase, and the particles collected will increasingly reflect their solar origin, coming from flares and coronal mass ejections, for which the instrument was designed. From January 20 to 25, ERNE measured the first small solar originated particle event. During this event, the number of protons counted rose about tenfold over the background. Initial analysis indicates that the high-energy protons detected originated in the shock front arising from a fast solar wind running into a slower part of the wind, with the collision region located several Sun-Earth radii outwards.

  Fig. 6: Element and isotope spectrum as obtained with the MTOF sensor of CELIAS. Many elements such as phosphorus, chlorine, potassium, titanium, chromium, and nickel are being measured in the solar wind for the first time (F. Ipavich, private communication).

  To determine the dynamic change of the large-scale Solar wind flow an instrument (SWAN) measures the neutral hydrogen in the Heliosphere and its evolution. SWAN (Bertaux et al., 1995) surveys the sky all around and sees an ultraviolet glow from hydrogen atoms lit by the Sun (Figure 7). These atoms come on a breeze from the stars that blows through the solar system. The competing wind of charged particles from the Sun breaks the incoming atoms, so that they no longer emit their characteristic wavelength. The result is a hole in the pattern of emissions downstream from the Sun, which gives the impression of the strength of the solar wind in different directions. At the present time of a quiet Sun, the SWAN sky maps indicate clearly a situation of increased solar wind around the Sun=s equator in the ecliptic plane (see Figure 7). We are anxious to see what will happen when the Sun becomes more active. Then we shall see important changes in the solar wind=s impact on the interstellar gas revealed by the changes in the sky maps.

 

  Fig. 7: Full sky Lyman-2 map in ecliptic co-ordinates as recorded by the SWAN instrument. Note the asymmetry between the northern and southern hemisphere. The U-shaped yellow band is the Milky Way (J.-L.Bertaux, private communication).

CONCLUSION

During the first couple of months, after the commissioning of the spacecraft and experiments, all the instruments have performed observations and measurements that demonstrate that SOHO is fully qualified to achieve the goals for which it was designed. In depth analysis of the data is just starting, but quick analysis of the early measurements have already provided new observations in all the fields addressed by the SOHO instruments. Of particular relevance are:

 In the field of helioseismology:

 In the solar atmosphere:

In the solar wind:

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