first results from soho
The First Results from SOHO
V. Domingo, B. Fleck
ESA Space Science Department at NASA/GSFC, Greenbelt (MD), USA A. Poland
Laboratory for Astronomy and Solar Physics, NASA/GSFC, Greenbelt (MD), USA and The SOHO Instrument Teams
The Solar and Heliospheric Observatory, SOHO, Is an international cooperative project between ESA and NASA to study the Sun. This space-based Observatory is viewing and investigating the Sun from its deep core, through its outer atmosphere - the 'corona' - and the domain of the solar wind, out to a distance ten times beyond the Earth's orbit. The spacecraft provides a highly-stabilised
SOHO, launched by an Atlas ll-AS from Cape Canaveral on 2 December 1995, was inserted into its halo orbit around the L1 Lagrangian point on 14 February, six weeks ahead of schedule. The launch and the orbital manoeuvres were so accurate and efficient that sufficient fuel remains on board to maintain the halo orbit for more than a decade, i.e. for at least twice as long as originally foreseen.
Already during their commissioning phase, the SOHO experiments have returned a wealth of data, impressive in terms of both its quality and diversity. Some of the images can be viewed via 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 home page.
Typical examples of the unique results being obtained with SOHO's instruments are presented here. Although they have been obtained with single instruments, it is worth noting that the main scientific advances from SOHO are expected to come from the joint analysis of coordinated observations.
platform for a complement of twelve sophisticated, state-of-the-art instruments, developed and furnished by twelve International consortia involving 39 institutes from fifteen countries (Belgium, Denmark, Finland, France, Germany, Ireland, Italy, Japan, Netherlands, Norway, Russia, Spain, Switzerland, the United Kingdom, and the United States).
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 extreme-ultraviolet (EUV), UV and visible-light imagers, spectrographs and coronagraphs, are currently giving 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 measuring the composition and energy of the solar wind and energetic particles at the spacecraft (Table 1).
The solar interior
Just as seismology reveals the Earth's interior by studying earthquake waves, solar physicists are probing the solar interior by the use of helioseismology. Oscillations detectable at the Sun's visible surface are due to sound waves reverberating through its stellar interior Using seismology techniques and wave measurements from SOHO's Michelson Doppler Imager (MDI), which records the vertical motion of the Sun's surface at a million different points once a minute. SOHO's investigators have already been 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 Sun's visible surface (Fig. 1). The convection zone lies directly beneath the photosphere, which forms the 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 the Earth.
MDI data have been used to calculate the time it takes for sound to travel between many different points on the solar surface. Because the paths of these sound waves loop down