Figure 3.2.6/1: A mosaic of SOHO results (1).
The SOHO Project Scientist Team:
SOHO homepages http://sohowww.nascom.nasa.gov and http://www.estec.esa.nl/spdwww/soho/html
SOHO, within a year of its launch, and after only 7 months of science operations, has taken centre stage in the discipline of Solar Physics. Evry Schatzman of the Observatoire de Meudon called it '. . . the most remarkable and successful spacecraft ever devoted to examining the star on which our lives depend....to fulfill its high promise, SOHO must continue operating at least until the maximum of sunspot activity around the year 2000.'
In its initial year, SOHO's helioseismology experiments (GOLF, VIRGO and MDI), while patiently pursuing their holy grail the detection of solar gravity modes have produced spectra cleaner by at least an order of magnitude than observable from the ground (Fig. 3.2.6/1, top left) and have, for the first time ever, provided a glimpse of convective flows below a stellar surface (Fig. 3.2.6/1, top right).
Figure 3.2.6/1: A mosaic of SOHO results (1).
The in situ particle detectors (ERNE and COSTEP) regularly detect high energy protons, electrons and alpha particles in the aftermath of filament eruptions and Coronal Mass Ejections (CMEs). CELIAS, SOHO's solar wind analyser, has doubled the number of elements and isotopes recorded in the solar wind (Fig. 3.2.6/1, middle left). During a Sun-Venus-Earth alignment, CELIAS observed for the first time from such a large distance pick-up ions from Venus' ionosphere, finding a much smaller diffusion cone than anticipated. SWAN, the solar wind anisotropy detector, has recorded full-sky Lyman-alfa maps at a resolution and signal to noise ratio of at least a factor 10 better than previous observations (Fig. 3.2.6/1, middle right). From these maps, the latitude distribution of the solar wind can be determined.
SOHO's solar visible light and UV imagers and spectrographs (MDI, SUMER, CDS, EIT, LASCO and UVCS) are producing a wealth of data, the analysis of which has barely begun. Among the first results, immediately clear from the time-lapse sequences, is that there is no such thing as the quiet Sun: SOHO's UV detectors show structure at all observable size and time scales in regions that are quite unremarkable in soft X-ray images, such as those from Yohkoh and Skylab. One example is the so-called polar plumes, columns of high temperature and density plasma in the polar open field regions (Fig. 3.2.6/1, bottom left).
The SOHO coronographs have found that CMEs are more frequent and carry more material than previously thought. In fact, the concept is now emerging that the 'slow' solar wind, which emanates from the Sun's equatorial regions, is nothing else but an almost uninterrupted series of small and large CMEs. The high cadence synoptic sequence of LASCO observations is perfectly suited to verifying this hypothesis. Fig. 3.2.6/1, bottom right, shows a remarkable LASCO image of a CME simultaneous with Comet Hyakutake approaching its perihelion.
The 'fast' solar wind, from coronal holes, is probably driven in a completely different manner. UVCS, SOHO's UV coronal spectrometer, has observed similar velocities for heavy ions and protons, indicating bulk motions, perhaps in the form of magnetohydrodynamic waves, that can drive the wind outflow. Fig. 3.2.6/2, top left, shows a composite UV image from EIT (inner region) and UVCS (outer region) tracing magnetic structures from the surface out to several solar radii.
Figure 3.2.6/2: A mosaic of SOHO results (2).
Closer to the surface, SOHO's twin extreme-UV spectrometers, SUMER and CDS, have produced maps of total emission, velocities, plasma densities and temperatures for a wide variety of solar structures and phenomena, such as prominences, coronal holes, the network, jets, plumes, bright points and active region loops. Preliminary spectral analysis confirms the impression from time- lapse series of images: there is ceaseless activity at all scales. Equally important are the synoptic programmes of SUMER and CDS, which have resulted in spectral atlases for various objects with unprecedented accuracy. More than 2000 emission lines have been observed by SUMER (Fig. 3.2.6/2, top right), while CDS through its unique combination of high angular spectral and temporal resolution has recorded, for the first time, reliable spectra for several small-scale features, such as jets, loops and prominences. Fig. 3.2.6/2, bottom left, for example, shows a prominence observed by CDS in the helium I line.
Finally, in addition to its helioseismology data, MDI provides about 15 full-disc magnetograms and white light images daily (Fig. 3.2.6/2, bottom right). While the resolution of these images is less than can be achieved from the ground, the continuity and absence of atmospheric interference leads to sequences that are extremely well-suited to observing the evolution of the surface magnetic field over the course of the solar cycle.
SOHO science is more than the sum of each experiment's contribution. From the very beginning, the instruments have devoted much of their observing time to Joint Observing Programs (JOPs), tackling specific physical problems such as the temperature and density structure in coronal holes, through coordinated observations with other spacecraft, such as Yohkoh and Ulysses, and numerous ground-based observatories worldwide. In a typical SOHO week, 3-5 different JOPs may be run, each several times.
SOHO operations are conducted from the Goddard Space Flight Center (GSFC), where the entire ESA SOHO Project Scientist Team is resident. In close collaboration with ESA SOHO Project personnel, it planned and coordinated SOHO's commissioning and now, in the operational phase, it plays a central role in planning and coordinating SOHO science operations. It maintains a highly visible Web page (about 8000 information requests daily in November 1996). Features include the latest solar images from SOHO and other observatories around the world, a gallery of SOHO images and data, and the latest information on SOHO observing sequences, schedules and targets. The Website ( http://sohowww.nascom.nasa. gov) has proved to be a very efficient means of communication among the 12 experiment teams spread over Europe and the US, as well as an innovative public relations link to the outside world.
The SOHO Project Scientist Team also supports and manages the development of the SOHO archives (one at GSFC, three in Europe). Perhaps 'data libraries' is a better description, as they provide a platform for SOHO-wide data analysis, using integrated software, as the data come down from the spacecraft. The first such data library is now operational at GSFC.
Other Project Scientist Team activities include the organisation of SOHO science meetings and workshops, as well as coordinating and editing SOHO publications, such as special issues of scientific journals, conference proceedings, press releases, articles in non-scientific magazines, brochures, and posters. In 1997 there will be two SOHO-dedicated issues of Solar Physics and one of Geophysical Research Letters, while numerous informal science work-shops have been planned, such as one on the results of the 'Whole Sun Month' and two on SOHO-Yohkoh collaborative science. Two formal SOHO workshops complement these: one on 'The Corona and Solar Wind near Minimum Activity' in June 1997 in Oslo, and one on helio- seismology in June 1998 in Boston.
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