Latest SOHO images -- a new service for the solar eclipse
4 August 1999The media and the web-surfing public, including teachers, now have instant access to the newest images of the Sun from the ESA-NASA SOHOspacecraft. In addition to still pictures from selected instruments, movies compiled from images of the previous 48 hours are available on-line. Although its introduction is timely, with a total eclipse of the Sun due on 11 August, this service of "The very latest SOHO images" will be used by solar physicists and space weather forecasters for years to come.
Previews of what the solar atmosphere, or corona, will look like during the eclipse are seen in the coronagraph images from LASCO C2 and C3. They show, by visible light, broad "streamers" and narrow "plumes". Here gas flows outwards along magnetic field lines to create the solar wind which blows through the Solar System. The lowest part of the atmosphere is hidden by a mask that blocks the direct light of the Sun, and a ring in the mask indicates the true size of the Sun as normally seen. The widths of the images are, for C2, 6 solar diameters (about 8 million kilometres) and for C3, 30 solar diameters (about 40 million kilometres).
From the EIT instrument on SOHO come ultraviolet images of the Sun's full face, revealing the present commotion in the atmosphere. Three EIT channels show gas at different temperatures, corresponding to different ultraviolet wavelengths in angstrom units. EIT 171 picks out gas at 1 million °C, whilst for EIT 195 the temperature is 1.5 million °C. EIT 304 shows mainly the lower part of the atmosphere at just 80 000 °C. (A fourth channel, EIT 284, is excluded from the "latest images" package but can be accessed via the SOHO home page.) The stormiest regions, seen as bright swirls in the EIT images, are often concentrated over dark sunspots on the visible face.
Pictures of the present distribution of sunspots come from the MDI instrument on SOHO, which measures visible light at a million points across the surface. MDI's main task is to study the Sun's interior by detecting sound waves at the surface, but the sunspot images are a helpful by- product. They show relatively cool patches created by intense concentrations of magnetic fields that penetrate the visible surface. At present the count of sunspots is increasing, as the Sun approaches a maximum of activity due next year.
The date and time of each observation appear in the format year/month/day and hr:min UT (or GMT). Add 2 hours for Central European Time (summer). Remember that the Sun rotates, so that features move across the visible face from far left to far right in two weeks. SOHO images show north at the top, while during the eclipse as seen in Western Europe the Sun's poles will slant a little clockwise from the vertical. SOHO is stationed far beyond the Moon's orbit, so it will see no eclipse itself.