On the source regions of the fast solar wind in polar coronal holes

Fast streams of the solar wind with speeds of up to ≈ 800 km s−1 at a distance of 1 AU (astronomical unit) from the Sun are known to originate in solar coronal holes. With the Solar and Heliospheric Observatory (SOHO) detailed studies of the solar wind source regions have been made possible for the first time. We show images of solar polar coronal holes in the extreme ultraviolet (EUV), which were obtained by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrograph on the SOHO spacecraft. The light is emitted in spectral lines of helium atoms and Ne7+ ions formed at temperatures of about 20 000 K and 630 000 K, respectively, in ionization equilibrium. The sources of the fast solar wind in polar coronal holes can clearly be seen in the chromospheric He i line and in the Ne viii line of the low corona, either as dark polar caps in radiance diagrams or as regions of predominant blue shift. The average blue shifts along the line of sight in coronal holes amount to speeds of ≈ 3 km s−1 for both He and Ne7+, if the Doppler shift can be interpreted as an indication of bulk motions. Bright points and polar plumes seen in Ne viii (λ770) do, however, not show signatures of outflow.