spase://ESA/NumericalData/SOHO/SWAN/SSA_SDAC

The SWAN Instrument is designed to observe the solar Lyman alpha photons (121.6 nm) backscattered by the neutral hydrogen atoms present in the interplanetary medium. The background images show the distribution pattern of the backscattered Lyman alpha photons as observed from SOHO. This image, obtained in approximately 24 hours, is then processed to reveal spatial variations of the solar illuminating flux. These spatial variations are correlated to the actual activity on the solar disk. Because SWAN observes backscattered photons, it is actually possible to 'see' those which are originating from the farside of the Sun.