Flare observation of the Sun as a star by SUMER/SOHO in the hydrogen Lyman continuum

During the execution of the programme "Sun as a star", while the SUMER (Solar Ultraviolet Emission of Emitted Radiation)/SOHO (SOlar and Heliospheric Observatory) slit was collecting the scattered radiation from the telescope mirror far away from the solar disk image, a class X5.3/3b flare erupted on the solar disk, on 25 August 2001. During the first phase of the flare a relative increase of a few percent was detected at the head of the hydrogen Lyman continuum. After correction from the instrumental parameters, the relative signal increase is 70% at the head of the Lyman continuum (910 Angstrom), and 190% in the C II 904 Angstrom multiplet. Accounting for the area of the flare region, the local increase of the radiance of the Lyman continuum and of the C II lines is estimated to be a factor of several thousands. We compare this result with other solar observations and models.