Authors: McMullin, D.R., D. L. Judge, E. Phillips, E. Moebius, P. Bochsler, P. Wurz, M. Hilchenbach and F. Ipavich
Reference: Proc. of the SOHO 11 Workshop, ed A. Wilson, ESA SP-508, 489. (2002)
While the absolute measurement of solar EUV flux is an important reason the SEM was included in the SOHO mission, the SEM measurements also provide a method to measure the absolute photoionization rate of neutral interstellar helium flowing into our solar system. After hydrogen, helium is the most abundant substance found in interplanetary space, and the interstellar medium. In the inner solar system photoionization of helium is the dominant ionization process of the inflowing interstellar helium. Thus, an accurate determination of the solar photoionization rate is a requirement in astrophysical research. The daily averaged photoionization rate of helium at 1 AU, derived from the SOHO CELIAS/SEM absolute solar extreme ultraviolet (EUV) flux values is presented for the time period since the launch of SOHO in December, 1995.