  The NIS He gas has been observed at a few AU to the Sun almost from the
beginning of the space age. To model its flow an estimate of the loss rates due
to ionization by solar extreme-ultraviolet (EUV) flux is needed. The EUV
irradiance has been measured directly from mid 1990-ties, but with high
temporal and spectral resolution only from 2002. Beforehand only EUV proxies
are available. A new method of reconstruction of the Carrington rotation
averaged photoionization rates for neutral interstellar helium (NIS He) in the
ecliptic at 1 AU to the Sun before 2002 is presented. We investigate the
relation between the solar rotation averaged time series of the ionization
rates for NIS He at 1 AU derived from TIMED measurements of EUV irradiance and
the solar 10.7 cm flux (F10.7) only. We perform a weighted iterative fit of a
nonlinear model to data split into sectors. The obtained formula allows to
reconstruct the solar rotation averages of photoionization rates for He between
~1947 and 2002 with an uncertainty ranging from less than 10% during solar
minimum up to 20% for solar maximum.
