In this paper, we perform numerical modeling of the interstellar hydrogen
fluxes measured by IBEX-Lo during orbit 23 (spring 2009) using a
state-of-the-art kinetic model of the interstellar neutral hydrogen
distribution in the heliosphere. This model takes into account the temporal and
heliolatitudinal variations of the solar parameters as well as non-Maxwellian
kinetic properties of the hydrogen distribution due to charge exchange in the
heliospheric interface. We found that there is a qualitative difference between
the IBEX-Lo data and the modeling results obtained with the three-dimensional,
time-dependent model. Namely, the model predicts a larger count rate in energy
bin~2 (20-41 eV) than in energy bin~1 (11-21 eV), while the data shows the
opposite case. We perform study of the model parameter effects on the IBEX-Lo
fluxes and the ratio of fluxes in two energy channels. We shown that the most
important parameter, which has a major influence on the ratio of the fluxes in
the two energy bins, is the solar radiation pressure. The parameter fitting
procedure shows that the best agreement between the model result and the data
occurs in the case when the ratio of the solar radiation pressure to the solar
gravitation, $\mu_0$, is 1.26$^{+0.06}_{-0.076}$, and the total ionization rate
of hydrogen at 1 AU is $\beta_{E,0}=3.7^{+0.39}_{-0.35}\times
10^{-7}$~s$^{-1}$. We have found that the value of $\mu_0$ is much larger than
$\mu_0=0.89$, which is the value derived from the integrated solar Lyman-alpha
flux data for the period of time studied. We discuss possible reasons for the
differences.
