  Solar ionizing factors are responsible for modulation of interstellar neutral
gas and its derivative populations inside the heliosphere. We provide an
overview of the current state of knowledge about them for heliospheric
particles inside the termination shock. We discuss charge exchange with solar
wind particles, photoionization, and electron impact ionization for hydrogen,
oxygen, neon, and helium from 1985 to 2018 both in the ecliptic plane and in
the polar regions. We discuss ionization rates as a function of time, distance
to the Sun, and latitude. We compare the total ionization rates among the
species within a consistent and homogeneous system of calculation of the
ionization rates. The highest total ionization rates at 1 au in the ecliptic
plane are for hydrogen and oxygen, and the lowest are for helium. In the polar
regions, the strongest ionization losses are for oxygen, regardless of the
solar activity. Photoionization is the dominant ionization reaction for helium
and neon, and a reaction of high significance for oxygen. Charge exchange with
solar wind particles is the dominant ionization reaction for hydrogen and the
second important ionization reaction for oxygen. Electron impact ionization is
an important ionization reaction for Ne and He, with the contribution to the
total ionization rates stronger within 1 au and smaller outside. The total
ionization rates for He and Ne vary in time with the solar activity, whereas
the total ionization rates for H and O follow the cyclic solar wind variations
out of the ecliptic plane and aperiodic variations in the ecliptic plane.
