THE NEUTRAL HYDROGEN ABUNDANCE AND IONIZATION IN THE INNER HELIOSPHERE

We have developed a new and self-consistent method to simultaneously d erive the abundance of interplanetary hydrogen and its rate of ionizat ion by the solar environment, Using the principle of invariance and th e solar Ly alpha activity, we show that some properties of the interpl anetary medium radiation held in the inner heliosphere depend uniquely on the ionization rate beta and the local density and can therefore b e derived by a self-consistent technique. Thanks to Voyager and IUE Ly alpha sky maps, we show that the H distribution in the solar neighbor hood is well described by a hot model, characterized by n(local) = 0.1 59 +/- 0.05 cm(-3) at 10 AU and beta = (5.50 +/- 1.65) x 10(-7) s(-1) near the period of maximum solar activity. When extrapolated to the ou ter region of the heliosphere, the same hot model provides a lower den sity limit of n(infinity) = 0.25 +/- 0.08 cm(-3). Any filtration effec t should increase the density. Although our estimate of the ionization rate corresponds to the solar activity of the analyzed period, the IP M density derived is far larger than previously claimed since the disc overy of the interplanetary medium 20 yr ago.