THE EFFECT OF THE TIME AND LATITUDE-DEPENDENT SOLAR IONIZATION RATE ON THE MEASURED LYMAN ALPHA-INTENSITY

We have studied how the varying solar ionisation rate and the net forc e affect the density distribution of interplanetary neutral hydrogen a nd interplanetary Lyman alpha radiation in the solar system. The ionis ation rate is assumed to be latitude- and time-dependent and the net f orce is assumed to vary in magnitude during the solar cycle. Therefore , both time-dependent and 3-dimensional effects can be studied for the first time. The main result is that the variation during the solar cy cle is significant both in density values and in Lyman alpha (Ly alpha ) radiation. The effects with reasonable modelling parameters show tha t differences near the Sun are measurable. They also show that intensi ties measured perpendicular to the wind can be modelled using an appro priate stationary solution. Then the errors are less than 7%. Solar ma ximum intensities are underestimated and solar minimum values are over estimated. However, the line-of-sight (LOS) toward the downwind direct ion is a special case; it cannot be modelled without taking into accou nt temporal effects. We have used a model where the latitudinal distri bution of the ionisation rate changes according to the phase of the so lar cycle. The ionisation rate is flat and high during the solar maxim um and toward the solar minimum it decreases near the poles. The decre ase from the equator to the poles is 40% at the time of minimum. We ha ve studied lines-of-sight directed parallel to the solar rotation axis and lines-of-sight lying in the equator plane. All LOS are perpendicu lar to the interstellar wind. The long term averages of measured Ly al pha-intensity vary significantly according to the solar cycle. Their i ncrease from the solar minimum to the solar maximum is between 15% and 80% using the ionisation rate explained above. However, the overall c hanges of scattered Ly alpha-intensity are highly dominated by the lon g term temporal variation of the solar Ly alpha flux. The highest incr ease at 2 AU on the upwind side of the Sun results because the assumed long term variation in the solar L alpha flux dominates the behaviour of the intensity curve. Therefore, time-dependent variations further away from the Sun seem to be strongly dependent on the modelling of th e solar flux. The time-dependent variations in densities on the lines- of-sight lying perpendicular to the wind axis disappear around 10 - 15 AU from the Sun. However, on the downwind axis the temporal variation s exist even 50 AU from the Sun. The intensity depends also on the LOS direction. Using this specific model the poleward intensity values ar e 20-40% higher than the equator type intensity values at 1 AU at sola r minimum. The exact value depends on the measuring point around the S un.