Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...314..663s&link_type=abstract
Astronomy and Astrophysics, v.314, p.663-671
Astronomy and Astrophysics
Astrophysics
11
Interplanetary Medium, Sun: Solar Wind, Galaxy: Solar Neighbourhood
Scientific paper
We have studied how the varying solar ionisation rate and the net force affect the density distribution of interplanetary neutral hydrogen and interplanetary Lyman α radiation in the solar system. The ionisation rate is assumed to be latitude- and time-dependent and the net force 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 cycle is significant both in density values and in Lyman α (Lyα) radiation. The effects with reasonable modelling parameters show that differences near the Sun are measurable. They also show that intensities measured perpendicular to the wind can be modelled using an appropriate stationary solution. Then the errors are less than 7%. Solar maximum intensities are underestimated and solar minimum values are overestimated. However, the line-of-sight (LOS) toward the downwind direction is a special case; it cannot be modelled without taking into account temporal effects. We have used a model where the latitudinal distribution of the ionisation rate changes according to the phase of the solar cycle. The ionisation rate is flat and high during the solar maximum and toward the solar minimum it decreases near the poles. The decrease from the equator to the poles is 40% at the time of minimum. We have studied lines-of-sight directed parallel to the solar rotation axis and lines-of-sight lying in the equator plane. All LOS are perpendicular to the interstellar wind. The long term averages of measured Lyα -intensity vary significantly according to the solar cycle. Their increase from the solar minimum to the solar maximum is between 15% and 80% using the ionisation rate explained above. However, the overall changes of scattered Lyα -intensity are highly dominated by the long term temporal variation of the solar Lyα flux. The highest increase at 2AU on the upwind side of the Sun results because the assumed long term variation in the solar Lα 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 the solar flux. The time-dependent variations in densities on the lines-of -sight lying perpendicular to the wind axis disappear around 10-15AU from the Sun. However, on the downwind axis the temporal variations exist even 50AU from the Sun. The intensity depends also on the LOS direction. Using this specific model the poleward intensity values are 20-40% higher than the equator type intensity values at 1AU at solar minimum. The exact value depends on the measuring point around the Sun.
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