VARIATION OF BULK VELOCITY AND TEMPERATURE ANISOTROPY OF NEUTRAL HELIOSPHERIC HYDROGEN DURING THE SOLAR-CYCLE

Using a time-dependent kinetic approach the density, bulk velocity vec tor, and temperature tensor of interstellar hydrogen in the inner Sola r System in several phases of solar cycle were computed. Model solar c ycle time-profiles of hydrogen ionisation rate and Lyman-cr radiation pressure were used, solar latiudinal effects and filtration at the hel iospheric interface were neglected. It was concluded that due to the j oint action of solar ionisation and radiation pressure the bulk veloci ty of the gas is strongly variable during solar cycle and, within 15-3 0 AU from the Sun, it significantly changes with the heliocentric dist ance both in magnituide and direction. The changes typically are about 15 km/s and exceed the thermal spread of the gas. The temperature is strongly anisotropic; the anisotropy is strongly variable in time and it fades off with the heliocentric distance. The projections of temper ature tensor on various lines can change from 5 000 to 11 000 K upwind and from similar to 12 000 to 45 000 K downwind at 1 AU and from 6 00 0 to 8 000 K upwind and from 8 000 to 15 000 K downwind at 10 AU. For optical observations an important quantity is the radial temperature. For lines of sight directed radially away from the Sun the change of r adial temperature along the sightline is strongest during solar minimu m and it is equal to about 3 000 K in the upwind direction and to abou t 5 000 K in the downwind direction. The smallest change occurs during solar maximum. The upwind-to-downwind ratio of intensity of backscatt ered radiation varies during the solar cycle by about 20% around the m ean value. A brief discussion of theoretical spectra of interplanetary lines is provided. The main conclusion is that for observations carri ed out from 1 AU the Doppler shift of interplanetary lines corresponds to the bulk speed ''in infinity'' for the lines of sight directed dow nwind; for the lines of sight directed upwind the Doppler shift corres ponds to the bulk speed increased by about 25% in comparison with the bulk speed ''in infinity'' except solar maximum epoch, when the increa se is only 1 to 2 km/s. The width of interplanetary lines returns the temperature of the gas ''in infinity'' only for the lines of sight dir ected upwind; for the lines of sight directed crosswind and downwind t he line width returns temperatures higher by about 3 000 IC and no sim ple seasonal correlation can be observed.