Astron. Astrophys. 326, 396-411 (1997)

Variation of bulk velocity and temperature anisotropy of neutral heliospheric hydrogen during the solar cycle

Maciej Bzowski ¹, Hans J. Fahr ², Daniel Ruciski ¹ and Horst Scherer <sup>2

1</sup> Space Research Centre of the Polish Academy of Sciences, Bartycka 18 A, PL-00-716 Warsaw, Poland
² Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany

Received 6 January 1997 / Accepted 24 April 1997

Abstract

Using a time-dependent kinetic approach the density, bulk velocity vector, and temperature tensor of interstellar hydrogen in the inner Solar System in several phases of solar cycle were computed. Model solar cycle time-profiles of hydrogen ionisation rate and radiation pressure were used, solar latiudinal effects and filtration at the heliospheric interface were neglected. It was concluded that due to the joint action of solar ionisation and radiation pressure the bulk velocity of the gas is strongly variable during solar cycle and, within 15-30 AU from the Sun, it significantly changes with the heliocentric distance 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 temperature tensor on various lines can change from 5 000 to 11 000 K upwind and from to 45 000 K downwind at 1 AU and from 6 000 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 radial temperature along the sightline is strongest during solar minimum and it is equal to about 3 000 K in the upwind direction and to about 5 000 K in the downwind direction. The smallest change occurs during solar maximum. The upwind-to-downwind ratio of intensity of backscattered radiation varies during the solar cycle by about 20% around the mean value. A brief discussion of theoretical spectra of interplanetary lines is provided. The main conclusion is that for observations carried out from 1 AU the Doppler shift of interplanetary lines corresponds to the bulk speed "in infinity" for the lines of sight directed downwind; for the lines of sight directed upwind the Doppler shift corresponds to the bulk speed increased by about 25% in comparison with the bulk speed "in infinity" except solar maximum epoch, when the increase 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 directed upwind; for the lines of sight directed crosswind and downwind the line width returns temperatures higher by about 3 000 K and no simple seasonal correlation can be observed.

Key words: solar system: interplanetary medium Sun: activity

Send offprint requests to: M. Bzowski

Contents

• 1. Introduction
• 2. Basic model assumptions
• 3. Computation of moments of the hydrogen distribution function
• 4. Solar cycle variation of local density, bulk velocity and temperature of interplanetary hydrogen
  • 4.1. Density
  • 4.2. Bulk velocity
  • 4.3. Temperature
• 5. Practical application: consequences on observations of backscattered Lyman- radiation
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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