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Astron. Astrophys. 363, 295-305 (2000)

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A thermokinetic study of wave-modulated solar wind electrons using truncated maxwellians

I.V. Chashei 1 and H.J. Fahr 2

1 P.N. Lebedev Institute, Russian Academy of Science, Leninskii Prospect 53, 117924 Moscow, Russia
2 Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received 1 December 1999 / Accepted 5 July 2000

Abstract

Classical kinetic solar wind theories reveal that the solar wind electron distribution function has a delicate influence on the acceleration of the coronal solar wind and its asymptotic velocity. In collisionless kinetics this results from the fact that no hyperbolic electrons with sunward velocities can be expected above the coronal exobase. This problem affects both the associated electron pressure and the effective electric polarisation potential helping protons to leave the solar gravitational potential. The actual electron distribution thus influences the asymptotic properties of the solar wind flow. Of importance for a comprehensive understanding of the solar wind acceleration thus is the actual mechanism to populate the sunward velocity branch of the distribution function in the hyperbolic energy regime which according to classical kinetic theory should be unpopulated. In view of the expected electron temperatures at the corona one finds that 30 to 50 percent contributions to the dynamic electron pressure results from this questionable regime. We study the influence of electron depletions in the sunward hemisphere of velocity space in terms of modified velocity moments like the resulting solar wind electron temperature, bulk velocity and heat flow. Parametrizing the electron distribution function by truncated Maxwellians we show that all higher moments of the distribution function can be generated based on knowledge of the three lowest moments. Using solar wind data on electron density, drift, and temperature, we derive an expression for the electron heat flow which perfectly fits the ULYSSES heatflow measurements both by its absolute magnitude and by its radial gradient. To justify truncated electron distribution functions by physical processes we also study the effect of an energy dissipation of fast magnetosonic waves cascading up to the range of whistler frequencies and consider the specific local heat source due to absorption of such wavepowers. An adequate representation of the electron temperature profile without the account of a heating due to wave energy transfer to solar wind electrons may not be achievable at regions beyond 1 AU. As we can also show wave-induced energy absorption occurs just with the adequate rate allowing for truncated Maxwellian electron distribution functions to be maintained in the expanding solar wind.

Key words: acceleration of particles – conduction – plasmas – turbulence – Sun: corona

Send offprint requests to: H.J. Fahr (hfahr@astro.uni-bonn.de)

This article contains no SIMBAD objects.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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