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Ann. Geophys., 20, 1509-1518, 2002
www.ann-geophys.net/20/1509/2002/
© European Geosciences Union 2002
Solar wind heating by an embedded quasi-isothermal pick-up ion fluid
H. J. Fahr
Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn Auf dem Hügel 71, D-53121 Bonn, Germany
Correspondence to: H. J. Fahr (hfahr@astro.uni-bonn.de)
Abstract. It is well known that the solar wind plasma
consists of primary ions of solar coronal origin and secondary ions of
interstellar origin. Interstellar H-atoms penetrate into the inner heliosphere
and when ionized there are converted into secondary ions. These are implanted
into the magnetized solar wind flow and are essentially enforced to co-move
with this flow. By nonlinear interactions with wind-entrained Alfvén waves the
latter are processed in the co-moving velocity space. This pick-up process,
however, also causes actions back upon the original solar wind flow, leading to
a deceleration, as well as a heating of the solar wind plasma. The resulting
deceleration is not only due to the loading effect, but also due to the action
of the pressure gradient. To calculate the latter, it is important to take into
account the stochastic acceleration that suffers at their convection out of the
inner heliosphere by the quasi-linear interactions with MHD turbulences. Only
then can the presently reported VOYAGER observations of solar wind
decelerations and heatings in the outer heliosphere be understood in terms of
the current, most likely values of interstellar gas parameters. In a consistent
view of the thermodynamics of the solar wind plasma, which is composed of
secondary ions and solar wind protons, we also derive that the latter are
globally heated at their motion to larger solar distances. The arising heat
transfer is due to the action of suprathermal ions which drive MHD waves that
are partially absorbed by solar wind protons and thereby establish their
observed quasi-polytropy. We obtain a quantitative expression for the solar
wind proton pressure as a function of solar distance. This expression clearly
shows the change from an adiabatic to a quasi-polytropic behaviour with a
decreasing polytropic index at increasing distances, as has been observed by
the VOYAGERS. This also allows one to calculate the average percentage of the
intitial energy fed into the thermal proton energy. In a first-order evaluation
of this expression we can estimate that under stationary flow conditions about
10% of the initial injection energy is eventually transfered to solar wind
protons, independent of the actual injection rate.
Key words. Interplanetary physics (energetic
particles; interstellar gas; solar wind plasma)
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