MPAe T1ex127-.45ex51275atigkeitsbericht 2002/2003 - Max-Planck ...

Solar wind and heliosphere
Solar corona and solar wind
Dynamic sun and solar wind
II.1. SONNE UND HELIOSPHÄRE/SUN AND HELIOSPHERE 37
As a contribution to the book Dynamic Sun published
by Cambridge University Press, a review was written
about the solar wind, discussing theory and observations.
The article content as summarized in the abstract
is presented in the subsequent paragraph.
There are three major types of solar wind: The steady
fast wind originating in coronal holes, the unsteady
slow wind (coming partly from opening streamers)
and the very fast transient wind in the form of coronal
mass ejections. The fast streams are, at least during
solar minimum, the normal modes of the solar
wind. Their basic properties can be reproduced by
multi-fluid models involving waves. We briefly review
the history of the subject and describe some of
the modern theories of the fast wind. We then discuss
the boundary conditions and in situ constraints which
are imposed on the models, in particular by Ulysses
at high latitudes. The recent SOHO observations have
brought a wealth of new informations on the state of
the wind in the inner corona as well as the plasma
source conditions prevailing in the transition region
and solar chromosphere. Some of these results are presented
here. Finally, problem areas are identified, and
future research perspectives are outlined.
(E. Marsch, W.I. Axford, J.F. McKenzie)
The microstate of the solar wind
In an invited review talk given at the Solar Wind Ten
conference held in Pisa (Italy) in 2002, the microscopic
state of the solar wind was reviewed, in particular
the measurements and models for the proton
and electron velocity distributions and kinetic features
of heavy ions in the fast solar wind and coronal holes.
It is now generally accepted that the electron distributions
are largely determined by Coulomb collisions.
Concerning the ions, there is mounting evidence that
pitch-angle diffusion in resonance with ion-cyclotron
waves is the main process forming the shape of ion
velocity distributions. Moreover, the absorption of
high-frequency waves seems to play a major role in
the heating of the corona and solar wind. Understanding
dispersive plasma waves and the associated
wave-particle interactions holds the key to this problem.
Plasma stability analyses and model calculations,
as well as observations adressing these subjects were
briefly reviewed, while focussing on the critical kinetic
physics issues.
(E. Marsch)
Ion velocity distributions and cyclotron wave
absorption or opacity
The wave absorption coefficient or opacity is a key
parameter in the phyiscs of wave heating of the solar
corona. In simulation results, obtained by solving our
model equations numerically, it is found that heavy
ions are preferentially heated, and that considerable
temperature anisotropies thereby develop. The velocity
distribution functions (VDFs) of the heavy ions deviate
strongly from Maxwellians, an effect which increases
with height in the corona due to the declining
efficiency of Coulomb collisions.
The corresponding wave spectra, as calculated numerically
in the diffusion model, show rather deep absorption
edges that occur at and below the ion gyrofrequencies
of the species involved. At a certain height,
all waves with frequencies above the lowest ion gyrofrequency
will have suffered severe damping. This
is the essence of the frequency sweeping mechanism
introduced and worked out in detail by the Lindau
group. Because of this strong absorption, it was argued
that the waves solely originating from the coronal
base would not suffice to heat coronal ions, but that
local wave production was required too.
Fig. 33: Normalized damping rate (per resonant particle)
as a function of the resonant speed of O 5+ ions
calculated at 2.46 Rs. The plot displays a distinct flatness
of the curve over a wide range of speeds, corresponding
to a quasilinear plateau in the VDF.
However, the wave damping rate γ is a sensitive function
of the VDF. For cyclotron waves propagating
parallel to the background magnetic field in a multicomponent
plasma, this normalized damping rate is
plotted in Fig. 33 for resonant oxygen ions. Over
a wide range of negative speeds γ is close to zero,
meaning that the VDF calculated from the diffusion
model has reached marginal stability. Apparently, a
major fraction of the oxygen ions then has a wave
absorption coefficient close to zero, i.e. their opacity
vanishes, whereupon wave heating of ions with
higher resonance frequencies (such as alpha particles
and protons) is facilitated in the extended corona.
(C. Vocks, E. Marsch)