116 Encyclopedia of the Solar System
reconnection, and turbulence and waves. Proof of the exis-
tence of the solar wind was one of the first great triumphs of
the space age, and much has been learned about the physi-
cal nature of the wind and related processes in intervening
years. Nevertheless, our understanding of the solar wind
is far from complete. For example, we still do not know
what physical processes heat and accelerate the solar wind
or what determines its flow speed. We do not yet know if
the low-speed wind arises primarily from quasi-stationary
processes or from a series of small transient solar events.
Likewise, the physical origins of coronal mass ejections are
still being debated; we do not yet fully understand why they
occur or how they relate to the long-term evolution of the
solar magnetic field and the structure of the solar corona.
We do not yet understand how a rough balance of mag-
netic flux is maintained in the solar wind in the presence of
ICMEs or how the magnetic topologies of ICMEs evolve
with time. In general, our ideas about the structure of the
heliospheric magnetic field are still developing and need
testing with observations. Ideas about the termination of
the solar wind in the outer heliosphere and the role of the
termination shock in accelerating anomalous cosmic rays
are just now being tested by in situ observations for the first
time. The physical origin of variations in elemental abun-
dances in the solar wind is just beginning to be understood,
as are temporal changes in the charge states of the heavier
elements. Origins of double ion beams and suprathermal
ion tails in the solar wind also remain unknown, and we do
not yet fully understand why different ionic species have
different speeds and temperatures in the solar wind. Fur-
ther analysis of existing data, new types of measurements,
and fresh theoretical insights should lead to understanding
in these and other areas of solar wind research in the years
ahead.Bibliography
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