The Solar Wind 115
and energetic particles associated with CIRs, CMEs, solar
flares, and the planetary bow shocks. All but the galactic
cosmic rays are energized within the heliosphere.
Shocks are particularly effective particle accelerators,
and all but one of the preceding populations have shock
origins. The physical process by which a collisionless shock
accelerates a small fraction of the ions it intercepts to high
energy is reasonably well understood, although complex
in detail. The effectiveness of the acceleration process de-
pends upon factors such as shock speed and strength, the
angle between the magnetic field and the shock, normal
time of field line connection to the shock, and the local
reservoir of particles available for acceleration. Recent work
indicates that shocks in the solar wind most easily accelerate
ions that already exceed solar wind thermal energies when
they encounter the shocks. These so-called seed particles
include the suprathermal ion tails always present in the
low-speed wind (Fig. 13b), but also “pickup ions” (see dis-
cussion that follows), and energetic particles remaining in
the heliosphere from previous solar flares and CME-driven
disturbances.
Anomalous cosmic rays have energies that are lower than
the galactic cosmic rays; are predominantly singly ionized H,
He, N, O, and Ne; and, like galactic cosmic rays, have an in-
tensity that varies slowly with time. They are associated with
a particularly interesting seed population—neutral particles
from the local interstellar cloud that penetrate deep into the
heliosphere. As the neutrals approach the Sun, they are ion-
ized by solar extreme ultraviolet radiation, electron impact,
or charge exchange with solar wind protons; are then picked
up by the solar wind magnetic field (the pickup process ac-
celerates them to∼4 keV/nucleon); and are swept into the
outer reaches of the heliosphere by the solar wind flow.
As they encounter the heliosphere’s termination shock, the
pickup ions are accelerated to high energies. After acceler-
ation, the energized particles diffuse back into the interior
of the heliosphere as anomalous cosmic rays.
Of the energetic ion populations in the heliosphere, that
associated directly with solar flares appears to be the only
population that is not obviously shock-associated, although
even in this case shock acceleration cannot be ruled out con-
clusively. Flare events are usually impulsive and short-lived
(hours), are overabundant in^3 He, appear to originate low in
the solar atmosphere, occur at a rate of∼1000 events/year
near solar activity maximum, and generally occur in asso-
ciation with impulsive energetic solar electron bursts. The
latter have energies ranging up to several hundred keV. Re-
cent work suggests that solar electron bursts originate at
a variety of altitudes in the solar atmosphere and can be
triggered by more than one process.
13. Waves and Turbulence
The solar wind is filled with waves and turbulence having
various spatial and temporal scales. Figure 15 illustrates that
FIGURE 15 Solar wind velocity data sampled at a cadence of
64s inr−,t−, andn-coordinates, where the+r-direction is
radial outward from the Sun, the+t-direction is in the direction
of the Earth’s motion about the Sun, and the+n-direction
completes a right-handed system. The high-frequency
fluctuations in this representative 25-day interval are caused by
waves and turbulence in the wind.fluctuations in solar wind velocity associated with waves and
turbulence are observed throughout much of the solar wind,
but fluctuation amplitudes tend to be greatest in high-speed
streams. Many of these fluctuations are Alfv ́enic in nature
(coupled changes in flow velocity and magnetic field vec-
tors) and the waves and turbulence usually are propagating
away from the Sun through the solar wind; together with
the fact that fluctuation amplitudes generally decrease with
distance from the Sun, this indicates that many of these fluc-
tuations originate close to the Sun. Indeed, it is commonly
thought that the fluctuations are largely remnants of waves
and turbulence in the solar corona that heat and acceler-
ate the solar wind. In addition to their probable, but poorly
understood, role in heating and accelerating the solar wind
plasma, waves and turbulence strongly affect energetic par-
ticle transport in the heliosphere and are essential elements
of most current models of particle energization at shocks in
the heliosphere.14. Conclusion
The solar wind is a magnificent natural laboratory for study-
ing and obtaining understanding of processes and phe-
nomena that also occur in a variety of other astrophysical
contexts. These include kinetic and fluid aspects of plas-
mas, plasma heating and acceleration, collisionless shock
formation, particle acceleration and transport, magnetic