100 Encyclopedia of the Solar System
declining years of solar activity. This observation led to the
suggestion that certain regions on the Sun, commonly called
M (for magnetic)-regions, occasionally produce long-lived
charged particle streams in interplanetary space. Further,
because some form of auroral and geomagnetic activity is
almost always present at high geomagnetic latitudes, it was
inferred that charged particles from the Sun almost contin-
uously impact and perturb the geomagnetic field.
Observations of modulations in galactic cosmic rays
(highly energetic charged particles that originate outside
the solar system) in the 1930s also suggested that plasma
and magnetic fields are ejected from the Sun during inter-
vals of high solar activity. For example, S. Forbush noted
that cosmic ray intensity often decreases suddenly during
large geomagnetic storms and then recovers slowly over a
period of several days. Moreover, cosmic ray intensity varies
in a cycle of∼11 years, but roughly 180◦out of phase with
thesolar activity cycle. One possible explanation of these
observations was that magnetic fields embedded in plasma
clouds from the Sun sweep cosmic rays away from the vicin-
ity of Earth.
In the early 1950s, L. Biermann concluded that there
must be a continuous outflow of charged particles from the
Sun to explain the fact that ionic tails of comets always point
away from the Sun. He estimated that a continuous particle
flux on the order of 10^10 protons cm−^2 s−^1 was needed at
1 AU to explain the comet tail observations. He later revised
his estimate downward to a value of∼ 109 protons cm−^2 s−^1 ,
closer to the average observed solar wind proton flux of∼3.8
× 108 protons cm−^2 s−^1 at 1 AU.
1.2 Parker’s Solar Wind Model
Apparently inspired by these diverse observations and in-
terpretations, E. Parker, in 1958, formulated a radically new
model of the solar corona in which the solar atmosphere is
continually expanding outward. Prior to Parker’s work most
theories of the solar atmosphere treated the corona as static
and gravitationally bound to the Sun except for sporadic out-
bursts of material into space at times of high solar activity.
S. Chapman had constructed a model of a static solar corona
in which heat transport was dominated by electron thermal
conduction. For a 10^6 K corona, Chapman found that even
a static solar corona must extend far out into space. Parker
realized, however, that a static model leads to pressures at
large distances from the Sun that are seven to eight orders of
magnitude larger than estimated pressures in the interstel-
lar plasma. Because of this mismatch in pressure at large
heliocentric distances, he reasoned that the solar corona
could not be in hydrostatic equilibrium and must therefore
be expanding. His consideration of the hydrodynamic (i.e.,
fluid) equations for mass, momentum, and energy conser-
vation for a hot solar corona led him to unique solutions for
the coronal expansion that depended on the coronal tem-
perature close to the surface of the Sun. Parker’s model
produced low flow speeds close to the Sun, supersonic flow
speeds far from the Sun, and vanishingly small pressures at
large heliocentric distances. In view of the fluid character
of the solutions, Parker called this continuous, supersonic,
coronal expansion the solar wind. The region of space filled
by the solar wind is now known as the heliosphere.1.3 First Direct Observations of the Solar Wind
Several Russian and American space probes in the 1959–
1961 era penetrated interplanetary space and found
tentative evidence for a solar wind. Firm proof of the wind’s
existence was provided by C. Snyder and M. Neugebauer,
who flew a plasma experiment onMariner 2during its
epic 3-month journey to Venus in late 1962. Their experi-
ment detected a continual outflow of plasma from the Sun
that was highly variable, being structured into alternating
streams of high- and low-speed flows that lasted for several
days each. Several of the high-speed streams recurred at
roughly the rotation period of the Sun. Average solar wind
proton densities (normalized for a 1 AU heliocentric dis-
tance), flow speeds, and temperatures during this 3-month
interval were 5.4 cm−^3 , 504 km/s, and 1.7× 105 K, respec-
tively, in essential agreement with Parker’s predictions. The
Mariner 2observations also showed that helium, in the
form of alpha particles, is present in the solar wind in vari-
able amounts; the average alpha particle abundance relative
to protons of 4.6% is about a factor of 2 lower than estimates
of the helium abundance within the Sun. Finally, measure-
ments made byMariner 2confirmed that the solar wind
carried a magnetic field whose strength and orientation in
the ecliptic plane were much as predicted by Parker (see
Section 3).
Despite the good agreement of observations with
Parker’s model, we still do not fully understand the pro-
cesses that heat the solar corona and accelerate the solar
wind. Parker simply assumed that the corona is heated to
a very high temperature, but he did not explain how the
heating was accomplished. Moreover, it is now known that
electron heat conduction is insufficient to power the coro-
nal expansion. Present models for heating the corona and
accelerating the solar wind generally fall into two classes: (1)
heating and acceleration by waves generated by convective
motions below the photosphere and (2) bulk acceleration
and heating associated with transient events in the solar
atmosphere such asmagnetic reconnection. Present ob-
servations are incapable of distinguishing among these and
other alternatives.2. Statistical Properties in the
Ecliptic Plane at 1 AU
Table 1 summarizes a number of statistical solar wind
properties derived from spacecraft measurements in the