The Solar Wind 107
50 km/s in some of the slowest events to greater than 2500
km/s in the fastest ones. The average CME speed at∼5 so-
lar radii is close to the median ecliptic solar wind speed of
∼440 km/s. Since observed solar wind speeds near 1 AU are
never less than∼280 km/s, the slowest CMEs are further
accelerated enroute to 1 AU.
7.2 Origins, Associations with Other Forms of Solar
Activity, and Frequency of Occurrence
The processes that trigger CMEs and that determine their
sizes and outward speeds are only poorly understood; there
is presently no consensus on the physical processes respon-
sible for initiating or accelerating these events, although
it is clear that stressed magnetic fields are the underlying
cause of these events and that CMEs play a fundamental
role in the long-term evolution of the structure of the so-
lar corona. They appear to be an essential part of the way
the corona responds to the evolution of the solar magnetic
field associated with the advance of the solar activity cycle.
Indeed, the release of a CME is one way that the solar at-
mosphere reconfigures itself in response to changes in the
solar magnetic field. CMEs are commonly, but not always,
observed in association with other forms of solar activity
such as eruptive prominences and solar flares. From a his-
torical perspective, one might be led to expect that large
solar flares are the prime cause of CMEs; however, it is
now clear that flares and CMEs are separate, but closely
related, phenomena associated with magnetic disturbances
on the Sun. Like other forms of solar activity, CMEs occur
with a frequency that varies in a cycle of∼11 years. On av-
erage, the Sun emits about 3.5 CMEs/day near the peak of
the solar activity cycle, but only about 0.1 CMEs/day near
solar activity minimum.
7.3 Heliospheric Disturbances Driven by Fast Coronal
Mass Ejections
As illustrated in Fig. 7b, fast CMEs produce transient solar
wind disturbances that, in turn, often are the cause of large,
geomagnetic storms. [SeeSun–EarthConnections.]
Figure 8 shows calculated radial speed and pressure pro-
files of a simulated solar wind disturbance driven by a fast
CME at the time the disturbance first reaches 1 AU. As
indicated by the insert in the top portion of the figure, the
disturbance was initiated at the inner boundary of the one-
dimensional fluid calculation by abruptly raising the flow
speed from 275 to 980 km/s, sustaining it at this level for 6
hours, and then returning it to its original value of 275 km/s.
The initial disturbance thus mimics a uniformly fast, spa-
tially limited CME with an internal pressure equal to that of
the surrounding solar wind plasma. A region of high pres-
sure develops on the leading edge of the disturbance as
the CME overtakes the slower wind ahead. This region of
FIGURE 8 Solar wind speed and pressure as functions of
heliocentric distance for a simple, one-dimensional gas-dynamic
simulation of a CME-driven disturbance. The dashed line
indicates the steady state prior to introduction of the temporal
variation in flow speed imposed at the inner boundary of 0.14
AU and shown at the top of the figure. The hatching identifies
material that was introduced with a speed of 980 km/s at the
inner boundary, and therefore identifies the CME in the
simulation. [Adapted originally from A. J. Hundhausen, 1985, in
“Collisionless Shocks in the Heliosphere: A Tutorial Review”
(R. G. Stone and B. T. Tsurutani, eds.), American Geophysical
Union, Washington, D.C.].higher pressure is bounded by a forward shock on its leading
edge that propagates into the ambient solar wind ahead and
by a reverse shock on its trailing edge that propagates back-
ward into and eventually through the CME. Both shocks
are, however, carried away from the Sun by the highly su-
personic flow of the solar wind. Observations and more de-
tailed calculations indicate, however, that reverse shocks in
CME-driven disturbances are ordinarily present only near
the central portions of the disturbances.