108 Encyclopedia of the Solar System
Except for the reverse shock, the simple calculation
shown in Fig. 8 is consistent with observations of many
solar wind disturbances obtained near 1 AU in the ecliptic
plane and illustrates to first order the radial and tempo-
ral evolution of an interplanetary disturbance driven by a
fast CME (now commonly called an interplanetary coronal
mass ejection, ICME, when observed in the solar wind).
The leading edge of the disturbance is a shock that stands
off ahead of the ICME (see also Fig. 7b). The ambient so-
lar wind ahead of the ICME is compressed, heated, and
accelerated as the shock passes by, and the leading por-
tion of the ICME is compressed, heated, and slowed as
a result of the interaction. In the example illustrated, the
ICME slows from an initial speed of 980 km/s to less than
600 km/s by the time the leading edge of the disturbance
reaches 1 AU. This slowing is a result of momentum transfer
to the ambient solar wind ahead and proceeds at an ever-
slower rate as the disturbance propagates outward. Figure 9
displays selected plasma and magnetic field data from a so-
lar wind disturbance driven by an ICME observed near
1 AU. The shock is distinguished in the data by discontinu-
ous increases in flow speed, density, temperature, and field
strength. The plasma identified as the ICME had a higher
flow speed than the ambient solar wind ahead of the shock.
In this case, it was also distinguished by counterstream-
ing suprathermal electrons (indicative of a closed magnetic
field topology, see Section 10.3), anomalously low proton
temperatures, somewhat elevated helium abundance, and
a strong, smoothly rotating magnetic field that indicates the
magnetic field topology was that of a nested helical structure
(i.e., a flux rope; see Fig. 7b).
7.4 Characteristics of Interplanetary
Coronal Mass Ejections
The identification of ICMEs in solar wind plasma and field
data is still something of an art; however, shocks serve as
useful fiducials for identifying fast ICMEs. Table 2 pro-
vides a summary of plasma and field signatures that qual-
ify as being unusual compared to the normal solar wind,
but that are commonly observed a number of hours after
shock passage. Most of these anomalous signatures are also
observed elsewhere in the solar wind where, presumably,
they serve to identify those numerous, relatively low-speed
ICMEs that do not drive shock disturbances. Few ICMEs
at 1 AU exhibit all of these characteristics, and some of
these signatures are more commonly observed than are
others.
Most ICMEs expand as they propagate outward through
the heliosphere. ICME radial thicknesses are variable; at
1 AU the typical ICME has a radial width of∼0.2 AU,
whereas at Jupiter’s orbit ICMEs can have radial widths
as large as 2.5 AU. Approximately one third of all ICMEs
FIGURE 9 A solar wind disturbance associated with moderately
fast ICME observed by theAdvanced Composition Explorerin
October 1998. From top to bottom the quantities plotted are
color-coded pitch angle distributions of 256–288 eV electrons,
proton density, proton temperature, bulk flow speed,
alpha-proton density ratio, and magnetic field strength, azimuth,
and polar angle in solar ecliptic coordinates. The color scale for
f(v) extends from 5× 10 −^32 (dark purple) to 2× 10 −^29 s^3 cm−^6
(dark red). Dashed and solid vertical lines respectively mark the
shock and the edges of the ICME. (From J. T. Gosling et al.,
2002,Geophys. Res. Lett. 29 , 12 10.1029/2001GL013949.)in the ecliptic plane have sufficiently high speeds relative
to the ambient solar wind to drive shock disturbances at 1
AU; the remainder do not and simply coast along with the
rest of the solar wind. Typically, ICMEs cannot be distin-
guished from the normal solar wind at 1 AU on the basis of
either their speed or density (the event in Fig. 9 is an exam-
ple). Near the solar activity maximum, ICMEs account for
15–20% of the solar wind in the ecliptic plane at 1 AU, while
they account for less than 1% near the solar activity mini-
mum. The Earth intercepts about 72 ICMEs/year near the
solar activity maximum and∼8 ICMEs/year near the solar
activity minimum. ICMEs are much less common at high
heliographic latitudes, particularly near activity minimum