218 Encyclopedia of the Solar System
FIGURE 7 Illustration of the interplanetary effects of a CME.
The CME produces an ejection of coronal material (ejecta) that
may include a helical magnetic field structure or flux rope
(illustrated by the black line). This structure plows into the
ambient solar wind and may produce a shock in the solar wind
plasma ahead of the ejecta. The region of compressed solar wind
between the shock and ejecta is referred to as the sheath. Some
solar wind particles are accelerated at the shock and speed out
ahead of it along interplanetary field lines (red, green).
(Luhmann, 2000,Physics World, p. 31–36, July 2000.)
In addition to the magnetized solar wind plasma, the he-
liosphere also contains a population of energetic (tens of
kiloelectron volts (keV) to hundreds of megaelectron volts
(MeV)) charged particles that varies with time. Ions and
electrons are accelerated at both flare sites on the Sun and
by the shock waves formed in the corona and interplane-
tary space by the fast-moving CME ejecta or by interacting
high- and low-speed solar wind streams. CME shocks pro-
duce the most intense and long-lived (several day) episodes.
The particles race ahead of their shock source along the
spiral interplanetary field lines, surrounding the magneto-
sphere with a sea of potentially hazardous radiation within
tens of minutes of the events at the Sun. Sometimes the
fluxes of these particles increase by several orders of mag-
nitude when the CME shock itself arrives several days after
the event in the corona. The contributions ofsolar en-
ergetic particlesto local interplanetary conditions are re-
lated to the level of flare and CME activity on the Sun. In an
interesting opposite effect of solar activity, other more per-
manently present energetic charged particles called cosmic
rays, which arrive at Earth from the heliospheric bound-
ary and beyond, show locally decreased fluxes when solar
activity is high. This is likely due to the sweeping action of
the highly structured solar wind around the time of solar ac-
tivity maximum, when interplanetary field disturbances car-
ried outward present effective barriers to incoming charged
particles. Under certain conditions solar energetic particles
can enter the magnetosphere where they contribute to the
radiation belts and produce layers of enhanced ionization
deep in the Earth’s polar atmosphere.2. The Geospace Role in the Sun–Earth
Connection
The Earth’s space environment is determined by its nearly
dipolar internal magnetic field that forms an obstacle to
the solar wind, creating the magnetosphere [seePlane-
tary Magnetospheres]. Spreiter and coworkers (1966)
and Axford and Hines (1961) were among the pioneering
researchers to recognize the fluid-like aspects of the solar
wind interaction with Earth’s compressible field, describing
it in terms of a blunt body in a hypersonic flow. The size and
shape of this blunt body, the magnetopause, can be calcu-
lated from the assumption of pressure balance between the
Earth’s internal magnetic field pressure and the incident dy-
namic pressure of the solar wind (Dynamic pressure=Mass
density×Velocity squared). It typically occurs at∼10 Earth
radii along the line connecting the centers of the Earth and
the Sun and at∼15 Earth radii in the terminator plane. In
contrast to the compressed, solar wind pressure-confined
day side, the night side magnetosphere stretches out into
an elongated structure called the magnetotail. These fea-
tures, confirmed by decades of observations, are illustrated
in Fig. 8. The magnetopause separates geospace and the
solar wind plasma-dominated regions outside. As seen in
Fig. 8, the outermost features associated with the Earth’s
magnetic obstacle are actually the bow shock that forms in
the solar wind∼5 Earth radii upstream of the day side mag-
netopause and the magnetosheath. The magnetosheath is
the slowed, deflected solar wind between the bow shock
and the magnetopause. Thus, when the Earth’s field inter-
acts with the solar wind, it does so through the altered solar
wind in the magnetosheath.
Dungey (1961) first recognized that the magnetopause is
not a complete barrier to the solar wind, and that magneto-
spheric field topology is also controlled by its interconnec-
tion, orreconnection,with the interplanetary field. This
leap of understanding revolutionized the study of solar wind
magnetosphere coupling and geomagnetic activity. Figure 9
reproduces Dungey’s original cartoon suggesting the dif-
ferent appearances of the magnetospheric field topology
for the extreme cases of steady northward and southward
interplanetary fields. Similar pictures can be obtained by
adding background uniform fields of both directions to a