The Sun–Earth Connection 217
FIGURE 6 Solar wind velocities at
solar minimum and maximum as
measured on theUlyssesspacecraft
which passes over the Sun’s poles.
The speed is shown in a polar
coordinate system with zero speed
at the center of the Sun. The blue
and red indicate interplanetary
magnetic field polarity. (McComas
et al., 2003,Geophys. Res. Lett.
v.30, doi 10.1029/2003 GLO
17136, 2003.)orientation of the near-ecliptic field in the heliospheric, and
its typical 45◦(from radial) orientation at 1 AU. [SeeThe
Solar Wind.]At solar minimum, adjacent streams of dif-
ferent speeds from different coronal source regions may
interact, producing spiral density and field ridges at their
boundaries. When these ridges, which are called stream
interaction regions or corotating interaction regions (CIRs),
rotate past the Earth, they can cause modest geomagnetic
activity. At maximum solar activity, the solar wind condi-
tions are more variable and structured, and less organized
by solar latitude. They also exhibit many transient distur-
bances caused by rapid changes in coronal structure and
CMEs, whose effects are described in more detail later. Fig-
ure 6 shows solar wind characteristics from periods around
solar minimum and solar maximum. These interplanetary
conditions shape the Earth’s magnetosphere, control its re-
sponses to the solar wind, and regulate states of internal
particles, energy, and stresses.
As the Sun becomes more active, as indicated by in-
creasing numbers of sunspots, it produces greater numbers
of both flares and CMEs. CMEs have the greatest effects
on geospace, and so we focus on them here. The details
of the CME initiation process, as well as CME structure,
are subjects of intensive current research. As seen in white
light images from coronagraphs like theSOHOLASCO in-
strument (an example of which is shown in Fig. 1), CMEs
appear to be eruptions of a magnetic bubble or twisted “flux
rope” of coronal magnetic fields. These structures, which
are referred to as drivers or ejecta, travel outward at speeds
ranging from tens to several thousand km/s. As they travel,
they interact with the surrounding solar wind, compressing
it ahead if they are moving faster. If they move fast enough
relative to their surroundings, they create an interplanetary
shock wave. Figure 7 illustrates the effect of a CME on the
solar wind and interplanetary magnetic field at 1 AU. These
propagating disturbances are experienced by the magneto-
sphere as sudden increases of solar wind density, velocity,
and magnetic field at the shock passage, followed by sev-
eral hours of enhanced solar wind parameters, and then
the ejecta passage characterized by a period with normal
densities but high magnetic field strengths and, often, in-
clinations. The entire structure may take hours to days to
pass Earth depending on its speed. Enhanced solar wind
pressure associated with a CME is usually from the sheath
portion between the shock front and the ejecta. The ejecta
fields can occasionally be modeled as a passing magnetic
flux rope configuration as suggested in Fig. 7. Around the
minimum of solar activity the local interplanetary medium
is disturbed by one of these interplanetary CMEs or ICMEs
once every few months, but at solar maximum they can oc-
cur about once a week. The most extreme (largest, fastest)
ICMEs usually follow CMEs associated with large, com-
plex active regions on the Sun that also produce solar
flares.