106 Encyclopedia of the Solar System
to solar rotation. These transverse deflections produce the
systematic west–east flow direction changes observed near
the leading edges of quasi-stationary, high-speed streams
(see Fig. 2).
There is an interesting three-dimensional aspect to
stream evolution, ultimately associated with the fact that the
solar magnetic dipole is tilted relative to the solar rotation
axis. That tilt causes CIRs in the northern and southern solar
hemispheres to have opposed meridional tilts that, partic-
ularly in the outer heliosphere, can be discerned in plasma
data as systematic north–south deflections of the flow at
CIRs. The meridional tilts are such that the forward waves in
both hemispheres initially propagate equatorward, whereas
the reverse waves in both hemispheres propagate poleward.
As a result, forward shocks in the outer heliosphere near the
solar minimum are generally confined to the low-latitude
band of solar wind variability, whereas the reverse shocks
are commonly observed both within the band of variabil-
ity and poleward of it. However, the reverse waves seldom
reach latitudes more than∼ 15 ◦above the low-latitude band
of variability.
7. Coronal Mass Ejections and Transient
Solar Wind Disturbances
7.1 Coronal Mass Ejections
The solar corona evolves on a variety of time scales closely
connected with the evolution of the coronal magnetic field.
[SeeTheSun.] The most rapid and dramatic evolution in
the corona occurs in events known ascoronal mass ejec-
tions,orCMEs(Fig. 7a). CMEs originate in closed field
regions in the corona where the magnetic field normally
is sufficiently strong to constrain the coronal plasma from
expanding outward. Typically these closed field regions are
found in the coronal streamer belt that encircles the Sun
and that underlies the heliospheric current sheet. The outer
edges of CMEs often have the optical appearance of closed
loops such as the event shown in Fig. 7a. Few, if any, CMEs
ever appear to sever completely their magnetic connection
with the Sun. During a typical CME, somewhere between
1015 and 10^16 g of solar material is ejected into the helio-
sphere. Ejection speeds near the Sun range from less thanFIGURE 7 (a) A coronal mass ejection as imaged by the LASCO/C3 coronagraph onSOHOon
April 20, 1998. The Sun, indicated by the white circle, has been occulted within the instrument.
The field of view of the image is 30 solar diameters. [The SOHO/LASCO data are produced by a
consortium of the Naval Research Laboratory (USA), Max-Planck-Institut fur
Sonnensystemforshung (Germany), Laboratoire d’Astronomie (France), and the University of
Birmingham (UK). SOHO is a project of international cooperation between ESA and NASA.] (b)
A sketch of a solar wind shock disturbance produced by a fast ICME directed toward Earth. Red
and magenta arrows indicate the ambient magnetic field and that threading the ICME,
respectively. Blue arrows indicate the suprathermal electron strahl flowing away from the Sun
along the magnetic field. The ambient magnetic field is compressed by its interaction with the
ICME and is forced to drape around the ICME. [To appear in T. H. Zurbuchen and I. G.
Richardson, 2006, in “Coronal Mass Ejections” (H. Kunow et al., eds.), Kluwer academic
Publishers, Dordrecht.]