220 Encyclopedia of the Solar System
FIGURE 9 Dungey’s original 1961 cartoon suggesting the
reason for the association of greater geomagnetic activity with
southward interplanetary magnetic fields. Southward
interplanetary fields can reconnect or merge with the Earth’s
dipole field at the day side magnetopause. Another reconnection
in the magnetotail returns the Earth’s opened fields to their
original “closed” state so that all the Earth’s field is not
permanently opened by day side reconnection. The process of
reconnection drives magnetospheric circulation (see Fig. 10) and
is thus a means by which solar wind energy is transferred to the
magnetosphere. (Reprinted figure with permission from J. W.
Dungey, 1961,Phys. Rev. Lett.,6,47. Copyright 2005 by the
American Physical Society.)
magnetometer measurements. Solar wind–magnetosphere
coupling is thus greatly enhanced at times when the inter-
planetary magnetic field is southward.
The physics of the reconnection or magnetic field merg-
ing process that results in this configuration change for
southward interplanetary fields is still a subject of intensive
research. Because space is not a vacuum, simple superpo-
sition of the external (interplanetary) and internal (Earth
dipole) fields is not a physically correct explanation. Some-
how the solar wind plasma that carries the interplanetary
field “frozen” into its flow must allow the field to merge with
the magnetospheric field at the magnetopause when the
two have antiparallel components. The interested reader is
referred to the review by Drake for further details on cur-
rent theories of magnetic field reconnection in space plas-
mas. When CME effects reach the Earth, the solar wind
dynamic pressure incident on the magnetosphere can in-
FIGURE 10 Illustration of magnetospheric circulation during
periods of southward interplanetary magnetic field. Various key
features of the solar wind interaction are shown, including
magnetospheric field line connections to the interplanetary field
and their mapping to the high-altitude atmosphere. The
numbers indicate a time sequence. The driven circulation occurs
all the way down to the polar ionosphere as shown by the inset,
which shows the dusk half of the double-celled ionosphere
convection pattern. The aurora occurs mainly in the regions of
convection reversals. (Kivelson and Russell, 1995, “Introduction
to Space Physics.”)crease by an order of magnitude or more, primarily due
to the compression of the ambient solar wind plasma by
the driver or ejecta from the CME. The onset of this in-
crease may be sudden if a leading shock is present. The so-
lar wind magnetic field is also compressed with the plasma
and can become significantly inclined with respect to the
Earth’s equatorial plane. The ejecta fields are also highly in-
clined, and often strong and steady, or slowly rotating over
intervals of about a day. Thus, larger than normal north-
ward and/or southward interplanetary field components re-
sult from both passing segments of the disturbance. The
magnetosphere’s response to these disturbed heliospheric
conditions includes increased compression of the dayside
magnetosphere, sometimes to within a few Earth radii of the
surface, and increased reconnection between the Earth’s
magnetic field and interplanetary field during the passage
of the southward-oriented portions. The time-dependent